linux/block/bfq-iosched.c
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   1// SPDX-License-Identifier: GPL-2.0-or-later
   2/*
   3 * Budget Fair Queueing (BFQ) I/O scheduler.
   4 *
   5 * Based on ideas and code from CFQ:
   6 * Copyright (C) 2003 Jens Axboe <axboe@kernel.dk>
   7 *
   8 * Copyright (C) 2008 Fabio Checconi <fabio@gandalf.sssup.it>
   9 *                    Paolo Valente <paolo.valente@unimore.it>
  10 *
  11 * Copyright (C) 2010 Paolo Valente <paolo.valente@unimore.it>
  12 *                    Arianna Avanzini <avanzini@google.com>
  13 *
  14 * Copyright (C) 2017 Paolo Valente <paolo.valente@linaro.org>
  15 *
  16 * BFQ is a proportional-share I/O scheduler, with some extra
  17 * low-latency capabilities. BFQ also supports full hierarchical
  18 * scheduling through cgroups. Next paragraphs provide an introduction
  19 * on BFQ inner workings. Details on BFQ benefits, usage and
  20 * limitations can be found in Documentation/block/bfq-iosched.rst.
  21 *
  22 * BFQ is a proportional-share storage-I/O scheduling algorithm based
  23 * on the slice-by-slice service scheme of CFQ. But BFQ assigns
  24 * budgets, measured in number of sectors, to processes instead of
  25 * time slices. The device is not granted to the in-service process
  26 * for a given time slice, but until it has exhausted its assigned
  27 * budget. This change from the time to the service domain enables BFQ
  28 * to distribute the device throughput among processes as desired,
  29 * without any distortion due to throughput fluctuations, or to device
  30 * internal queueing. BFQ uses an ad hoc internal scheduler, called
  31 * B-WF2Q+, to schedule processes according to their budgets. More
  32 * precisely, BFQ schedules queues associated with processes. Each
  33 * process/queue is assigned a user-configurable weight, and B-WF2Q+
  34 * guarantees that each queue receives a fraction of the throughput
  35 * proportional to its weight. Thanks to the accurate policy of
  36 * B-WF2Q+, BFQ can afford to assign high budgets to I/O-bound
  37 * processes issuing sequential requests (to boost the throughput),
  38 * and yet guarantee a low latency to interactive and soft real-time
  39 * applications.
  40 *
  41 * In particular, to provide these low-latency guarantees, BFQ
  42 * explicitly privileges the I/O of two classes of time-sensitive
  43 * applications: interactive and soft real-time. In more detail, BFQ
  44 * behaves this way if the low_latency parameter is set (default
  45 * configuration). This feature enables BFQ to provide applications in
  46 * these classes with a very low latency.
  47 *
  48 * To implement this feature, BFQ constantly tries to detect whether
  49 * the I/O requests in a bfq_queue come from an interactive or a soft
  50 * real-time application. For brevity, in these cases, the queue is
  51 * said to be interactive or soft real-time. In both cases, BFQ
  52 * privileges the service of the queue, over that of non-interactive
  53 * and non-soft-real-time queues. This privileging is performed,
  54 * mainly, by raising the weight of the queue. So, for brevity, we
  55 * call just weight-raising periods the time periods during which a
  56 * queue is privileged, because deemed interactive or soft real-time.
  57 *
  58 * The detection of soft real-time queues/applications is described in
  59 * detail in the comments on the function
  60 * bfq_bfqq_softrt_next_start. On the other hand, the detection of an
  61 * interactive queue works as follows: a queue is deemed interactive
  62 * if it is constantly non empty only for a limited time interval,
  63 * after which it does become empty. The queue may be deemed
  64 * interactive again (for a limited time), if it restarts being
  65 * constantly non empty, provided that this happens only after the
  66 * queue has remained empty for a given minimum idle time.
  67 *
  68 * By default, BFQ computes automatically the above maximum time
  69 * interval, i.e., the time interval after which a constantly
  70 * non-empty queue stops being deemed interactive. Since a queue is
  71 * weight-raised while it is deemed interactive, this maximum time
  72 * interval happens to coincide with the (maximum) duration of the
  73 * weight-raising for interactive queues.
  74 *
  75 * Finally, BFQ also features additional heuristics for
  76 * preserving both a low latency and a high throughput on NCQ-capable,
  77 * rotational or flash-based devices, and to get the job done quickly
  78 * for applications consisting in many I/O-bound processes.
  79 *
  80 * NOTE: if the main or only goal, with a given device, is to achieve
  81 * the maximum-possible throughput at all times, then do switch off
  82 * all low-latency heuristics for that device, by setting low_latency
  83 * to 0.
  84 *
  85 * BFQ is described in [1], where also a reference to the initial,
  86 * more theoretical paper on BFQ can be found. The interested reader
  87 * can find in the latter paper full details on the main algorithm, as
  88 * well as formulas of the guarantees and formal proofs of all the
  89 * properties.  With respect to the version of BFQ presented in these
  90 * papers, this implementation adds a few more heuristics, such as the
  91 * ones that guarantee a low latency to interactive and soft real-time
  92 * applications, and a hierarchical extension based on H-WF2Q+.
  93 *
  94 * B-WF2Q+ is based on WF2Q+, which is described in [2], together with
  95 * H-WF2Q+, while the augmented tree used here to implement B-WF2Q+
  96 * with O(log N) complexity derives from the one introduced with EEVDF
  97 * in [3].
  98 *
  99 * [1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
 100 *     Scheduler", Proceedings of the First Workshop on Mobile System
 101 *     Technologies (MST-2015), May 2015.
 102 *     http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf
 103 *
 104 * [2] Jon C.R. Bennett and H. Zhang, "Hierarchical Packet Fair Queueing
 105 *     Algorithms", IEEE/ACM Transactions on Networking, 5(5):675-689,
 106 *     Oct 1997.
 107 *
 108 * http://www.cs.cmu.edu/~hzhang/papers/TON-97-Oct.ps.gz
 109 *
 110 * [3] I. Stoica and H. Abdel-Wahab, "Earliest Eligible Virtual Deadline
 111 *     First: A Flexible and Accurate Mechanism for Proportional Share
 112 *     Resource Allocation", technical report.
 113 *
 114 * http://www.cs.berkeley.edu/~istoica/papers/eevdf-tr-95.pdf
 115 */
 116#include <linux/module.h>
 117#include <linux/slab.h>
 118#include <linux/blkdev.h>
 119#include <linux/cgroup.h>
 120#include <linux/elevator.h>
 121#include <linux/ktime.h>
 122#include <linux/rbtree.h>
 123#include <linux/ioprio.h>
 124#include <linux/sbitmap.h>
 125#include <linux/delay.h>
 126#include <linux/backing-dev.h>
 127
 128#include <trace/events/block.h>
 129
 130#include "blk.h"
 131#include "blk-mq.h"
 132#include "blk-mq-tag.h"
 133#include "blk-mq-sched.h"
 134#include "bfq-iosched.h"
 135#include "blk-wbt.h"
 136
 137#define BFQ_BFQQ_FNS(name)                                              \
 138void bfq_mark_bfqq_##name(struct bfq_queue *bfqq)                       \
 139{                                                                       \
 140        __set_bit(BFQQF_##name, &(bfqq)->flags);                        \
 141}                                                                       \
 142void bfq_clear_bfqq_##name(struct bfq_queue *bfqq)                      \
 143{                                                                       \
 144        __clear_bit(BFQQF_##name, &(bfqq)->flags);              \
 145}                                                                       \
 146int bfq_bfqq_##name(const struct bfq_queue *bfqq)                       \
 147{                                                                       \
 148        return test_bit(BFQQF_##name, &(bfqq)->flags);          \
 149}
 150
 151BFQ_BFQQ_FNS(just_created);
 152BFQ_BFQQ_FNS(busy);
 153BFQ_BFQQ_FNS(wait_request);
 154BFQ_BFQQ_FNS(non_blocking_wait_rq);
 155BFQ_BFQQ_FNS(fifo_expire);
 156BFQ_BFQQ_FNS(has_short_ttime);
 157BFQ_BFQQ_FNS(sync);
 158BFQ_BFQQ_FNS(IO_bound);
 159BFQ_BFQQ_FNS(in_large_burst);
 160BFQ_BFQQ_FNS(coop);
 161BFQ_BFQQ_FNS(split_coop);
 162BFQ_BFQQ_FNS(softrt_update);
 163#undef BFQ_BFQQ_FNS                                             \
 164
 165/* Expiration time of async (0) and sync (1) requests, in ns. */
 166static const u64 bfq_fifo_expire[2] = { NSEC_PER_SEC / 4, NSEC_PER_SEC / 8 };
 167
 168/* Maximum backwards seek (magic number lifted from CFQ), in KiB. */
 169static const int bfq_back_max = 16 * 1024;
 170
 171/* Penalty of a backwards seek, in number of sectors. */
 172static const int bfq_back_penalty = 2;
 173
 174/* Idling period duration, in ns. */
 175static u64 bfq_slice_idle = NSEC_PER_SEC / 125;
 176
 177/* Minimum number of assigned budgets for which stats are safe to compute. */
 178static const int bfq_stats_min_budgets = 194;
 179
 180/* Default maximum budget values, in sectors and number of requests. */
 181static const int bfq_default_max_budget = 16 * 1024;
 182
 183/*
 184 * When a sync request is dispatched, the queue that contains that
 185 * request, and all the ancestor entities of that queue, are charged
 186 * with the number of sectors of the request. In contrast, if the
 187 * request is async, then the queue and its ancestor entities are
 188 * charged with the number of sectors of the request, multiplied by
 189 * the factor below. This throttles the bandwidth for async I/O,
 190 * w.r.t. to sync I/O, and it is done to counter the tendency of async
 191 * writes to steal I/O throughput to reads.
 192 *
 193 * The current value of this parameter is the result of a tuning with
 194 * several hardware and software configurations. We tried to find the
 195 * lowest value for which writes do not cause noticeable problems to
 196 * reads. In fact, the lower this parameter, the stabler I/O control,
 197 * in the following respect.  The lower this parameter is, the less
 198 * the bandwidth enjoyed by a group decreases
 199 * - when the group does writes, w.r.t. to when it does reads;
 200 * - when other groups do reads, w.r.t. to when they do writes.
 201 */
 202static const int bfq_async_charge_factor = 3;
 203
 204/* Default timeout values, in jiffies, approximating CFQ defaults. */
 205const int bfq_timeout = HZ / 8;
 206
 207/*
 208 * Time limit for merging (see comments in bfq_setup_cooperator). Set
 209 * to the slowest value that, in our tests, proved to be effective in
 210 * removing false positives, while not causing true positives to miss
 211 * queue merging.
 212 *
 213 * As can be deduced from the low time limit below, queue merging, if
 214 * successful, happens at the very beginning of the I/O of the involved
 215 * cooperating processes, as a consequence of the arrival of the very
 216 * first requests from each cooperator.  After that, there is very
 217 * little chance to find cooperators.
 218 */
 219static const unsigned long bfq_merge_time_limit = HZ/10;
 220
 221static struct kmem_cache *bfq_pool;
 222
 223/* Below this threshold (in ns), we consider thinktime immediate. */
 224#define BFQ_MIN_TT              (2 * NSEC_PER_MSEC)
 225
 226/* hw_tag detection: parallel requests threshold and min samples needed. */
 227#define BFQ_HW_QUEUE_THRESHOLD  3
 228#define BFQ_HW_QUEUE_SAMPLES    32
 229
 230#define BFQQ_SEEK_THR           (sector_t)(8 * 100)
 231#define BFQQ_SECT_THR_NONROT    (sector_t)(2 * 32)
 232#define BFQ_RQ_SEEKY(bfqd, last_pos, rq) \
 233        (get_sdist(last_pos, rq) >                      \
 234         BFQQ_SEEK_THR &&                               \
 235         (!blk_queue_nonrot(bfqd->queue) ||             \
 236          blk_rq_sectors(rq) < BFQQ_SECT_THR_NONROT))
 237#define BFQQ_CLOSE_THR          (sector_t)(8 * 1024)
 238#define BFQQ_SEEKY(bfqq)        (hweight32(bfqq->seek_history) > 19)
 239/*
 240 * Sync random I/O is likely to be confused with soft real-time I/O,
 241 * because it is characterized by limited throughput and apparently
 242 * isochronous arrival pattern. To avoid false positives, queues
 243 * containing only random (seeky) I/O are prevented from being tagged
 244 * as soft real-time.
 245 */
 246#define BFQQ_TOTALLY_SEEKY(bfqq)        (bfqq->seek_history == -1)
 247
 248/* Min number of samples required to perform peak-rate update */
 249#define BFQ_RATE_MIN_SAMPLES    32
 250/* Min observation time interval required to perform a peak-rate update (ns) */
 251#define BFQ_RATE_MIN_INTERVAL   (300*NSEC_PER_MSEC)
 252/* Target observation time interval for a peak-rate update (ns) */
 253#define BFQ_RATE_REF_INTERVAL   NSEC_PER_SEC
 254
 255/*
 256 * Shift used for peak-rate fixed precision calculations.
 257 * With
 258 * - the current shift: 16 positions
 259 * - the current type used to store rate: u32
 260 * - the current unit of measure for rate: [sectors/usec], or, more precisely,
 261 *   [(sectors/usec) / 2^BFQ_RATE_SHIFT] to take into account the shift,
 262 * the range of rates that can be stored is
 263 * [1 / 2^BFQ_RATE_SHIFT, 2^(32 - BFQ_RATE_SHIFT)] sectors/usec =
 264 * [1 / 2^16, 2^16] sectors/usec = [15e-6, 65536] sectors/usec =
 265 * [15, 65G] sectors/sec
 266 * Which, assuming a sector size of 512B, corresponds to a range of
 267 * [7.5K, 33T] B/sec
 268 */
 269#define BFQ_RATE_SHIFT          16
 270
 271/*
 272 * When configured for computing the duration of the weight-raising
 273 * for interactive queues automatically (see the comments at the
 274 * beginning of this file), BFQ does it using the following formula:
 275 * duration = (ref_rate / r) * ref_wr_duration,
 276 * where r is the peak rate of the device, and ref_rate and
 277 * ref_wr_duration are two reference parameters.  In particular,
 278 * ref_rate is the peak rate of the reference storage device (see
 279 * below), and ref_wr_duration is about the maximum time needed, with
 280 * BFQ and while reading two files in parallel, to load typical large
 281 * applications on the reference device (see the comments on
 282 * max_service_from_wr below, for more details on how ref_wr_duration
 283 * is obtained).  In practice, the slower/faster the device at hand
 284 * is, the more/less it takes to load applications with respect to the
 285 * reference device.  Accordingly, the longer/shorter BFQ grants
 286 * weight raising to interactive applications.
 287 *
 288 * BFQ uses two different reference pairs (ref_rate, ref_wr_duration),
 289 * depending on whether the device is rotational or non-rotational.
 290 *
 291 * In the following definitions, ref_rate[0] and ref_wr_duration[0]
 292 * are the reference values for a rotational device, whereas
 293 * ref_rate[1] and ref_wr_duration[1] are the reference values for a
 294 * non-rotational device. The reference rates are not the actual peak
 295 * rates of the devices used as a reference, but slightly lower
 296 * values. The reason for using slightly lower values is that the
 297 * peak-rate estimator tends to yield slightly lower values than the
 298 * actual peak rate (it can yield the actual peak rate only if there
 299 * is only one process doing I/O, and the process does sequential
 300 * I/O).
 301 *
 302 * The reference peak rates are measured in sectors/usec, left-shifted
 303 * by BFQ_RATE_SHIFT.
 304 */
 305static int ref_rate[2] = {14000, 33000};
 306/*
 307 * To improve readability, a conversion function is used to initialize
 308 * the following array, which entails that the array can be
 309 * initialized only in a function.
 310 */
 311static int ref_wr_duration[2];
 312
 313/*
 314 * BFQ uses the above-detailed, time-based weight-raising mechanism to
 315 * privilege interactive tasks. This mechanism is vulnerable to the
 316 * following false positives: I/O-bound applications that will go on
 317 * doing I/O for much longer than the duration of weight
 318 * raising. These applications have basically no benefit from being
 319 * weight-raised at the beginning of their I/O. On the opposite end,
 320 * while being weight-raised, these applications
 321 * a) unjustly steal throughput to applications that may actually need
 322 * low latency;
 323 * b) make BFQ uselessly perform device idling; device idling results
 324 * in loss of device throughput with most flash-based storage, and may
 325 * increase latencies when used purposelessly.
 326 *
 327 * BFQ tries to reduce these problems, by adopting the following
 328 * countermeasure. To introduce this countermeasure, we need first to
 329 * finish explaining how the duration of weight-raising for
 330 * interactive tasks is computed.
 331 *
 332 * For a bfq_queue deemed as interactive, the duration of weight
 333 * raising is dynamically adjusted, as a function of the estimated
 334 * peak rate of the device, so as to be equal to the time needed to
 335 * execute the 'largest' interactive task we benchmarked so far. By
 336 * largest task, we mean the task for which each involved process has
 337 * to do more I/O than for any of the other tasks we benchmarked. This
 338 * reference interactive task is the start-up of LibreOffice Writer,
 339 * and in this task each process/bfq_queue needs to have at most ~110K
 340 * sectors transferred.
 341 *
 342 * This last piece of information enables BFQ to reduce the actual
 343 * duration of weight-raising for at least one class of I/O-bound
 344 * applications: those doing sequential or quasi-sequential I/O. An
 345 * example is file copy. In fact, once started, the main I/O-bound
 346 * processes of these applications usually consume the above 110K
 347 * sectors in much less time than the processes of an application that
 348 * is starting, because these I/O-bound processes will greedily devote
 349 * almost all their CPU cycles only to their target,
 350 * throughput-friendly I/O operations. This is even more true if BFQ
 351 * happens to be underestimating the device peak rate, and thus
 352 * overestimating the duration of weight raising. But, according to
 353 * our measurements, once transferred 110K sectors, these processes
 354 * have no right to be weight-raised any longer.
 355 *
 356 * Basing on the last consideration, BFQ ends weight-raising for a
 357 * bfq_queue if the latter happens to have received an amount of
 358 * service at least equal to the following constant. The constant is
 359 * set to slightly more than 110K, to have a minimum safety margin.
 360 *
 361 * This early ending of weight-raising reduces the amount of time
 362 * during which interactive false positives cause the two problems
 363 * described at the beginning of these comments.
 364 */
 365static const unsigned long max_service_from_wr = 120000;
 366
 367#define RQ_BIC(rq)              icq_to_bic((rq)->elv.priv[0])
 368#define RQ_BFQQ(rq)             ((rq)->elv.priv[1])
 369
 370struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync)
 371{
 372        return bic->bfqq[is_sync];
 373}
 374
 375static void bfq_put_stable_ref(struct bfq_queue *bfqq);
 376
 377void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync)
 378{
 379        /*
 380         * If bfqq != NULL, then a non-stable queue merge between
 381         * bic->bfqq and bfqq is happening here. This causes troubles
 382         * in the following case: bic->bfqq has also been scheduled
 383         * for a possible stable merge with bic->stable_merge_bfqq,
 384         * and bic->stable_merge_bfqq == bfqq happens to
 385         * hold. Troubles occur because bfqq may then undergo a split,
 386         * thereby becoming eligible for a stable merge. Yet, if
 387         * bic->stable_merge_bfqq points exactly to bfqq, then bfqq
 388         * would be stably merged with itself. To avoid this anomaly,
 389         * we cancel the stable merge if
 390         * bic->stable_merge_bfqq == bfqq.
 391         */
 392        bic->bfqq[is_sync] = bfqq;
 393
 394        if (bfqq && bic->stable_merge_bfqq == bfqq) {
 395                /*
 396                 * Actually, these same instructions are executed also
 397                 * in bfq_setup_cooperator, in case of abort or actual
 398                 * execution of a stable merge. We could avoid
 399                 * repeating these instructions there too, but if we
 400                 * did so, we would nest even more complexity in this
 401                 * function.
 402                 */
 403                bfq_put_stable_ref(bic->stable_merge_bfqq);
 404
 405                bic->stable_merge_bfqq = NULL;
 406        }
 407}
 408
 409struct bfq_data *bic_to_bfqd(struct bfq_io_cq *bic)
 410{
 411        return bic->icq.q->elevator->elevator_data;
 412}
 413
 414/**
 415 * icq_to_bic - convert iocontext queue structure to bfq_io_cq.
 416 * @icq: the iocontext queue.
 417 */
 418static struct bfq_io_cq *icq_to_bic(struct io_cq *icq)
 419{
 420        /* bic->icq is the first member, %NULL will convert to %NULL */
 421        return container_of(icq, struct bfq_io_cq, icq);
 422}
 423
 424/**
 425 * bfq_bic_lookup - search into @ioc a bic associated to @bfqd.
 426 * @bfqd: the lookup key.
 427 * @ioc: the io_context of the process doing I/O.
 428 * @q: the request queue.
 429 */
 430static struct bfq_io_cq *bfq_bic_lookup(struct bfq_data *bfqd,
 431                                        struct io_context *ioc,
 432                                        struct request_queue *q)
 433{
 434        if (ioc) {
 435                unsigned long flags;
 436                struct bfq_io_cq *icq;
 437
 438                spin_lock_irqsave(&q->queue_lock, flags);
 439                icq = icq_to_bic(ioc_lookup_icq(ioc, q));
 440                spin_unlock_irqrestore(&q->queue_lock, flags);
 441
 442                return icq;
 443        }
 444
 445        return NULL;
 446}
 447
 448/*
 449 * Scheduler run of queue, if there are requests pending and no one in the
 450 * driver that will restart queueing.
 451 */
 452void bfq_schedule_dispatch(struct bfq_data *bfqd)
 453{
 454        if (bfqd->queued != 0) {
 455                bfq_log(bfqd, "schedule dispatch");
 456                blk_mq_run_hw_queues(bfqd->queue, true);
 457        }
 458}
 459
 460#define bfq_class_idle(bfqq)    ((bfqq)->ioprio_class == IOPRIO_CLASS_IDLE)
 461
 462#define bfq_sample_valid(samples)       ((samples) > 80)
 463
 464/*
 465 * Lifted from AS - choose which of rq1 and rq2 that is best served now.
 466 * We choose the request that is closer to the head right now.  Distance
 467 * behind the head is penalized and only allowed to a certain extent.
 468 */
 469static struct request *bfq_choose_req(struct bfq_data *bfqd,
 470                                      struct request *rq1,
 471                                      struct request *rq2,
 472                                      sector_t last)
 473{
 474        sector_t s1, s2, d1 = 0, d2 = 0;
 475        unsigned long back_max;
 476#define BFQ_RQ1_WRAP    0x01 /* request 1 wraps */
 477#define BFQ_RQ2_WRAP    0x02 /* request 2 wraps */
 478        unsigned int wrap = 0; /* bit mask: requests behind the disk head? */
 479
 480        if (!rq1 || rq1 == rq2)
 481                return rq2;
 482        if (!rq2)
 483                return rq1;
 484
 485        if (rq_is_sync(rq1) && !rq_is_sync(rq2))
 486                return rq1;
 487        else if (rq_is_sync(rq2) && !rq_is_sync(rq1))
 488                return rq2;
 489        if ((rq1->cmd_flags & REQ_META) && !(rq2->cmd_flags & REQ_META))
 490                return rq1;
 491        else if ((rq2->cmd_flags & REQ_META) && !(rq1->cmd_flags & REQ_META))
 492                return rq2;
 493
 494        s1 = blk_rq_pos(rq1);
 495        s2 = blk_rq_pos(rq2);
 496
 497        /*
 498         * By definition, 1KiB is 2 sectors.
 499         */
 500        back_max = bfqd->bfq_back_max * 2;
 501
 502        /*
 503         * Strict one way elevator _except_ in the case where we allow
 504         * short backward seeks which are biased as twice the cost of a
 505         * similar forward seek.
 506         */
 507        if (s1 >= last)
 508                d1 = s1 - last;
 509        else if (s1 + back_max >= last)
 510                d1 = (last - s1) * bfqd->bfq_back_penalty;
 511        else
 512                wrap |= BFQ_RQ1_WRAP;
 513
 514        if (s2 >= last)
 515                d2 = s2 - last;
 516        else if (s2 + back_max >= last)
 517                d2 = (last - s2) * bfqd->bfq_back_penalty;
 518        else
 519                wrap |= BFQ_RQ2_WRAP;
 520
 521        /* Found required data */
 522
 523        /*
 524         * By doing switch() on the bit mask "wrap" we avoid having to
 525         * check two variables for all permutations: --> faster!
 526         */
 527        switch (wrap) {
 528        case 0: /* common case for CFQ: rq1 and rq2 not wrapped */
 529                if (d1 < d2)
 530                        return rq1;
 531                else if (d2 < d1)
 532                        return rq2;
 533
 534                if (s1 >= s2)
 535                        return rq1;
 536                else
 537                        return rq2;
 538
 539        case BFQ_RQ2_WRAP:
 540                return rq1;
 541        case BFQ_RQ1_WRAP:
 542                return rq2;
 543        case BFQ_RQ1_WRAP|BFQ_RQ2_WRAP: /* both rqs wrapped */
 544        default:
 545                /*
 546                 * Since both rqs are wrapped,
 547                 * start with the one that's further behind head
 548                 * (--> only *one* back seek required),
 549                 * since back seek takes more time than forward.
 550                 */
 551                if (s1 <= s2)
 552                        return rq1;
 553                else
 554                        return rq2;
 555        }
 556}
 557
 558/*
 559 * Async I/O can easily starve sync I/O (both sync reads and sync
 560 * writes), by consuming all tags. Similarly, storms of sync writes,
 561 * such as those that sync(2) may trigger, can starve sync reads.
 562 * Limit depths of async I/O and sync writes so as to counter both
 563 * problems.
 564 */
 565static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
 566{
 567        struct bfq_data *bfqd = data->q->elevator->elevator_data;
 568
 569        if (op_is_sync(op) && !op_is_write(op))
 570                return;
 571
 572        data->shallow_depth =
 573                bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)];
 574
 575        bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
 576                        __func__, bfqd->wr_busy_queues, op_is_sync(op),
 577                        data->shallow_depth);
 578}
 579
 580static struct bfq_queue *
 581bfq_rq_pos_tree_lookup(struct bfq_data *bfqd, struct rb_root *root,
 582                     sector_t sector, struct rb_node **ret_parent,
 583                     struct rb_node ***rb_link)
 584{
 585        struct rb_node **p, *parent;
 586        struct bfq_queue *bfqq = NULL;
 587
 588        parent = NULL;
 589        p = &root->rb_node;
 590        while (*p) {
 591                struct rb_node **n;
 592
 593                parent = *p;
 594                bfqq = rb_entry(parent, struct bfq_queue, pos_node);
 595
 596                /*
 597                 * Sort strictly based on sector. Smallest to the left,
 598                 * largest to the right.
 599                 */
 600                if (sector > blk_rq_pos(bfqq->next_rq))
 601                        n = &(*p)->rb_right;
 602                else if (sector < blk_rq_pos(bfqq->next_rq))
 603                        n = &(*p)->rb_left;
 604                else
 605                        break;
 606                p = n;
 607                bfqq = NULL;
 608        }
 609
 610        *ret_parent = parent;
 611        if (rb_link)
 612                *rb_link = p;
 613
 614        bfq_log(bfqd, "rq_pos_tree_lookup %llu: returning %d",
 615                (unsigned long long)sector,
 616                bfqq ? bfqq->pid : 0);
 617
 618        return bfqq;
 619}
 620
 621static bool bfq_too_late_for_merging(struct bfq_queue *bfqq)
 622{
 623        return bfqq->service_from_backlogged > 0 &&
 624                time_is_before_jiffies(bfqq->first_IO_time +
 625                                       bfq_merge_time_limit);
 626}
 627
 628/*
 629 * The following function is not marked as __cold because it is
 630 * actually cold, but for the same performance goal described in the
 631 * comments on the likely() at the beginning of
 632 * bfq_setup_cooperator(). Unexpectedly, to reach an even lower
 633 * execution time for the case where this function is not invoked, we
 634 * had to add an unlikely() in each involved if().
 635 */
 636void __cold
 637bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 638{
 639        struct rb_node **p, *parent;
 640        struct bfq_queue *__bfqq;
 641
 642        if (bfqq->pos_root) {
 643                rb_erase(&bfqq->pos_node, bfqq->pos_root);
 644                bfqq->pos_root = NULL;
 645        }
 646
 647        /* oom_bfqq does not participate in queue merging */
 648        if (bfqq == &bfqd->oom_bfqq)
 649                return;
 650
 651        /*
 652         * bfqq cannot be merged any longer (see comments in
 653         * bfq_setup_cooperator): no point in adding bfqq into the
 654         * position tree.
 655         */
 656        if (bfq_too_late_for_merging(bfqq))
 657                return;
 658
 659        if (bfq_class_idle(bfqq))
 660                return;
 661        if (!bfqq->next_rq)
 662                return;
 663
 664        bfqq->pos_root = &bfq_bfqq_to_bfqg(bfqq)->rq_pos_tree;
 665        __bfqq = bfq_rq_pos_tree_lookup(bfqd, bfqq->pos_root,
 666                        blk_rq_pos(bfqq->next_rq), &parent, &p);
 667        if (!__bfqq) {
 668                rb_link_node(&bfqq->pos_node, parent, p);
 669                rb_insert_color(&bfqq->pos_node, bfqq->pos_root);
 670        } else
 671                bfqq->pos_root = NULL;
 672}
 673
 674/*
 675 * The following function returns false either if every active queue
 676 * must receive the same share of the throughput (symmetric scenario),
 677 * or, as a special case, if bfqq must receive a share of the
 678 * throughput lower than or equal to the share that every other active
 679 * queue must receive.  If bfqq does sync I/O, then these are the only
 680 * two cases where bfqq happens to be guaranteed its share of the
 681 * throughput even if I/O dispatching is not plugged when bfqq remains
 682 * temporarily empty (for more details, see the comments in the
 683 * function bfq_better_to_idle()). For this reason, the return value
 684 * of this function is used to check whether I/O-dispatch plugging can
 685 * be avoided.
 686 *
 687 * The above first case (symmetric scenario) occurs when:
 688 * 1) all active queues have the same weight,
 689 * 2) all active queues belong to the same I/O-priority class,
 690 * 3) all active groups at the same level in the groups tree have the same
 691 *    weight,
 692 * 4) all active groups at the same level in the groups tree have the same
 693 *    number of children.
 694 *
 695 * Unfortunately, keeping the necessary state for evaluating exactly
 696 * the last two symmetry sub-conditions above would be quite complex
 697 * and time consuming. Therefore this function evaluates, instead,
 698 * only the following stronger three sub-conditions, for which it is
 699 * much easier to maintain the needed state:
 700 * 1) all active queues have the same weight,
 701 * 2) all active queues belong to the same I/O-priority class,
 702 * 3) there are no active groups.
 703 * In particular, the last condition is always true if hierarchical
 704 * support or the cgroups interface are not enabled, thus no state
 705 * needs to be maintained in this case.
 706 */
 707static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
 708                                   struct bfq_queue *bfqq)
 709{
 710        bool smallest_weight = bfqq &&
 711                bfqq->weight_counter &&
 712                bfqq->weight_counter ==
 713                container_of(
 714                        rb_first_cached(&bfqd->queue_weights_tree),
 715                        struct bfq_weight_counter,
 716                        weights_node);
 717
 718        /*
 719         * For queue weights to differ, queue_weights_tree must contain
 720         * at least two nodes.
 721         */
 722        bool varied_queue_weights = !smallest_weight &&
 723                !RB_EMPTY_ROOT(&bfqd->queue_weights_tree.rb_root) &&
 724                (bfqd->queue_weights_tree.rb_root.rb_node->rb_left ||
 725                 bfqd->queue_weights_tree.rb_root.rb_node->rb_right);
 726
 727        bool multiple_classes_busy =
 728                (bfqd->busy_queues[0] && bfqd->busy_queues[1]) ||
 729                (bfqd->busy_queues[0] && bfqd->busy_queues[2]) ||
 730                (bfqd->busy_queues[1] && bfqd->busy_queues[2]);
 731
 732        return varied_queue_weights || multiple_classes_busy
 733#ifdef CONFIG_BFQ_GROUP_IOSCHED
 734               || bfqd->num_groups_with_pending_reqs > 0
 735#endif
 736                ;
 737}
 738
 739/*
 740 * If the weight-counter tree passed as input contains no counter for
 741 * the weight of the input queue, then add that counter; otherwise just
 742 * increment the existing counter.
 743 *
 744 * Note that weight-counter trees contain few nodes in mostly symmetric
 745 * scenarios. For example, if all queues have the same weight, then the
 746 * weight-counter tree for the queues may contain at most one node.
 747 * This holds even if low_latency is on, because weight-raised queues
 748 * are not inserted in the tree.
 749 * In most scenarios, the rate at which nodes are created/destroyed
 750 * should be low too.
 751 */
 752void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 753                          struct rb_root_cached *root)
 754{
 755        struct bfq_entity *entity = &bfqq->entity;
 756        struct rb_node **new = &(root->rb_root.rb_node), *parent = NULL;
 757        bool leftmost = true;
 758
 759        /*
 760         * Do not insert if the queue is already associated with a
 761         * counter, which happens if:
 762         *   1) a request arrival has caused the queue to become both
 763         *      non-weight-raised, and hence change its weight, and
 764         *      backlogged; in this respect, each of the two events
 765         *      causes an invocation of this function,
 766         *   2) this is the invocation of this function caused by the
 767         *      second event. This second invocation is actually useless,
 768         *      and we handle this fact by exiting immediately. More
 769         *      efficient or clearer solutions might possibly be adopted.
 770         */
 771        if (bfqq->weight_counter)
 772                return;
 773
 774        while (*new) {
 775                struct bfq_weight_counter *__counter = container_of(*new,
 776                                                struct bfq_weight_counter,
 777                                                weights_node);
 778                parent = *new;
 779
 780                if (entity->weight == __counter->weight) {
 781                        bfqq->weight_counter = __counter;
 782                        goto inc_counter;
 783                }
 784                if (entity->weight < __counter->weight)
 785                        new = &((*new)->rb_left);
 786                else {
 787                        new = &((*new)->rb_right);
 788                        leftmost = false;
 789                }
 790        }
 791
 792        bfqq->weight_counter = kzalloc(sizeof(struct bfq_weight_counter),
 793                                       GFP_ATOMIC);
 794
 795        /*
 796         * In the unlucky event of an allocation failure, we just
 797         * exit. This will cause the weight of queue to not be
 798         * considered in bfq_asymmetric_scenario, which, in its turn,
 799         * causes the scenario to be deemed wrongly symmetric in case
 800         * bfqq's weight would have been the only weight making the
 801         * scenario asymmetric.  On the bright side, no unbalance will
 802         * however occur when bfqq becomes inactive again (the
 803         * invocation of this function is triggered by an activation
 804         * of queue).  In fact, bfq_weights_tree_remove does nothing
 805         * if !bfqq->weight_counter.
 806         */
 807        if (unlikely(!bfqq->weight_counter))
 808                return;
 809
 810        bfqq->weight_counter->weight = entity->weight;
 811        rb_link_node(&bfqq->weight_counter->weights_node, parent, new);
 812        rb_insert_color_cached(&bfqq->weight_counter->weights_node, root,
 813                                leftmost);
 814
 815inc_counter:
 816        bfqq->weight_counter->num_active++;
 817        bfqq->ref++;
 818}
 819
 820/*
 821 * Decrement the weight counter associated with the queue, and, if the
 822 * counter reaches 0, remove the counter from the tree.
 823 * See the comments to the function bfq_weights_tree_add() for considerations
 824 * about overhead.
 825 */
 826void __bfq_weights_tree_remove(struct bfq_data *bfqd,
 827                               struct bfq_queue *bfqq,
 828                               struct rb_root_cached *root)
 829{
 830        if (!bfqq->weight_counter)
 831                return;
 832
 833        bfqq->weight_counter->num_active--;
 834        if (bfqq->weight_counter->num_active > 0)
 835                goto reset_entity_pointer;
 836
 837        rb_erase_cached(&bfqq->weight_counter->weights_node, root);
 838        kfree(bfqq->weight_counter);
 839
 840reset_entity_pointer:
 841        bfqq->weight_counter = NULL;
 842        bfq_put_queue(bfqq);
 843}
 844
 845/*
 846 * Invoke __bfq_weights_tree_remove on bfqq and decrement the number
 847 * of active groups for each queue's inactive parent entity.
 848 */
 849void bfq_weights_tree_remove(struct bfq_data *bfqd,
 850                             struct bfq_queue *bfqq)
 851{
 852        struct bfq_entity *entity = bfqq->entity.parent;
 853
 854        for_each_entity(entity) {
 855                struct bfq_sched_data *sd = entity->my_sched_data;
 856
 857                if (sd->next_in_service || sd->in_service_entity) {
 858                        /*
 859                         * entity is still active, because either
 860                         * next_in_service or in_service_entity is not
 861                         * NULL (see the comments on the definition of
 862                         * next_in_service for details on why
 863                         * in_service_entity must be checked too).
 864                         *
 865                         * As a consequence, its parent entities are
 866                         * active as well, and thus this loop must
 867                         * stop here.
 868                         */
 869                        break;
 870                }
 871
 872                /*
 873                 * The decrement of num_groups_with_pending_reqs is
 874                 * not performed immediately upon the deactivation of
 875                 * entity, but it is delayed to when it also happens
 876                 * that the first leaf descendant bfqq of entity gets
 877                 * all its pending requests completed. The following
 878                 * instructions perform this delayed decrement, if
 879                 * needed. See the comments on
 880                 * num_groups_with_pending_reqs for details.
 881                 */
 882                if (entity->in_groups_with_pending_reqs) {
 883                        entity->in_groups_with_pending_reqs = false;
 884                        bfqd->num_groups_with_pending_reqs--;
 885                }
 886        }
 887
 888        /*
 889         * Next function is invoked last, because it causes bfqq to be
 890         * freed if the following holds: bfqq is not in service and
 891         * has no dispatched request. DO NOT use bfqq after the next
 892         * function invocation.
 893         */
 894        __bfq_weights_tree_remove(bfqd, bfqq,
 895                                  &bfqd->queue_weights_tree);
 896}
 897
 898/*
 899 * Return expired entry, or NULL to just start from scratch in rbtree.
 900 */
 901static struct request *bfq_check_fifo(struct bfq_queue *bfqq,
 902                                      struct request *last)
 903{
 904        struct request *rq;
 905
 906        if (bfq_bfqq_fifo_expire(bfqq))
 907                return NULL;
 908
 909        bfq_mark_bfqq_fifo_expire(bfqq);
 910
 911        rq = rq_entry_fifo(bfqq->fifo.next);
 912
 913        if (rq == last || ktime_get_ns() < rq->fifo_time)
 914                return NULL;
 915
 916        bfq_log_bfqq(bfqq->bfqd, bfqq, "check_fifo: returned %p", rq);
 917        return rq;
 918}
 919
 920static struct request *bfq_find_next_rq(struct bfq_data *bfqd,
 921                                        struct bfq_queue *bfqq,
 922                                        struct request *last)
 923{
 924        struct rb_node *rbnext = rb_next(&last->rb_node);
 925        struct rb_node *rbprev = rb_prev(&last->rb_node);
 926        struct request *next, *prev = NULL;
 927
 928        /* Follow expired path, else get first next available. */
 929        next = bfq_check_fifo(bfqq, last);
 930        if (next)
 931                return next;
 932
 933        if (rbprev)
 934                prev = rb_entry_rq(rbprev);
 935
 936        if (rbnext)
 937                next = rb_entry_rq(rbnext);
 938        else {
 939                rbnext = rb_first(&bfqq->sort_list);
 940                if (rbnext && rbnext != &last->rb_node)
 941                        next = rb_entry_rq(rbnext);
 942        }
 943
 944        return bfq_choose_req(bfqd, next, prev, blk_rq_pos(last));
 945}
 946
 947/* see the definition of bfq_async_charge_factor for details */
 948static unsigned long bfq_serv_to_charge(struct request *rq,
 949                                        struct bfq_queue *bfqq)
 950{
 951        if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1 ||
 952            bfq_asymmetric_scenario(bfqq->bfqd, bfqq))
 953                return blk_rq_sectors(rq);
 954
 955        return blk_rq_sectors(rq) * bfq_async_charge_factor;
 956}
 957
 958/**
 959 * bfq_updated_next_req - update the queue after a new next_rq selection.
 960 * @bfqd: the device data the queue belongs to.
 961 * @bfqq: the queue to update.
 962 *
 963 * If the first request of a queue changes we make sure that the queue
 964 * has enough budget to serve at least its first request (if the
 965 * request has grown).  We do this because if the queue has not enough
 966 * budget for its first request, it has to go through two dispatch
 967 * rounds to actually get it dispatched.
 968 */
 969static void bfq_updated_next_req(struct bfq_data *bfqd,
 970                                 struct bfq_queue *bfqq)
 971{
 972        struct bfq_entity *entity = &bfqq->entity;
 973        struct request *next_rq = bfqq->next_rq;
 974        unsigned long new_budget;
 975
 976        if (!next_rq)
 977                return;
 978
 979        if (bfqq == bfqd->in_service_queue)
 980                /*
 981                 * In order not to break guarantees, budgets cannot be
 982                 * changed after an entity has been selected.
 983                 */
 984                return;
 985
 986        new_budget = max_t(unsigned long,
 987                           max_t(unsigned long, bfqq->max_budget,
 988                                 bfq_serv_to_charge(next_rq, bfqq)),
 989                           entity->service);
 990        if (entity->budget != new_budget) {
 991                entity->budget = new_budget;
 992                bfq_log_bfqq(bfqd, bfqq, "updated next rq: new budget %lu",
 993                                         new_budget);
 994                bfq_requeue_bfqq(bfqd, bfqq, false);
 995        }
 996}
 997
 998static unsigned int bfq_wr_duration(struct bfq_data *bfqd)
 999{
1000        u64 dur;
1001
1002        if (bfqd->bfq_wr_max_time > 0)
1003                return bfqd->bfq_wr_max_time;
1004
1005        dur = bfqd->rate_dur_prod;
1006        do_div(dur, bfqd->peak_rate);
1007
1008        /*
1009         * Limit duration between 3 and 25 seconds. The upper limit
1010         * has been conservatively set after the following worst case:
1011         * on a QEMU/KVM virtual machine
1012         * - running in a slow PC
1013         * - with a virtual disk stacked on a slow low-end 5400rpm HDD
1014         * - serving a heavy I/O workload, such as the sequential reading
1015         *   of several files
1016         * mplayer took 23 seconds to start, if constantly weight-raised.
1017         *
1018         * As for higher values than that accommodating the above bad
1019         * scenario, tests show that higher values would often yield
1020         * the opposite of the desired result, i.e., would worsen
1021         * responsiveness by allowing non-interactive applications to
1022         * preserve weight raising for too long.
1023         *
1024         * On the other end, lower values than 3 seconds make it
1025         * difficult for most interactive tasks to complete their jobs
1026         * before weight-raising finishes.
1027         */
1028        return clamp_val(dur, msecs_to_jiffies(3000), msecs_to_jiffies(25000));
1029}
1030
1031/* switch back from soft real-time to interactive weight raising */
1032static void switch_back_to_interactive_wr(struct bfq_queue *bfqq,
1033                                          struct bfq_data *bfqd)
1034{
1035        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
1036        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
1037        bfqq->last_wr_start_finish = bfqq->wr_start_at_switch_to_srt;
1038}
1039
1040static void
1041bfq_bfqq_resume_state(struct bfq_queue *bfqq, struct bfq_data *bfqd,
1042                      struct bfq_io_cq *bic, bool bfq_already_existing)
1043{
1044        unsigned int old_wr_coeff = 1;
1045        bool busy = bfq_already_existing && bfq_bfqq_busy(bfqq);
1046
1047        if (bic->saved_has_short_ttime)
1048                bfq_mark_bfqq_has_short_ttime(bfqq);
1049        else
1050                bfq_clear_bfqq_has_short_ttime(bfqq);
1051
1052        if (bic->saved_IO_bound)
1053                bfq_mark_bfqq_IO_bound(bfqq);
1054        else
1055                bfq_clear_bfqq_IO_bound(bfqq);
1056
1057        bfqq->last_serv_time_ns = bic->saved_last_serv_time_ns;
1058        bfqq->inject_limit = bic->saved_inject_limit;
1059        bfqq->decrease_time_jif = bic->saved_decrease_time_jif;
1060
1061        bfqq->entity.new_weight = bic->saved_weight;
1062        bfqq->ttime = bic->saved_ttime;
1063        bfqq->io_start_time = bic->saved_io_start_time;
1064        bfqq->tot_idle_time = bic->saved_tot_idle_time;
1065        /*
1066         * Restore weight coefficient only if low_latency is on
1067         */
1068        if (bfqd->low_latency) {
1069                old_wr_coeff = bfqq->wr_coeff;
1070                bfqq->wr_coeff = bic->saved_wr_coeff;
1071        }
1072        bfqq->service_from_wr = bic->saved_service_from_wr;
1073        bfqq->wr_start_at_switch_to_srt = bic->saved_wr_start_at_switch_to_srt;
1074        bfqq->last_wr_start_finish = bic->saved_last_wr_start_finish;
1075        bfqq->wr_cur_max_time = bic->saved_wr_cur_max_time;
1076
1077        if (bfqq->wr_coeff > 1 && (bfq_bfqq_in_large_burst(bfqq) ||
1078            time_is_before_jiffies(bfqq->last_wr_start_finish +
1079                                   bfqq->wr_cur_max_time))) {
1080                if (bfqq->wr_cur_max_time == bfqd->bfq_wr_rt_max_time &&
1081                    !bfq_bfqq_in_large_burst(bfqq) &&
1082                    time_is_after_eq_jiffies(bfqq->wr_start_at_switch_to_srt +
1083                                             bfq_wr_duration(bfqd))) {
1084                        switch_back_to_interactive_wr(bfqq, bfqd);
1085                } else {
1086                        bfqq->wr_coeff = 1;
1087                        bfq_log_bfqq(bfqq->bfqd, bfqq,
1088                                     "resume state: switching off wr");
1089                }
1090        }
1091
1092        /* make sure weight will be updated, however we got here */
1093        bfqq->entity.prio_changed = 1;
1094
1095        if (likely(!busy))
1096                return;
1097
1098        if (old_wr_coeff == 1 && bfqq->wr_coeff > 1)
1099                bfqd->wr_busy_queues++;
1100        else if (old_wr_coeff > 1 && bfqq->wr_coeff == 1)
1101                bfqd->wr_busy_queues--;
1102}
1103
1104static int bfqq_process_refs(struct bfq_queue *bfqq)
1105{
1106        return bfqq->ref - bfqq->allocated - bfqq->entity.on_st_or_in_serv -
1107                (bfqq->weight_counter != NULL) - bfqq->stable_ref;
1108}
1109
1110/* Empty burst list and add just bfqq (see comments on bfq_handle_burst) */
1111static void bfq_reset_burst_list(struct bfq_data *bfqd, struct bfq_queue *bfqq)
1112{
1113        struct bfq_queue *item;
1114        struct hlist_node *n;
1115
1116        hlist_for_each_entry_safe(item, n, &bfqd->burst_list, burst_list_node)
1117                hlist_del_init(&item->burst_list_node);
1118
1119        /*
1120         * Start the creation of a new burst list only if there is no
1121         * active queue. See comments on the conditional invocation of
1122         * bfq_handle_burst().
1123         */
1124        if (bfq_tot_busy_queues(bfqd) == 0) {
1125                hlist_add_head(&bfqq->burst_list_node, &bfqd->burst_list);
1126                bfqd->burst_size = 1;
1127        } else
1128                bfqd->burst_size = 0;
1129
1130        bfqd->burst_parent_entity = bfqq->entity.parent;
1131}
1132
1133/* Add bfqq to the list of queues in current burst (see bfq_handle_burst) */
1134static void bfq_add_to_burst(struct bfq_data *bfqd, struct bfq_queue *bfqq)
1135{
1136        /* Increment burst size to take into account also bfqq */
1137        bfqd->burst_size++;
1138
1139        if (bfqd->burst_size == bfqd->bfq_large_burst_thresh) {
1140                struct bfq_queue *pos, *bfqq_item;
1141                struct hlist_node *n;
1142
1143                /*
1144                 * Enough queues have been activated shortly after each
1145                 * other to consider this burst as large.
1146                 */
1147                bfqd->large_burst = true;
1148
1149                /*
1150                 * We can now mark all queues in the burst list as
1151                 * belonging to a large burst.
1152                 */
1153                hlist_for_each_entry(bfqq_item, &bfqd->burst_list,
1154                                     burst_list_node)
1155                        bfq_mark_bfqq_in_large_burst(bfqq_item);
1156                bfq_mark_bfqq_in_large_burst(bfqq);
1157
1158                /*
1159                 * From now on, and until the current burst finishes, any
1160                 * new queue being activated shortly after the last queue
1161                 * was inserted in the burst can be immediately marked as
1162                 * belonging to a large burst. So the burst list is not
1163                 * needed any more. Remove it.
1164                 */
1165                hlist_for_each_entry_safe(pos, n, &bfqd->burst_list,
1166                                          burst_list_node)
1167                        hlist_del_init(&pos->burst_list_node);
1168        } else /*
1169                * Burst not yet large: add bfqq to the burst list. Do
1170                * not increment the ref counter for bfqq, because bfqq
1171                * is removed from the burst list before freeing bfqq
1172                * in put_queue.
1173                */
1174                hlist_add_head(&bfqq->burst_list_node, &bfqd->burst_list);
1175}
1176
1177/*
1178 * If many queues belonging to the same group happen to be created
1179 * shortly after each other, then the processes associated with these
1180 * queues have typically a common goal. In particular, bursts of queue
1181 * creations are usually caused by services or applications that spawn
1182 * many parallel threads/processes. Examples are systemd during boot,
1183 * or git grep. To help these processes get their job done as soon as
1184 * possible, it is usually better to not grant either weight-raising
1185 * or device idling to their queues, unless these queues must be
1186 * protected from the I/O flowing through other active queues.
1187 *
1188 * In this comment we describe, firstly, the reasons why this fact
1189 * holds, and, secondly, the next function, which implements the main
1190 * steps needed to properly mark these queues so that they can then be
1191 * treated in a different way.
1192 *
1193 * The above services or applications benefit mostly from a high
1194 * throughput: the quicker the requests of the activated queues are
1195 * cumulatively served, the sooner the target job of these queues gets
1196 * completed. As a consequence, weight-raising any of these queues,
1197 * which also implies idling the device for it, is almost always
1198 * counterproductive, unless there are other active queues to isolate
1199 * these new queues from. If there no other active queues, then
1200 * weight-raising these new queues just lowers throughput in most
1201 * cases.
1202 *
1203 * On the other hand, a burst of queue creations may be caused also by
1204 * the start of an application that does not consist of a lot of
1205 * parallel I/O-bound threads. In fact, with a complex application,
1206 * several short processes may need to be executed to start-up the
1207 * application. In this respect, to start an application as quickly as
1208 * possible, the best thing to do is in any case to privilege the I/O
1209 * related to the application with respect to all other
1210 * I/O. Therefore, the best strategy to start as quickly as possible
1211 * an application that causes a burst of queue creations is to
1212 * weight-raise all the queues created during the burst. This is the
1213 * exact opposite of the best strategy for the other type of bursts.
1214 *
1215 * In the end, to take the best action for each of the two cases, the
1216 * two types of bursts need to be distinguished. Fortunately, this
1217 * seems relatively easy, by looking at the sizes of the bursts. In
1218 * particular, we found a threshold such that only bursts with a
1219 * larger size than that threshold are apparently caused by
1220 * services or commands such as systemd or git grep. For brevity,
1221 * hereafter we call just 'large' these bursts. BFQ *does not*
1222 * weight-raise queues whose creation occurs in a large burst. In
1223 * addition, for each of these queues BFQ performs or does not perform
1224 * idling depending on which choice boosts the throughput more. The
1225 * exact choice depends on the device and request pattern at
1226 * hand.
1227 *
1228 * Unfortunately, false positives may occur while an interactive task
1229 * is starting (e.g., an application is being started). The
1230 * consequence is that the queues associated with the task do not
1231 * enjoy weight raising as expected. Fortunately these false positives
1232 * are very rare. They typically occur if some service happens to
1233 * start doing I/O exactly when the interactive task starts.
1234 *
1235 * Turning back to the next function, it is invoked only if there are
1236 * no active queues (apart from active queues that would belong to the
1237 * same, possible burst bfqq would belong to), and it implements all
1238 * the steps needed to detect the occurrence of a large burst and to
1239 * properly mark all the queues belonging to it (so that they can then
1240 * be treated in a different way). This goal is achieved by
1241 * maintaining a "burst list" that holds, temporarily, the queues that
1242 * belong to the burst in progress. The list is then used to mark
1243 * these queues as belonging to a large burst if the burst does become
1244 * large. The main steps are the following.
1245 *
1246 * . when the very first queue is created, the queue is inserted into the
1247 *   list (as it could be the first queue in a possible burst)
1248 *
1249 * . if the current burst has not yet become large, and a queue Q that does
1250 *   not yet belong to the burst is activated shortly after the last time
1251 *   at which a new queue entered the burst list, then the function appends
1252 *   Q to the burst list
1253 *
1254 * . if, as a consequence of the previous step, the burst size reaches
1255 *   the large-burst threshold, then
1256 *
1257 *     . all the queues in the burst list are marked as belonging to a
1258 *       large burst
1259 *
1260 *     . the burst list is deleted; in fact, the burst list already served
1261 *       its purpose (keeping temporarily track of the queues in a burst,
1262 *       so as to be able to mark them as belonging to a large burst in the
1263 *       previous sub-step), and now is not needed any more
1264 *
1265 *     . the device enters a large-burst mode
1266 *
1267 * . if a queue Q that does not belong to the burst is created while
1268 *   the device is in large-burst mode and shortly after the last time
1269 *   at which a queue either entered the burst list or was marked as
1270 *   belonging to the current large burst, then Q is immediately marked
1271 *   as belonging to a large burst.
1272 *
1273 * . if a queue Q that does not belong to the burst is created a while
1274 *   later, i.e., not shortly after, than the last time at which a queue
1275 *   either entered the burst list or was marked as belonging to the
1276 *   current large burst, then the current burst is deemed as finished and:
1277 *
1278 *        . the large-burst mode is reset if set
1279 *
1280 *        . the burst list is emptied
1281 *
1282 *        . Q is inserted in the burst list, as Q may be the first queue
1283 *          in a possible new burst (then the burst list contains just Q
1284 *          after this step).
1285 */
1286static void bfq_handle_burst(struct bfq_data *bfqd, struct bfq_queue *bfqq)
1287{
1288        /*
1289         * If bfqq is already in the burst list or is part of a large
1290         * burst, or finally has just been split, then there is
1291         * nothing else to do.
1292         */
1293        if (!hlist_unhashed(&bfqq->burst_list_node) ||
1294            bfq_bfqq_in_large_burst(bfqq) ||
1295            time_is_after_eq_jiffies(bfqq->split_time +
1296                                     msecs_to_jiffies(10)))
1297                return;
1298
1299        /*
1300         * If bfqq's creation happens late enough, or bfqq belongs to
1301         * a different group than the burst group, then the current
1302         * burst is finished, and related data structures must be
1303         * reset.
1304         *
1305         * In this respect, consider the special case where bfqq is
1306         * the very first queue created after BFQ is selected for this
1307         * device. In this case, last_ins_in_burst and
1308         * burst_parent_entity are not yet significant when we get
1309         * here. But it is easy to verify that, whether or not the
1310         * following condition is true, bfqq will end up being
1311         * inserted into the burst list. In particular the list will
1312         * happen to contain only bfqq. And this is exactly what has
1313         * to happen, as bfqq may be the first queue of the first
1314         * burst.
1315         */
1316        if (time_is_before_jiffies(bfqd->last_ins_in_burst +
1317            bfqd->bfq_burst_interval) ||
1318            bfqq->entity.parent != bfqd->burst_parent_entity) {
1319                bfqd->large_burst = false;
1320                bfq_reset_burst_list(bfqd, bfqq);
1321                goto end;
1322        }
1323
1324        /*
1325         * If we get here, then bfqq is being activated shortly after the
1326         * last queue. So, if the current burst is also large, we can mark
1327         * bfqq as belonging to this large burst immediately.
1328         */
1329        if (bfqd->large_burst) {
1330                bfq_mark_bfqq_in_large_burst(bfqq);
1331                goto end;
1332        }
1333
1334        /*
1335         * If we get here, then a large-burst state has not yet been
1336         * reached, but bfqq is being activated shortly after the last
1337         * queue. Then we add bfqq to the burst.
1338         */
1339        bfq_add_to_burst(bfqd, bfqq);
1340end:
1341        /*
1342         * At this point, bfqq either has been added to the current
1343         * burst or has caused the current burst to terminate and a
1344         * possible new burst to start. In particular, in the second
1345         * case, bfqq has become the first queue in the possible new
1346         * burst.  In both cases last_ins_in_burst needs to be moved
1347         * forward.
1348         */
1349        bfqd->last_ins_in_burst = jiffies;
1350}
1351
1352static int bfq_bfqq_budget_left(struct bfq_queue *bfqq)
1353{
1354        struct bfq_entity *entity = &bfqq->entity;
1355
1356        return entity->budget - entity->service;
1357}
1358
1359/*
1360 * If enough samples have been computed, return the current max budget
1361 * stored in bfqd, which is dynamically updated according to the
1362 * estimated disk peak rate; otherwise return the default max budget
1363 */
1364static int bfq_max_budget(struct bfq_data *bfqd)
1365{
1366        if (bfqd->budgets_assigned < bfq_stats_min_budgets)
1367                return bfq_default_max_budget;
1368        else
1369                return bfqd->bfq_max_budget;
1370}
1371
1372/*
1373 * Return min budget, which is a fraction of the current or default
1374 * max budget (trying with 1/32)
1375 */
1376static int bfq_min_budget(struct bfq_data *bfqd)
1377{
1378        if (bfqd->budgets_assigned < bfq_stats_min_budgets)
1379                return bfq_default_max_budget / 32;
1380        else
1381                return bfqd->bfq_max_budget / 32;
1382}
1383
1384/*
1385 * The next function, invoked after the input queue bfqq switches from
1386 * idle to busy, updates the budget of bfqq. The function also tells
1387 * whether the in-service queue should be expired, by returning
1388 * true. The purpose of expiring the in-service queue is to give bfqq
1389 * the chance to possibly preempt the in-service queue, and the reason
1390 * for preempting the in-service queue is to achieve one of the two
1391 * goals below.
1392 *
1393 * 1. Guarantee to bfqq its reserved bandwidth even if bfqq has
1394 * expired because it has remained idle. In particular, bfqq may have
1395 * expired for one of the following two reasons:
1396 *
1397 * - BFQQE_NO_MORE_REQUESTS bfqq did not enjoy any device idling
1398 *   and did not make it to issue a new request before its last
1399 *   request was served;
1400 *
1401 * - BFQQE_TOO_IDLE bfqq did enjoy device idling, but did not issue
1402 *   a new request before the expiration of the idling-time.
1403 *
1404 * Even if bfqq has expired for one of the above reasons, the process
1405 * associated with the queue may be however issuing requests greedily,
1406 * and thus be sensitive to the bandwidth it receives (bfqq may have
1407 * remained idle for other reasons: CPU high load, bfqq not enjoying
1408 * idling, I/O throttling somewhere in the path from the process to
1409 * the I/O scheduler, ...). But if, after every expiration for one of
1410 * the above two reasons, bfqq has to wait for the service of at least
1411 * one full budget of another queue before being served again, then
1412 * bfqq is likely to get a much lower bandwidth or resource time than
1413 * its reserved ones. To address this issue, two countermeasures need
1414 * to be taken.
1415 *
1416 * First, the budget and the timestamps of bfqq need to be updated in
1417 * a special way on bfqq reactivation: they need to be updated as if
1418 * bfqq did not remain idle and did not expire. In fact, if they are
1419 * computed as if bfqq expired and remained idle until reactivation,
1420 * then the process associated with bfqq is treated as if, instead of
1421 * being greedy, it stopped issuing requests when bfqq remained idle,
1422 * and restarts issuing requests only on this reactivation. In other
1423 * words, the scheduler does not help the process recover the "service
1424 * hole" between bfqq expiration and reactivation. As a consequence,
1425 * the process receives a lower bandwidth than its reserved one. In
1426 * contrast, to recover this hole, the budget must be updated as if
1427 * bfqq was not expired at all before this reactivation, i.e., it must
1428 * be set to the value of the remaining budget when bfqq was
1429 * expired. Along the same line, timestamps need to be assigned the
1430 * value they had the last time bfqq was selected for service, i.e.,
1431 * before last expiration. Thus timestamps need to be back-shifted
1432 * with respect to their normal computation (see [1] for more details
1433 * on this tricky aspect).
1434 *
1435 * Secondly, to allow the process to recover the hole, the in-service
1436 * queue must be expired too, to give bfqq the chance to preempt it
1437 * immediately. In fact, if bfqq has to wait for a full budget of the
1438 * in-service queue to be completed, then it may become impossible to
1439 * let the process recover the hole, even if the back-shifted
1440 * timestamps of bfqq are lower than those of the in-service queue. If
1441 * this happens for most or all of the holes, then the process may not
1442 * receive its reserved bandwidth. In this respect, it is worth noting
1443 * that, being the service of outstanding requests unpreemptible, a
1444 * little fraction of the holes may however be unrecoverable, thereby
1445 * causing a little loss of bandwidth.
1446 *
1447 * The last important point is detecting whether bfqq does need this
1448 * bandwidth recovery. In this respect, the next function deems the
1449 * process associated with bfqq greedy, and thus allows it to recover
1450 * the hole, if: 1) the process is waiting for the arrival of a new
1451 * request (which implies that bfqq expired for one of the above two
1452 * reasons), and 2) such a request has arrived soon. The first
1453 * condition is controlled through the flag non_blocking_wait_rq,
1454 * while the second through the flag arrived_in_time. If both
1455 * conditions hold, then the function computes the budget in the
1456 * above-described special way, and signals that the in-service queue
1457 * should be expired. Timestamp back-shifting is done later in
1458 * __bfq_activate_entity.
1459 *
1460 * 2. Reduce latency. Even if timestamps are not backshifted to let
1461 * the process associated with bfqq recover a service hole, bfqq may
1462 * however happen to have, after being (re)activated, a lower finish
1463 * timestamp than the in-service queue.  That is, the next budget of
1464 * bfqq may have to be completed before the one of the in-service
1465 * queue. If this is the case, then preempting the in-service queue
1466 * allows this goal to be achieved, apart from the unpreemptible,
1467 * outstanding requests mentioned above.
1468 *
1469 * Unfortunately, regardless of which of the above two goals one wants
1470 * to achieve, service trees need first to be updated to know whether
1471 * the in-service queue must be preempted. To have service trees
1472 * correctly updated, the in-service queue must be expired and
1473 * rescheduled, and bfqq must be scheduled too. This is one of the
1474 * most costly operations (in future versions, the scheduling
1475 * mechanism may be re-designed in such a way to make it possible to
1476 * know whether preemption is needed without needing to update service
1477 * trees). In addition, queue preemptions almost always cause random
1478 * I/O, which may in turn cause loss of throughput. Finally, there may
1479 * even be no in-service queue when the next function is invoked (so,
1480 * no queue to compare timestamps with). Because of these facts, the
1481 * next function adopts the following simple scheme to avoid costly
1482 * operations, too frequent preemptions and too many dependencies on
1483 * the state of the scheduler: it requests the expiration of the
1484 * in-service queue (unconditionally) only for queues that need to
1485 * recover a hole. Then it delegates to other parts of the code the
1486 * responsibility of handling the above case 2.
1487 */
1488static bool bfq_bfqq_update_budg_for_activation(struct bfq_data *bfqd,
1489                                                struct bfq_queue *bfqq,
1490                                                bool arrived_in_time)
1491{
1492        struct bfq_entity *entity = &bfqq->entity;
1493
1494        /*
1495         * In the next compound condition, we check also whether there
1496         * is some budget left, because otherwise there is no point in
1497         * trying to go on serving bfqq with this same budget: bfqq
1498         * would be expired immediately after being selected for
1499         * service. This would only cause useless overhead.
1500         */
1501        if (bfq_bfqq_non_blocking_wait_rq(bfqq) && arrived_in_time &&
1502            bfq_bfqq_budget_left(bfqq) > 0) {
1503                /*
1504                 * We do not clear the flag non_blocking_wait_rq here, as
1505                 * the latter is used in bfq_activate_bfqq to signal
1506                 * that timestamps need to be back-shifted (and is
1507                 * cleared right after).
1508                 */
1509
1510                /*
1511                 * In next assignment we rely on that either
1512                 * entity->service or entity->budget are not updated
1513                 * on expiration if bfqq is empty (see
1514                 * __bfq_bfqq_recalc_budget). Thus both quantities
1515                 * remain unchanged after such an expiration, and the
1516                 * following statement therefore assigns to
1517                 * entity->budget the remaining budget on such an
1518                 * expiration.
1519                 */
1520                entity->budget = min_t(unsigned long,
1521                                       bfq_bfqq_budget_left(bfqq),
1522                                       bfqq->max_budget);
1523
1524                /*
1525                 * At this point, we have used entity->service to get
1526                 * the budget left (needed for updating
1527                 * entity->budget). Thus we finally can, and have to,
1528                 * reset entity->service. The latter must be reset
1529                 * because bfqq would otherwise be charged again for
1530                 * the service it has received during its previous
1531                 * service slot(s).
1532                 */
1533                entity->service = 0;
1534
1535                return true;
1536        }
1537
1538        /*
1539         * We can finally complete expiration, by setting service to 0.
1540         */
1541        entity->service = 0;
1542        entity->budget = max_t(unsigned long, bfqq->max_budget,
1543                               bfq_serv_to_charge(bfqq->next_rq, bfqq));
1544        bfq_clear_bfqq_non_blocking_wait_rq(bfqq);
1545        return false;
1546}
1547
1548/*
1549 * Return the farthest past time instant according to jiffies
1550 * macros.
1551 */
1552static unsigned long bfq_smallest_from_now(void)
1553{
1554        return jiffies - MAX_JIFFY_OFFSET;
1555}
1556
1557static void bfq_update_bfqq_wr_on_rq_arrival(struct bfq_data *bfqd,
1558                                             struct bfq_queue *bfqq,
1559                                             unsigned int old_wr_coeff,
1560                                             bool wr_or_deserves_wr,
1561                                             bool interactive,
1562                                             bool in_burst,
1563                                             bool soft_rt)
1564{
1565        if (old_wr_coeff == 1 && wr_or_deserves_wr) {
1566                /* start a weight-raising period */
1567                if (interactive) {
1568                        bfqq->service_from_wr = 0;
1569                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
1570                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
1571                } else {
1572                        /*
1573                         * No interactive weight raising in progress
1574                         * here: assign minus infinity to
1575                         * wr_start_at_switch_to_srt, to make sure
1576                         * that, at the end of the soft-real-time
1577                         * weight raising periods that is starting
1578                         * now, no interactive weight-raising period
1579                         * may be wrongly considered as still in
1580                         * progress (and thus actually started by
1581                         * mistake).
1582                         */
1583                        bfqq->wr_start_at_switch_to_srt =
1584                                bfq_smallest_from_now();
1585                        bfqq->wr_coeff = bfqd->bfq_wr_coeff *
1586                                BFQ_SOFTRT_WEIGHT_FACTOR;
1587                        bfqq->wr_cur_max_time =
1588                                bfqd->bfq_wr_rt_max_time;
1589                }
1590
1591                /*
1592                 * If needed, further reduce budget to make sure it is
1593                 * close to bfqq's backlog, so as to reduce the
1594                 * scheduling-error component due to a too large
1595                 * budget. Do not care about throughput consequences,
1596                 * but only about latency. Finally, do not assign a
1597                 * too small budget either, to avoid increasing
1598                 * latency by causing too frequent expirations.
1599                 */
1600                bfqq->entity.budget = min_t(unsigned long,
1601                                            bfqq->entity.budget,
1602                                            2 * bfq_min_budget(bfqd));
1603        } else if (old_wr_coeff > 1) {
1604                if (interactive) { /* update wr coeff and duration */
1605                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
1606                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
1607                } else if (in_burst)
1608                        bfqq->wr_coeff = 1;
1609                else if (soft_rt) {
1610                        /*
1611                         * The application is now or still meeting the
1612                         * requirements for being deemed soft rt.  We
1613                         * can then correctly and safely (re)charge
1614                         * the weight-raising duration for the
1615                         * application with the weight-raising
1616                         * duration for soft rt applications.
1617                         *
1618                         * In particular, doing this recharge now, i.e.,
1619                         * before the weight-raising period for the
1620                         * application finishes, reduces the probability
1621                         * of the following negative scenario:
1622                         * 1) the weight of a soft rt application is
1623                         *    raised at startup (as for any newly
1624                         *    created application),
1625                         * 2) since the application is not interactive,
1626                         *    at a certain time weight-raising is
1627                         *    stopped for the application,
1628                         * 3) at that time the application happens to
1629                         *    still have pending requests, and hence
1630                         *    is destined to not have a chance to be
1631                         *    deemed soft rt before these requests are
1632                         *    completed (see the comments to the
1633                         *    function bfq_bfqq_softrt_next_start()
1634                         *    for details on soft rt detection),
1635                         * 4) these pending requests experience a high
1636                         *    latency because the application is not
1637                         *    weight-raised while they are pending.
1638                         */
1639                        if (bfqq->wr_cur_max_time !=
1640                                bfqd->bfq_wr_rt_max_time) {
1641                                bfqq->wr_start_at_switch_to_srt =
1642                                        bfqq->last_wr_start_finish;
1643
1644                                bfqq->wr_cur_max_time =
1645                                        bfqd->bfq_wr_rt_max_time;
1646                                bfqq->wr_coeff = bfqd->bfq_wr_coeff *
1647                                        BFQ_SOFTRT_WEIGHT_FACTOR;
1648                        }
1649                        bfqq->last_wr_start_finish = jiffies;
1650                }
1651        }
1652}
1653
1654static bool bfq_bfqq_idle_for_long_time(struct bfq_data *bfqd,
1655                                        struct bfq_queue *bfqq)
1656{
1657        return bfqq->dispatched == 0 &&
1658                time_is_before_jiffies(
1659                        bfqq->budget_timeout +
1660                        bfqd->bfq_wr_min_idle_time);
1661}
1662
1663
1664/*
1665 * Return true if bfqq is in a higher priority class, or has a higher
1666 * weight than the in-service queue.
1667 */
1668static bool bfq_bfqq_higher_class_or_weight(struct bfq_queue *bfqq,
1669                                            struct bfq_queue *in_serv_bfqq)
1670{
1671        int bfqq_weight, in_serv_weight;
1672
1673        if (bfqq->ioprio_class < in_serv_bfqq->ioprio_class)
1674                return true;
1675
1676        if (in_serv_bfqq->entity.parent == bfqq->entity.parent) {
1677                bfqq_weight = bfqq->entity.weight;
1678                in_serv_weight = in_serv_bfqq->entity.weight;
1679        } else {
1680                if (bfqq->entity.parent)
1681                        bfqq_weight = bfqq->entity.parent->weight;
1682                else
1683                        bfqq_weight = bfqq->entity.weight;
1684                if (in_serv_bfqq->entity.parent)
1685                        in_serv_weight = in_serv_bfqq->entity.parent->weight;
1686                else
1687                        in_serv_weight = in_serv_bfqq->entity.weight;
1688        }
1689
1690        return bfqq_weight > in_serv_weight;
1691}
1692
1693static bool bfq_better_to_idle(struct bfq_queue *bfqq);
1694
1695static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
1696                                             struct bfq_queue *bfqq,
1697                                             int old_wr_coeff,
1698                                             struct request *rq,
1699                                             bool *interactive)
1700{
1701        bool soft_rt, in_burst, wr_or_deserves_wr,
1702                bfqq_wants_to_preempt,
1703                idle_for_long_time = bfq_bfqq_idle_for_long_time(bfqd, bfqq),
1704                /*
1705                 * See the comments on
1706                 * bfq_bfqq_update_budg_for_activation for
1707                 * details on the usage of the next variable.
1708                 */
1709                arrived_in_time =  ktime_get_ns() <=
1710                        bfqq->ttime.last_end_request +
1711                        bfqd->bfq_slice_idle * 3;
1712
1713
1714        /*
1715         * bfqq deserves to be weight-raised if:
1716         * - it is sync,
1717         * - it does not belong to a large burst,
1718         * - it has been idle for enough time or is soft real-time,
1719         * - is linked to a bfq_io_cq (it is not shared in any sense),
1720         * - has a default weight (otherwise we assume the user wanted
1721         *   to control its weight explicitly)
1722         */
1723        in_burst = bfq_bfqq_in_large_burst(bfqq);
1724        soft_rt = bfqd->bfq_wr_max_softrt_rate > 0 &&
1725                !BFQQ_TOTALLY_SEEKY(bfqq) &&
1726                !in_burst &&
1727                time_is_before_jiffies(bfqq->soft_rt_next_start) &&
1728                bfqq->dispatched == 0 &&
1729                bfqq->entity.new_weight == 40;
1730        *interactive = !in_burst && idle_for_long_time &&
1731                bfqq->entity.new_weight == 40;
1732        wr_or_deserves_wr = bfqd->low_latency &&
1733                (bfqq->wr_coeff > 1 ||
1734                 (bfq_bfqq_sync(bfqq) &&
1735                  bfqq->bic && (*interactive || soft_rt)));
1736
1737        /*
1738         * Using the last flag, update budget and check whether bfqq
1739         * may want to preempt the in-service queue.
1740         */
1741        bfqq_wants_to_preempt =
1742                bfq_bfqq_update_budg_for_activation(bfqd, bfqq,
1743                                                    arrived_in_time);
1744
1745        /*
1746         * If bfqq happened to be activated in a burst, but has been
1747         * idle for much more than an interactive queue, then we
1748         * assume that, in the overall I/O initiated in the burst, the
1749         * I/O associated with bfqq is finished. So bfqq does not need
1750         * to be treated as a queue belonging to a burst
1751         * anymore. Accordingly, we reset bfqq's in_large_burst flag
1752         * if set, and remove bfqq from the burst list if it's
1753         * there. We do not decrement burst_size, because the fact
1754         * that bfqq does not need to belong to the burst list any
1755         * more does not invalidate the fact that bfqq was created in
1756         * a burst.
1757         */
1758        if (likely(!bfq_bfqq_just_created(bfqq)) &&
1759            idle_for_long_time &&
1760            time_is_before_jiffies(
1761                    bfqq->budget_timeout +
1762                    msecs_to_jiffies(10000))) {
1763                hlist_del_init(&bfqq->burst_list_node);
1764                bfq_clear_bfqq_in_large_burst(bfqq);
1765        }
1766
1767        bfq_clear_bfqq_just_created(bfqq);
1768
1769        if (bfqd->low_latency) {
1770                if (unlikely(time_is_after_jiffies(bfqq->split_time)))
1771                        /* wraparound */
1772                        bfqq->split_time =
1773                                jiffies - bfqd->bfq_wr_min_idle_time - 1;
1774
1775                if (time_is_before_jiffies(bfqq->split_time +
1776                                           bfqd->bfq_wr_min_idle_time)) {
1777                        bfq_update_bfqq_wr_on_rq_arrival(bfqd, bfqq,
1778                                                         old_wr_coeff,
1779                                                         wr_or_deserves_wr,
1780                                                         *interactive,
1781                                                         in_burst,
1782                                                         soft_rt);
1783
1784                        if (old_wr_coeff != bfqq->wr_coeff)
1785                                bfqq->entity.prio_changed = 1;
1786                }
1787        }
1788
1789        bfqq->last_idle_bklogged = jiffies;
1790        bfqq->service_from_backlogged = 0;
1791        bfq_clear_bfqq_softrt_update(bfqq);
1792
1793        bfq_add_bfqq_busy(bfqd, bfqq);
1794
1795        /*
1796         * Expire in-service queue if preemption may be needed for
1797         * guarantees or throughput. As for guarantees, we care
1798         * explicitly about two cases. The first is that bfqq has to
1799         * recover a service hole, as explained in the comments on
1800         * bfq_bfqq_update_budg_for_activation(), i.e., that
1801         * bfqq_wants_to_preempt is true. However, if bfqq does not
1802         * carry time-critical I/O, then bfqq's bandwidth is less
1803         * important than that of queues that carry time-critical I/O.
1804         * So, as a further constraint, we consider this case only if
1805         * bfqq is at least as weight-raised, i.e., at least as time
1806         * critical, as the in-service queue.
1807         *
1808         * The second case is that bfqq is in a higher priority class,
1809         * or has a higher weight than the in-service queue. If this
1810         * condition does not hold, we don't care because, even if
1811         * bfqq does not start to be served immediately, the resulting
1812         * delay for bfqq's I/O is however lower or much lower than
1813         * the ideal completion time to be guaranteed to bfqq's I/O.
1814         *
1815         * In both cases, preemption is needed only if, according to
1816         * the timestamps of both bfqq and of the in-service queue,
1817         * bfqq actually is the next queue to serve. So, to reduce
1818         * useless preemptions, the return value of
1819         * next_queue_may_preempt() is considered in the next compound
1820         * condition too. Yet next_queue_may_preempt() just checks a
1821         * simple, necessary condition for bfqq to be the next queue
1822         * to serve. In fact, to evaluate a sufficient condition, the
1823         * timestamps of the in-service queue would need to be
1824         * updated, and this operation is quite costly (see the
1825         * comments on bfq_bfqq_update_budg_for_activation()).
1826         *
1827         * As for throughput, we ask bfq_better_to_idle() whether we
1828         * still need to plug I/O dispatching. If bfq_better_to_idle()
1829         * says no, then plugging is not needed any longer, either to
1830         * boost throughput or to perserve service guarantees. Then
1831         * the best option is to stop plugging I/O, as not doing so
1832         * would certainly lower throughput. We may end up in this
1833         * case if: (1) upon a dispatch attempt, we detected that it
1834         * was better to plug I/O dispatch, and to wait for a new
1835         * request to arrive for the currently in-service queue, but
1836         * (2) this switch of bfqq to busy changes the scenario.
1837         */
1838        if (bfqd->in_service_queue &&
1839            ((bfqq_wants_to_preempt &&
1840              bfqq->wr_coeff >= bfqd->in_service_queue->wr_coeff) ||
1841             bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue) ||
1842             !bfq_better_to_idle(bfqd->in_service_queue)) &&
1843            next_queue_may_preempt(bfqd))
1844                bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
1845                                false, BFQQE_PREEMPTED);
1846}
1847
1848static void bfq_reset_inject_limit(struct bfq_data *bfqd,
1849                                   struct bfq_queue *bfqq)
1850{
1851        /* invalidate baseline total service time */
1852        bfqq->last_serv_time_ns = 0;
1853
1854        /*
1855         * Reset pointer in case we are waiting for
1856         * some request completion.
1857         */
1858        bfqd->waited_rq = NULL;
1859
1860        /*
1861         * If bfqq has a short think time, then start by setting the
1862         * inject limit to 0 prudentially, because the service time of
1863         * an injected I/O request may be higher than the think time
1864         * of bfqq, and therefore, if one request was injected when
1865         * bfqq remains empty, this injected request might delay the
1866         * service of the next I/O request for bfqq significantly. In
1867         * case bfqq can actually tolerate some injection, then the
1868         * adaptive update will however raise the limit soon. This
1869         * lucky circumstance holds exactly because bfqq has a short
1870         * think time, and thus, after remaining empty, is likely to
1871         * get new I/O enqueued---and then completed---before being
1872         * expired. This is the very pattern that gives the
1873         * limit-update algorithm the chance to measure the effect of
1874         * injection on request service times, and then to update the
1875         * limit accordingly.
1876         *
1877         * However, in the following special case, the inject limit is
1878         * left to 1 even if the think time is short: bfqq's I/O is
1879         * synchronized with that of some other queue, i.e., bfqq may
1880         * receive new I/O only after the I/O of the other queue is
1881         * completed. Keeping the inject limit to 1 allows the
1882         * blocking I/O to be served while bfqq is in service. And
1883         * this is very convenient both for bfqq and for overall
1884         * throughput, as explained in detail in the comments in
1885         * bfq_update_has_short_ttime().
1886         *
1887         * On the opposite end, if bfqq has a long think time, then
1888         * start directly by 1, because:
1889         * a) on the bright side, keeping at most one request in
1890         * service in the drive is unlikely to cause any harm to the
1891         * latency of bfqq's requests, as the service time of a single
1892         * request is likely to be lower than the think time of bfqq;
1893         * b) on the downside, after becoming empty, bfqq is likely to
1894         * expire before getting its next request. With this request
1895         * arrival pattern, it is very hard to sample total service
1896         * times and update the inject limit accordingly (see comments
1897         * on bfq_update_inject_limit()). So the limit is likely to be
1898         * never, or at least seldom, updated.  As a consequence, by
1899         * setting the limit to 1, we avoid that no injection ever
1900         * occurs with bfqq. On the downside, this proactive step
1901         * further reduces chances to actually compute the baseline
1902         * total service time. Thus it reduces chances to execute the
1903         * limit-update algorithm and possibly raise the limit to more
1904         * than 1.
1905         */
1906        if (bfq_bfqq_has_short_ttime(bfqq))
1907                bfqq->inject_limit = 0;
1908        else
1909                bfqq->inject_limit = 1;
1910
1911        bfqq->decrease_time_jif = jiffies;
1912}
1913
1914static void bfq_update_io_intensity(struct bfq_queue *bfqq, u64 now_ns)
1915{
1916        u64 tot_io_time = now_ns - bfqq->io_start_time;
1917
1918        if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfqq->dispatched == 0)
1919                bfqq->tot_idle_time +=
1920                        now_ns - bfqq->ttime.last_end_request;
1921
1922        if (unlikely(bfq_bfqq_just_created(bfqq)))
1923                return;
1924
1925        /*
1926         * Must be busy for at least about 80% of the time to be
1927         * considered I/O bound.
1928         */
1929        if (bfqq->tot_idle_time * 5 > tot_io_time)
1930                bfq_clear_bfqq_IO_bound(bfqq);
1931        else
1932                bfq_mark_bfqq_IO_bound(bfqq);
1933
1934        /*
1935         * Keep an observation window of at most 200 ms in the past
1936         * from now.
1937         */
1938        if (tot_io_time > 200 * NSEC_PER_MSEC) {
1939                bfqq->io_start_time = now_ns - (tot_io_time>>1);
1940                bfqq->tot_idle_time >>= 1;
1941        }
1942}
1943
1944/*
1945 * Detect whether bfqq's I/O seems synchronized with that of some
1946 * other queue, i.e., whether bfqq, after remaining empty, happens to
1947 * receive new I/O only right after some I/O request of the other
1948 * queue has been completed. We call waker queue the other queue, and
1949 * we assume, for simplicity, that bfqq may have at most one waker
1950 * queue.
1951 *
1952 * A remarkable throughput boost can be reached by unconditionally
1953 * injecting the I/O of the waker queue, every time a new
1954 * bfq_dispatch_request happens to be invoked while I/O is being
1955 * plugged for bfqq.  In addition to boosting throughput, this
1956 * unblocks bfqq's I/O, thereby improving bandwidth and latency for
1957 * bfqq. Note that these same results may be achieved with the general
1958 * injection mechanism, but less effectively. For details on this
1959 * aspect, see the comments on the choice of the queue for injection
1960 * in bfq_select_queue().
1961 *
1962 * Turning back to the detection of a waker queue, a queue Q is deemed
1963 * as a waker queue for bfqq if, for three consecutive times, bfqq
1964 * happens to become non empty right after a request of Q has been
1965 * completed. In particular, on the first time, Q is tentatively set
1966 * as a candidate waker queue, while on the third consecutive time
1967 * that Q is detected, the field waker_bfqq is set to Q, to confirm
1968 * that Q is a waker queue for bfqq. These detection steps are
1969 * performed only if bfqq has a long think time, so as to make it more
1970 * likely that bfqq's I/O is actually being blocked by a
1971 * synchronization. This last filter, plus the above three-times
1972 * requirement, make false positives less likely.
1973 *
1974 * NOTE
1975 *
1976 * The sooner a waker queue is detected, the sooner throughput can be
1977 * boosted by injecting I/O from the waker queue. Fortunately,
1978 * detection is likely to be actually fast, for the following
1979 * reasons. While blocked by synchronization, bfqq has a long think
1980 * time. This implies that bfqq's inject limit is at least equal to 1
1981 * (see the comments in bfq_update_inject_limit()). So, thanks to
1982 * injection, the waker queue is likely to be served during the very
1983 * first I/O-plugging time interval for bfqq. This triggers the first
1984 * step of the detection mechanism. Thanks again to injection, the
1985 * candidate waker queue is then likely to be confirmed no later than
1986 * during the next I/O-plugging interval for bfqq.
1987 *
1988 * ISSUE
1989 *
1990 * On queue merging all waker information is lost.
1991 */
1992static void bfq_check_waker(struct bfq_data *bfqd, struct bfq_queue *bfqq,
1993                            u64 now_ns)
1994{
1995        if (!bfqd->last_completed_rq_bfqq ||
1996            bfqd->last_completed_rq_bfqq == bfqq ||
1997            bfq_bfqq_has_short_ttime(bfqq) ||
1998            now_ns - bfqd->last_completion >= 4 * NSEC_PER_MSEC ||
1999            bfqd->last_completed_rq_bfqq == bfqq->waker_bfqq)
2000                return;
2001
2002        if (bfqd->last_completed_rq_bfqq !=
2003            bfqq->tentative_waker_bfqq) {
2004                /*
2005                 * First synchronization detected with a
2006                 * candidate waker queue, or with a different
2007                 * candidate waker queue from the current one.
2008                 */
2009                bfqq->tentative_waker_bfqq =
2010                        bfqd->last_completed_rq_bfqq;
2011                bfqq->num_waker_detections = 1;
2012        } else /* Same tentative waker queue detected again */
2013                bfqq->num_waker_detections++;
2014
2015        if (bfqq->num_waker_detections == 3) {
2016                bfqq->waker_bfqq = bfqd->last_completed_rq_bfqq;
2017                bfqq->tentative_waker_bfqq = NULL;
2018
2019                /*
2020                 * If the waker queue disappears, then
2021                 * bfqq->waker_bfqq must be reset. To
2022                 * this goal, we maintain in each
2023                 * waker queue a list, woken_list, of
2024                 * all the queues that reference the
2025                 * waker queue through their
2026                 * waker_bfqq pointer. When the waker
2027                 * queue exits, the waker_bfqq pointer
2028                 * of all the queues in the woken_list
2029                 * is reset.
2030                 *
2031                 * In addition, if bfqq is already in
2032                 * the woken_list of a waker queue,
2033                 * then, before being inserted into
2034                 * the woken_list of a new waker
2035                 * queue, bfqq must be removed from
2036                 * the woken_list of the old waker
2037                 * queue.
2038                 */
2039                if (!hlist_unhashed(&bfqq->woken_list_node))
2040                        hlist_del_init(&bfqq->woken_list_node);
2041                hlist_add_head(&bfqq->woken_list_node,
2042                               &bfqd->last_completed_rq_bfqq->woken_list);
2043        }
2044}
2045
2046static void bfq_add_request(struct request *rq)
2047{
2048        struct bfq_queue *bfqq = RQ_BFQQ(rq);
2049        struct bfq_data *bfqd = bfqq->bfqd;
2050        struct request *next_rq, *prev;
2051        unsigned int old_wr_coeff = bfqq->wr_coeff;
2052        bool interactive = false;
2053        u64 now_ns = ktime_get_ns();
2054
2055        bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
2056        bfqq->queued[rq_is_sync(rq)]++;
2057        bfqd->queued++;
2058
2059        if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
2060                bfq_check_waker(bfqd, bfqq, now_ns);
2061
2062                /*
2063                 * Periodically reset inject limit, to make sure that
2064                 * the latter eventually drops in case workload
2065                 * changes, see step (3) in the comments on
2066                 * bfq_update_inject_limit().
2067                 */
2068                if (time_is_before_eq_jiffies(bfqq->decrease_time_jif +
2069                                             msecs_to_jiffies(1000)))
2070                        bfq_reset_inject_limit(bfqd, bfqq);
2071
2072                /*
2073                 * The following conditions must hold to setup a new
2074                 * sampling of total service time, and then a new
2075                 * update of the inject limit:
2076                 * - bfqq is in service, because the total service
2077                 *   time is evaluated only for the I/O requests of
2078                 *   the queues in service;
2079                 * - this is the right occasion to compute or to
2080                 *   lower the baseline total service time, because
2081                 *   there are actually no requests in the drive,
2082                 *   or
2083                 *   the baseline total service time is available, and
2084                 *   this is the right occasion to compute the other
2085                 *   quantity needed to update the inject limit, i.e.,
2086                 *   the total service time caused by the amount of
2087                 *   injection allowed by the current value of the
2088                 *   limit. It is the right occasion because injection
2089                 *   has actually been performed during the service
2090                 *   hole, and there are still in-flight requests,
2091                 *   which are very likely to be exactly the injected
2092                 *   requests, or part of them;
2093                 * - the minimum interval for sampling the total
2094                 *   service time and updating the inject limit has
2095                 *   elapsed.
2096                 */
2097                if (bfqq == bfqd->in_service_queue &&
2098                    (bfqd->rq_in_driver == 0 ||
2099                     (bfqq->last_serv_time_ns > 0 &&
2100                      bfqd->rqs_injected && bfqd->rq_in_driver > 0)) &&
2101                    time_is_before_eq_jiffies(bfqq->decrease_time_jif +
2102                                              msecs_to_jiffies(10))) {
2103                        bfqd->last_empty_occupied_ns = ktime_get_ns();
2104                        /*
2105                         * Start the state machine for measuring the
2106                         * total service time of rq: setting
2107                         * wait_dispatch will cause bfqd->waited_rq to
2108                         * be set when rq will be dispatched.
2109                         */
2110                        bfqd->wait_dispatch = true;
2111                        /*
2112                         * If there is no I/O in service in the drive,
2113                         * then possible injection occurred before the
2114                         * arrival of rq will not affect the total
2115                         * service time of rq. So the injection limit
2116                         * must not be updated as a function of such
2117                         * total service time, unless new injection
2118                         * occurs before rq is completed. To have the
2119                         * injection limit updated only in the latter
2120                         * case, reset rqs_injected here (rqs_injected
2121                         * will be set in case injection is performed
2122                         * on bfqq before rq is completed).
2123                         */
2124                        if (bfqd->rq_in_driver == 0)
2125                                bfqd->rqs_injected = false;
2126                }
2127        }
2128
2129        if (bfq_bfqq_sync(bfqq))
2130                bfq_update_io_intensity(bfqq, now_ns);
2131
2132        elv_rb_add(&bfqq->sort_list, rq);
2133
2134        /*
2135         * Check if this request is a better next-serve candidate.
2136         */
2137        prev = bfqq->next_rq;
2138        next_rq = bfq_choose_req(bfqd, bfqq->next_rq, rq, bfqd->last_position);
2139        bfqq->next_rq = next_rq;
2140
2141        /*
2142         * Adjust priority tree position, if next_rq changes.
2143         * See comments on bfq_pos_tree_add_move() for the unlikely().
2144         */
2145        if (unlikely(!bfqd->nonrot_with_queueing && prev != bfqq->next_rq))
2146                bfq_pos_tree_add_move(bfqd, bfqq);
2147
2148        if (!bfq_bfqq_busy(bfqq)) /* switching to busy ... */
2149                bfq_bfqq_handle_idle_busy_switch(bfqd, bfqq, old_wr_coeff,
2150                                                 rq, &interactive);
2151        else {
2152                if (bfqd->low_latency && old_wr_coeff == 1 && !rq_is_sync(rq) &&
2153                    time_is_before_jiffies(
2154                                bfqq->last_wr_start_finish +
2155                                bfqd->bfq_wr_min_inter_arr_async)) {
2156                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
2157                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
2158
2159                        bfqd->wr_busy_queues++;
2160                        bfqq->entity.prio_changed = 1;
2161                }
2162                if (prev != bfqq->next_rq)
2163                        bfq_updated_next_req(bfqd, bfqq);
2164        }
2165
2166        /*
2167         * Assign jiffies to last_wr_start_finish in the following
2168         * cases:
2169         *
2170         * . if bfqq is not going to be weight-raised, because, for
2171         *   non weight-raised queues, last_wr_start_finish stores the
2172         *   arrival time of the last request; as of now, this piece
2173         *   of information is used only for deciding whether to
2174         *   weight-raise async queues
2175         *
2176         * . if bfqq is not weight-raised, because, if bfqq is now
2177         *   switching to weight-raised, then last_wr_start_finish
2178         *   stores the time when weight-raising starts
2179         *
2180         * . if bfqq is interactive, because, regardless of whether
2181         *   bfqq is currently weight-raised, the weight-raising
2182         *   period must start or restart (this case is considered
2183         *   separately because it is not detected by the above
2184         *   conditions, if bfqq is already weight-raised)
2185         *
2186         * last_wr_start_finish has to be updated also if bfqq is soft
2187         * real-time, because the weight-raising period is constantly
2188         * restarted on idle-to-busy transitions for these queues, but
2189         * this is already done in bfq_bfqq_handle_idle_busy_switch if
2190         * needed.
2191         */
2192        if (bfqd->low_latency &&
2193                (old_wr_coeff == 1 || bfqq->wr_coeff == 1 || interactive))
2194                bfqq->last_wr_start_finish = jiffies;
2195}
2196
2197static struct request *bfq_find_rq_fmerge(struct bfq_data *bfqd,
2198                                          struct bio *bio,
2199                                          struct request_queue *q)
2200{
2201        struct bfq_queue *bfqq = bfqd->bio_bfqq;
2202
2203
2204        if (bfqq)
2205                return elv_rb_find(&bfqq->sort_list, bio_end_sector(bio));
2206
2207        return NULL;
2208}
2209
2210static sector_t get_sdist(sector_t last_pos, struct request *rq)
2211{
2212        if (last_pos)
2213                return abs(blk_rq_pos(rq) - last_pos);
2214
2215        return 0;
2216}
2217
2218#if 0 /* Still not clear if we can do without next two functions */
2219static void bfq_activate_request(struct request_queue *q, struct request *rq)
2220{
2221        struct bfq_data *bfqd = q->elevator->elevator_data;
2222
2223        bfqd->rq_in_driver++;
2224}
2225
2226static void bfq_deactivate_request(struct request_queue *q, struct request *rq)
2227{
2228        struct bfq_data *bfqd = q->elevator->elevator_data;
2229
2230        bfqd->rq_in_driver--;
2231}
2232#endif
2233
2234static void bfq_remove_request(struct request_queue *q,
2235                               struct request *rq)
2236{
2237        struct bfq_queue *bfqq = RQ_BFQQ(rq);
2238        struct bfq_data *bfqd = bfqq->bfqd;
2239        const int sync = rq_is_sync(rq);
2240
2241        if (bfqq->next_rq == rq) {
2242                bfqq->next_rq = bfq_find_next_rq(bfqd, bfqq, rq);
2243                bfq_updated_next_req(bfqd, bfqq);
2244        }
2245
2246        if (rq->queuelist.prev != &rq->queuelist)
2247                list_del_init(&rq->queuelist);
2248        bfqq->queued[sync]--;
2249        bfqd->queued--;
2250        elv_rb_del(&bfqq->sort_list, rq);
2251
2252        elv_rqhash_del(q, rq);
2253        if (q->last_merge == rq)
2254                q->last_merge = NULL;
2255
2256        if (RB_EMPTY_ROOT(&bfqq->sort_list)) {
2257                bfqq->next_rq = NULL;
2258
2259                if (bfq_bfqq_busy(bfqq) && bfqq != bfqd->in_service_queue) {
2260                        bfq_del_bfqq_busy(bfqd, bfqq, false);
2261                        /*
2262                         * bfqq emptied. In normal operation, when
2263                         * bfqq is empty, bfqq->entity.service and
2264                         * bfqq->entity.budget must contain,
2265                         * respectively, the service received and the
2266                         * budget used last time bfqq emptied. These
2267                         * facts do not hold in this case, as at least
2268                         * this last removal occurred while bfqq is
2269                         * not in service. To avoid inconsistencies,
2270                         * reset both bfqq->entity.service and
2271                         * bfqq->entity.budget, if bfqq has still a
2272                         * process that may issue I/O requests to it.
2273                         */
2274                        bfqq->entity.budget = bfqq->entity.service = 0;
2275                }
2276
2277                /*
2278                 * Remove queue from request-position tree as it is empty.
2279                 */
2280                if (bfqq->pos_root) {
2281                        rb_erase(&bfqq->pos_node, bfqq->pos_root);
2282                        bfqq->pos_root = NULL;
2283                }
2284        } else {
2285                /* see comments on bfq_pos_tree_add_move() for the unlikely() */
2286                if (unlikely(!bfqd->nonrot_with_queueing))
2287                        bfq_pos_tree_add_move(bfqd, bfqq);
2288        }
2289
2290        if (rq->cmd_flags & REQ_META)
2291                bfqq->meta_pending--;
2292
2293}
2294
2295static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
2296                unsigned int nr_segs)
2297{
2298        struct bfq_data *bfqd = q->elevator->elevator_data;
2299        struct request *free = NULL;
2300        /*
2301         * bfq_bic_lookup grabs the queue_lock: invoke it now and
2302         * store its return value for later use, to avoid nesting
2303         * queue_lock inside the bfqd->lock. We assume that the bic
2304         * returned by bfq_bic_lookup does not go away before
2305         * bfqd->lock is taken.
2306         */
2307        struct bfq_io_cq *bic = bfq_bic_lookup(bfqd, current->io_context, q);
2308        bool ret;
2309
2310        spin_lock_irq(&bfqd->lock);
2311
2312        if (bic)
2313                bfqd->bio_bfqq = bic_to_bfqq(bic, op_is_sync(bio->bi_opf));
2314        else
2315                bfqd->bio_bfqq = NULL;
2316        bfqd->bio_bic = bic;
2317
2318        ret = blk_mq_sched_try_merge(q, bio, nr_segs, &free);
2319
2320        if (free)
2321                blk_mq_free_request(free);
2322        spin_unlock_irq(&bfqd->lock);
2323
2324        return ret;
2325}
2326
2327static int bfq_request_merge(struct request_queue *q, struct request **req,
2328                             struct bio *bio)
2329{
2330        struct bfq_data *bfqd = q->elevator->elevator_data;
2331        struct request *__rq;
2332
2333        __rq = bfq_find_rq_fmerge(bfqd, bio, q);
2334        if (__rq && elv_bio_merge_ok(__rq, bio)) {
2335                *req = __rq;
2336
2337                if (blk_discard_mergable(__rq))
2338                        return ELEVATOR_DISCARD_MERGE;
2339                return ELEVATOR_FRONT_MERGE;
2340        }
2341
2342        return ELEVATOR_NO_MERGE;
2343}
2344
2345static struct bfq_queue *bfq_init_rq(struct request *rq);
2346
2347static void bfq_request_merged(struct request_queue *q, struct request *req,
2348                               enum elv_merge type)
2349{
2350        if (type == ELEVATOR_FRONT_MERGE &&
2351            rb_prev(&req->rb_node) &&
2352            blk_rq_pos(req) <
2353            blk_rq_pos(container_of(rb_prev(&req->rb_node),
2354                                    struct request, rb_node))) {
2355                struct bfq_queue *bfqq = bfq_init_rq(req);
2356                struct bfq_data *bfqd;
2357                struct request *prev, *next_rq;
2358
2359                if (!bfqq)
2360                        return;
2361
2362                bfqd = bfqq->bfqd;
2363
2364                /* Reposition request in its sort_list */
2365                elv_rb_del(&bfqq->sort_list, req);
2366                elv_rb_add(&bfqq->sort_list, req);
2367
2368                /* Choose next request to be served for bfqq */
2369                prev = bfqq->next_rq;
2370                next_rq = bfq_choose_req(bfqd, bfqq->next_rq, req,
2371                                         bfqd->last_position);
2372                bfqq->next_rq = next_rq;
2373                /*
2374                 * If next_rq changes, update both the queue's budget to
2375                 * fit the new request and the queue's position in its
2376                 * rq_pos_tree.
2377                 */
2378                if (prev != bfqq->next_rq) {
2379                        bfq_updated_next_req(bfqd, bfqq);
2380                        /*
2381                         * See comments on bfq_pos_tree_add_move() for
2382                         * the unlikely().
2383                         */
2384                        if (unlikely(!bfqd->nonrot_with_queueing))
2385                                bfq_pos_tree_add_move(bfqd, bfqq);
2386                }
2387        }
2388}
2389
2390/*
2391 * This function is called to notify the scheduler that the requests
2392 * rq and 'next' have been merged, with 'next' going away.  BFQ
2393 * exploits this hook to address the following issue: if 'next' has a
2394 * fifo_time lower that rq, then the fifo_time of rq must be set to
2395 * the value of 'next', to not forget the greater age of 'next'.
2396 *
2397 * NOTE: in this function we assume that rq is in a bfq_queue, basing
2398 * on that rq is picked from the hash table q->elevator->hash, which,
2399 * in its turn, is filled only with I/O requests present in
2400 * bfq_queues, while BFQ is in use for the request queue q. In fact,
2401 * the function that fills this hash table (elv_rqhash_add) is called
2402 * only by bfq_insert_request.
2403 */
2404static void bfq_requests_merged(struct request_queue *q, struct request *rq,
2405                                struct request *next)
2406{
2407        struct bfq_queue *bfqq = bfq_init_rq(rq),
2408                *next_bfqq = bfq_init_rq(next);
2409
2410        if (!bfqq)
2411                return;
2412
2413        /*
2414         * If next and rq belong to the same bfq_queue and next is older
2415         * than rq, then reposition rq in the fifo (by substituting next
2416         * with rq). Otherwise, if next and rq belong to different
2417         * bfq_queues, never reposition rq: in fact, we would have to
2418         * reposition it with respect to next's position in its own fifo,
2419         * which would most certainly be too expensive with respect to
2420         * the benefits.
2421         */
2422        if (bfqq == next_bfqq &&
2423            !list_empty(&rq->queuelist) && !list_empty(&next->queuelist) &&
2424            next->fifo_time < rq->fifo_time) {
2425                list_del_init(&rq->queuelist);
2426                list_replace_init(&next->queuelist, &rq->queuelist);
2427                rq->fifo_time = next->fifo_time;
2428        }
2429
2430        if (bfqq->next_rq == next)
2431                bfqq->next_rq = rq;
2432
2433        bfqg_stats_update_io_merged(bfqq_group(bfqq), next->cmd_flags);
2434}
2435
2436/* Must be called with bfqq != NULL */
2437static void bfq_bfqq_end_wr(struct bfq_queue *bfqq)
2438{
2439        /*
2440         * If bfqq has been enjoying interactive weight-raising, then
2441         * reset soft_rt_next_start. We do it for the following
2442         * reason. bfqq may have been conveying the I/O needed to load
2443         * a soft real-time application. Such an application actually
2444         * exhibits a soft real-time I/O pattern after it finishes
2445         * loading, and finally starts doing its job. But, if bfqq has
2446         * been receiving a lot of bandwidth so far (likely to happen
2447         * on a fast device), then soft_rt_next_start now contains a
2448         * high value that. So, without this reset, bfqq would be
2449         * prevented from being possibly considered as soft_rt for a
2450         * very long time.
2451         */
2452
2453        if (bfqq->wr_cur_max_time !=
2454            bfqq->bfqd->bfq_wr_rt_max_time)
2455                bfqq->soft_rt_next_start = jiffies;
2456
2457        if (bfq_bfqq_busy(bfqq))
2458                bfqq->bfqd->wr_busy_queues--;
2459        bfqq->wr_coeff = 1;
2460        bfqq->wr_cur_max_time = 0;
2461        bfqq->last_wr_start_finish = jiffies;
2462        /*
2463         * Trigger a weight change on the next invocation of
2464         * __bfq_entity_update_weight_prio.
2465         */
2466        bfqq->entity.prio_changed = 1;
2467}
2468
2469void bfq_end_wr_async_queues(struct bfq_data *bfqd,
2470                             struct bfq_group *bfqg)
2471{
2472        int i, j;
2473
2474        for (i = 0; i < 2; i++)
2475                for (j = 0; j < IOPRIO_BE_NR; j++)
2476                        if (bfqg->async_bfqq[i][j])
2477                                bfq_bfqq_end_wr(bfqg->async_bfqq[i][j]);
2478        if (bfqg->async_idle_bfqq)
2479                bfq_bfqq_end_wr(bfqg->async_idle_bfqq);
2480}
2481
2482static void bfq_end_wr(struct bfq_data *bfqd)
2483{
2484        struct bfq_queue *bfqq;
2485
2486        spin_lock_irq(&bfqd->lock);
2487
2488        list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list)
2489                bfq_bfqq_end_wr(bfqq);
2490        list_for_each_entry(bfqq, &bfqd->idle_list, bfqq_list)
2491                bfq_bfqq_end_wr(bfqq);
2492        bfq_end_wr_async(bfqd);
2493
2494        spin_unlock_irq(&bfqd->lock);
2495}
2496
2497static sector_t bfq_io_struct_pos(void *io_struct, bool request)
2498{
2499        if (request)
2500                return blk_rq_pos(io_struct);
2501        else
2502                return ((struct bio *)io_struct)->bi_iter.bi_sector;
2503}
2504
2505static int bfq_rq_close_to_sector(void *io_struct, bool request,
2506                                  sector_t sector)
2507{
2508        return abs(bfq_io_struct_pos(io_struct, request) - sector) <=
2509               BFQQ_CLOSE_THR;
2510}
2511
2512static struct bfq_queue *bfqq_find_close(struct bfq_data *bfqd,
2513                                         struct bfq_queue *bfqq,
2514                                         sector_t sector)
2515{
2516        struct rb_root *root = &bfq_bfqq_to_bfqg(bfqq)->rq_pos_tree;
2517        struct rb_node *parent, *node;
2518        struct bfq_queue *__bfqq;
2519
2520        if (RB_EMPTY_ROOT(root))
2521                return NULL;
2522
2523        /*
2524         * First, if we find a request starting at the end of the last
2525         * request, choose it.
2526         */
2527        __bfqq = bfq_rq_pos_tree_lookup(bfqd, root, sector, &parent, NULL);
2528        if (__bfqq)
2529                return __bfqq;
2530
2531        /*
2532         * If the exact sector wasn't found, the parent of the NULL leaf
2533         * will contain the closest sector (rq_pos_tree sorted by
2534         * next_request position).
2535         */
2536        __bfqq = rb_entry(parent, struct bfq_queue, pos_node);
2537        if (bfq_rq_close_to_sector(__bfqq->next_rq, true, sector))
2538                return __bfqq;
2539
2540        if (blk_rq_pos(__bfqq->next_rq) < sector)
2541                node = rb_next(&__bfqq->pos_node);
2542        else
2543                node = rb_prev(&__bfqq->pos_node);
2544        if (!node)
2545                return NULL;
2546
2547        __bfqq = rb_entry(node, struct bfq_queue, pos_node);
2548        if (bfq_rq_close_to_sector(__bfqq->next_rq, true, sector))
2549                return __bfqq;
2550
2551        return NULL;
2552}
2553
2554static struct bfq_queue *bfq_find_close_cooperator(struct bfq_data *bfqd,
2555                                                   struct bfq_queue *cur_bfqq,
2556                                                   sector_t sector)
2557{
2558        struct bfq_queue *bfqq;
2559
2560        /*
2561         * We shall notice if some of the queues are cooperating,
2562         * e.g., working closely on the same area of the device. In
2563         * that case, we can group them together and: 1) don't waste
2564         * time idling, and 2) serve the union of their requests in
2565         * the best possible order for throughput.
2566         */
2567        bfqq = bfqq_find_close(bfqd, cur_bfqq, sector);
2568        if (!bfqq || bfqq == cur_bfqq)
2569                return NULL;
2570
2571        return bfqq;
2572}
2573
2574static struct bfq_queue *
2575bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
2576{
2577        int process_refs, new_process_refs;
2578        struct bfq_queue *__bfqq;
2579
2580        /*
2581         * If there are no process references on the new_bfqq, then it is
2582         * unsafe to follow the ->new_bfqq chain as other bfqq's in the chain
2583         * may have dropped their last reference (not just their last process
2584         * reference).
2585         */
2586        if (!bfqq_process_refs(new_bfqq))
2587                return NULL;
2588
2589        /* Avoid a circular list and skip interim queue merges. */
2590        while ((__bfqq = new_bfqq->new_bfqq)) {
2591                if (__bfqq == bfqq)
2592                        return NULL;
2593                new_bfqq = __bfqq;
2594        }
2595
2596        process_refs = bfqq_process_refs(bfqq);
2597        new_process_refs = bfqq_process_refs(new_bfqq);
2598        /*
2599         * If the process for the bfqq has gone away, there is no
2600         * sense in merging the queues.
2601         */
2602        if (process_refs == 0 || new_process_refs == 0)
2603                return NULL;
2604
2605        bfq_log_bfqq(bfqq->bfqd, bfqq, "scheduling merge with queue %d",
2606                new_bfqq->pid);
2607
2608        /*
2609         * Merging is just a redirection: the requests of the process
2610         * owning one of the two queues are redirected to the other queue.
2611         * The latter queue, in its turn, is set as shared if this is the
2612         * first time that the requests of some process are redirected to
2613         * it.
2614         *
2615         * We redirect bfqq to new_bfqq and not the opposite, because
2616         * we are in the context of the process owning bfqq, thus we
2617         * have the io_cq of this process. So we can immediately
2618         * configure this io_cq to redirect the requests of the
2619         * process to new_bfqq. In contrast, the io_cq of new_bfqq is
2620         * not available any more (new_bfqq->bic == NULL).
2621         *
2622         * Anyway, even in case new_bfqq coincides with the in-service
2623         * queue, redirecting requests the in-service queue is the
2624         * best option, as we feed the in-service queue with new
2625         * requests close to the last request served and, by doing so,
2626         * are likely to increase the throughput.
2627         */
2628        bfqq->new_bfqq = new_bfqq;
2629        new_bfqq->ref += process_refs;
2630        return new_bfqq;
2631}
2632
2633static bool bfq_may_be_close_cooperator(struct bfq_queue *bfqq,
2634                                        struct bfq_queue *new_bfqq)
2635{
2636        if (bfq_too_late_for_merging(new_bfqq))
2637                return false;
2638
2639        if (bfq_class_idle(bfqq) || bfq_class_idle(new_bfqq) ||
2640            (bfqq->ioprio_class != new_bfqq->ioprio_class))
2641                return false;
2642
2643        /*
2644         * If either of the queues has already been detected as seeky,
2645         * then merging it with the other queue is unlikely to lead to
2646         * sequential I/O.
2647         */
2648        if (BFQQ_SEEKY(bfqq) || BFQQ_SEEKY(new_bfqq))
2649                return false;
2650
2651        /*
2652         * Interleaved I/O is known to be done by (some) applications
2653         * only for reads, so it does not make sense to merge async
2654         * queues.
2655         */
2656        if (!bfq_bfqq_sync(bfqq) || !bfq_bfqq_sync(new_bfqq))
2657                return false;
2658
2659        return true;
2660}
2661
2662static bool idling_boosts_thr_without_issues(struct bfq_data *bfqd,
2663                                             struct bfq_queue *bfqq);
2664
2665/*
2666 * Attempt to schedule a merge of bfqq with the currently in-service
2667 * queue or with a close queue among the scheduled queues.  Return
2668 * NULL if no merge was scheduled, a pointer to the shared bfq_queue
2669 * structure otherwise.
2670 *
2671 * The OOM queue is not allowed to participate to cooperation: in fact, since
2672 * the requests temporarily redirected to the OOM queue could be redirected
2673 * again to dedicated queues at any time, the state needed to correctly
2674 * handle merging with the OOM queue would be quite complex and expensive
2675 * to maintain. Besides, in such a critical condition as an out of memory,
2676 * the benefits of queue merging may be little relevant, or even negligible.
2677 *
2678 * WARNING: queue merging may impair fairness among non-weight raised
2679 * queues, for at least two reasons: 1) the original weight of a
2680 * merged queue may change during the merged state, 2) even being the
2681 * weight the same, a merged queue may be bloated with many more
2682 * requests than the ones produced by its originally-associated
2683 * process.
2684 */
2685static struct bfq_queue *
2686bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
2687                     void *io_struct, bool request, struct bfq_io_cq *bic)
2688{
2689        struct bfq_queue *in_service_bfqq, *new_bfqq;
2690
2691        /*
2692         * Check delayed stable merge for rotational or non-queueing
2693         * devs. For this branch to be executed, bfqq must not be
2694         * currently merged with some other queue (i.e., bfqq->bic
2695         * must be non null). If we considered also merged queues,
2696         * then we should also check whether bfqq has already been
2697         * merged with bic->stable_merge_bfqq. But this would be
2698         * costly and complicated.
2699         */
2700        if (unlikely(!bfqd->nonrot_with_queueing)) {
2701                /*
2702                 * Make sure also that bfqq is sync, because
2703                 * bic->stable_merge_bfqq may point to some queue (for
2704                 * stable merging) also if bic is associated with a
2705                 * sync queue, but this bfqq is async
2706                 */
2707                if (bfq_bfqq_sync(bfqq) && bic->stable_merge_bfqq &&
2708                    !bfq_bfqq_just_created(bfqq) &&
2709                    time_is_before_jiffies(bfqq->split_time +
2710                                          msecs_to_jiffies(200))) {
2711                        struct bfq_queue *stable_merge_bfqq =
2712                                bic->stable_merge_bfqq;
2713                        int proc_ref = min(bfqq_process_refs(bfqq),
2714                                           bfqq_process_refs(stable_merge_bfqq));
2715
2716                        /* deschedule stable merge, because done or aborted here */
2717                        bfq_put_stable_ref(stable_merge_bfqq);
2718
2719                        bic->stable_merge_bfqq = NULL;
2720
2721                        if (!idling_boosts_thr_without_issues(bfqd, bfqq) &&
2722                            proc_ref > 0) {
2723                                /* next function will take at least one ref */
2724                                struct bfq_queue *new_bfqq =
2725                                        bfq_setup_merge(bfqq, stable_merge_bfqq);
2726
2727                                bic->stably_merged = true;
2728                                if (new_bfqq && new_bfqq->bic)
2729                                        new_bfqq->bic->stably_merged = true;
2730                                return new_bfqq;
2731                        } else
2732                                return NULL;
2733                }
2734        }
2735
2736        /*
2737         * Do not perform queue merging if the device is non
2738         * rotational and performs internal queueing. In fact, such a
2739         * device reaches a high speed through internal parallelism
2740         * and pipelining. This means that, to reach a high
2741         * throughput, it must have many requests enqueued at the same
2742         * time. But, in this configuration, the internal scheduling
2743         * algorithm of the device does exactly the job of queue
2744         * merging: it reorders requests so as to obtain as much as
2745         * possible a sequential I/O pattern. As a consequence, with
2746         * the workload generated by processes doing interleaved I/O,
2747         * the throughput reached by the device is likely to be the
2748         * same, with and without queue merging.
2749         *
2750         * Disabling merging also provides a remarkable benefit in
2751         * terms of throughput. Merging tends to make many workloads
2752         * artificially more uneven, because of shared queues
2753         * remaining non empty for incomparably more time than
2754         * non-merged queues. This may accentuate workload
2755         * asymmetries. For example, if one of the queues in a set of
2756         * merged queues has a higher weight than a normal queue, then
2757         * the shared queue may inherit such a high weight and, by
2758         * staying almost always active, may force BFQ to perform I/O
2759         * plugging most of the time. This evidently makes it harder
2760         * for BFQ to let the device reach a high throughput.
2761         *
2762         * Finally, the likely() macro below is not used because one
2763         * of the two branches is more likely than the other, but to
2764         * have the code path after the following if() executed as
2765         * fast as possible for the case of a non rotational device
2766         * with queueing. We want it because this is the fastest kind
2767         * of device. On the opposite end, the likely() may lengthen
2768         * the execution time of BFQ for the case of slower devices
2769         * (rotational or at least without queueing). But in this case
2770         * the execution time of BFQ matters very little, if not at
2771         * all.
2772         */
2773        if (likely(bfqd->nonrot_with_queueing))
2774                return NULL;
2775
2776        /*
2777         * Prevent bfqq from being merged if it has been created too
2778         * long ago. The idea is that true cooperating processes, and
2779         * thus their associated bfq_queues, are supposed to be
2780         * created shortly after each other. This is the case, e.g.,
2781         * for KVM/QEMU and dump I/O threads. Basing on this
2782         * assumption, the following filtering greatly reduces the
2783         * probability that two non-cooperating processes, which just
2784         * happen to do close I/O for some short time interval, have
2785         * their queues merged by mistake.
2786         */
2787        if (bfq_too_late_for_merging(bfqq))
2788                return NULL;
2789
2790        if (bfqq->new_bfqq)
2791                return bfqq->new_bfqq;
2792
2793        if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
2794                return NULL;
2795
2796        /* If there is only one backlogged queue, don't search. */
2797        if (bfq_tot_busy_queues(bfqd) == 1)
2798                return NULL;
2799
2800        in_service_bfqq = bfqd->in_service_queue;
2801
2802        if (in_service_bfqq && in_service_bfqq != bfqq &&
2803            likely(in_service_bfqq != &bfqd->oom_bfqq) &&
2804            bfq_rq_close_to_sector(io_struct, request,
2805                                   bfqd->in_serv_last_pos) &&
2806            bfqq->entity.parent == in_service_bfqq->entity.parent &&
2807            bfq_may_be_close_cooperator(bfqq, in_service_bfqq)) {
2808                new_bfqq = bfq_setup_merge(bfqq, in_service_bfqq);
2809                if (new_bfqq)
2810                        return new_bfqq;
2811        }
2812        /*
2813         * Check whether there is a cooperator among currently scheduled
2814         * queues. The only thing we need is that the bio/request is not
2815         * NULL, as we need it to establish whether a cooperator exists.
2816         */
2817        new_bfqq = bfq_find_close_cooperator(bfqd, bfqq,
2818                        bfq_io_struct_pos(io_struct, request));
2819
2820        if (new_bfqq && likely(new_bfqq != &bfqd->oom_bfqq) &&
2821            bfq_may_be_close_cooperator(bfqq, new_bfqq))
2822                return bfq_setup_merge(bfqq, new_bfqq);
2823
2824        return NULL;
2825}
2826
2827static void bfq_bfqq_save_state(struct bfq_queue *bfqq)
2828{
2829        struct bfq_io_cq *bic = bfqq->bic;
2830
2831        /*
2832         * If !bfqq->bic, the queue is already shared or its requests
2833         * have already been redirected to a shared queue; both idle window
2834         * and weight raising state have already been saved. Do nothing.
2835         */
2836        if (!bic)
2837                return;
2838
2839        bic->saved_last_serv_time_ns = bfqq->last_serv_time_ns;
2840        bic->saved_inject_limit = bfqq->inject_limit;
2841        bic->saved_decrease_time_jif = bfqq->decrease_time_jif;
2842
2843        bic->saved_weight = bfqq->entity.orig_weight;
2844        bic->saved_ttime = bfqq->ttime;
2845        bic->saved_has_short_ttime = bfq_bfqq_has_short_ttime(bfqq);
2846        bic->saved_IO_bound = bfq_bfqq_IO_bound(bfqq);
2847        bic->saved_io_start_time = bfqq->io_start_time;
2848        bic->saved_tot_idle_time = bfqq->tot_idle_time;
2849        bic->saved_in_large_burst = bfq_bfqq_in_large_burst(bfqq);
2850        bic->was_in_burst_list = !hlist_unhashed(&bfqq->burst_list_node);
2851        if (unlikely(bfq_bfqq_just_created(bfqq) &&
2852                     !bfq_bfqq_in_large_burst(bfqq) &&
2853                     bfqq->bfqd->low_latency)) {
2854                /*
2855                 * bfqq being merged right after being created: bfqq
2856                 * would have deserved interactive weight raising, but
2857                 * did not make it to be set in a weight-raised state,
2858                 * because of this early merge. Store directly the
2859                 * weight-raising state that would have been assigned
2860                 * to bfqq, so that to avoid that bfqq unjustly fails
2861                 * to enjoy weight raising if split soon.
2862                 */
2863                bic->saved_wr_coeff = bfqq->bfqd->bfq_wr_coeff;
2864                bic->saved_wr_start_at_switch_to_srt = bfq_smallest_from_now();
2865                bic->saved_wr_cur_max_time = bfq_wr_duration(bfqq->bfqd);
2866                bic->saved_last_wr_start_finish = jiffies;
2867        } else {
2868                bic->saved_wr_coeff = bfqq->wr_coeff;
2869                bic->saved_wr_start_at_switch_to_srt =
2870                        bfqq->wr_start_at_switch_to_srt;
2871                bic->saved_service_from_wr = bfqq->service_from_wr;
2872                bic->saved_last_wr_start_finish = bfqq->last_wr_start_finish;
2873                bic->saved_wr_cur_max_time = bfqq->wr_cur_max_time;
2874        }
2875}
2876
2877
2878static void
2879bfq_reassign_last_bfqq(struct bfq_queue *cur_bfqq, struct bfq_queue *new_bfqq)
2880{
2881        if (cur_bfqq->entity.parent &&
2882            cur_bfqq->entity.parent->last_bfqq_created == cur_bfqq)
2883                cur_bfqq->entity.parent->last_bfqq_created = new_bfqq;
2884        else if (cur_bfqq->bfqd && cur_bfqq->bfqd->last_bfqq_created == cur_bfqq)
2885                cur_bfqq->bfqd->last_bfqq_created = new_bfqq;
2886}
2887
2888void bfq_release_process_ref(struct bfq_data *bfqd, struct bfq_queue *bfqq)
2889{
2890        /*
2891         * To prevent bfqq's service guarantees from being violated,
2892         * bfqq may be left busy, i.e., queued for service, even if
2893         * empty (see comments in __bfq_bfqq_expire() for
2894         * details). But, if no process will send requests to bfqq any
2895         * longer, then there is no point in keeping bfqq queued for
2896         * service. In addition, keeping bfqq queued for service, but
2897         * with no process ref any longer, may have caused bfqq to be
2898         * freed when dequeued from service. But this is assumed to
2899         * never happen.
2900         */
2901        if (bfq_bfqq_busy(bfqq) && RB_EMPTY_ROOT(&bfqq->sort_list) &&
2902            bfqq != bfqd->in_service_queue)
2903                bfq_del_bfqq_busy(bfqd, bfqq, false);
2904
2905        bfq_reassign_last_bfqq(bfqq, NULL);
2906
2907        bfq_put_queue(bfqq);
2908}
2909
2910static void
2911bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
2912                struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
2913{
2914        bfq_log_bfqq(bfqd, bfqq, "merging with queue %lu",
2915                (unsigned long)new_bfqq->pid);
2916        /* Save weight raising and idle window of the merged queues */
2917        bfq_bfqq_save_state(bfqq);
2918        bfq_bfqq_save_state(new_bfqq);
2919        if (bfq_bfqq_IO_bound(bfqq))
2920                bfq_mark_bfqq_IO_bound(new_bfqq);
2921        bfq_clear_bfqq_IO_bound(bfqq);
2922
2923        /*
2924         * The processes associated with bfqq are cooperators of the
2925         * processes associated with new_bfqq. So, if bfqq has a
2926         * waker, then assume that all these processes will be happy
2927         * to let bfqq's waker freely inject I/O when they have no
2928         * I/O.
2929         */
2930        if (bfqq->waker_bfqq && !new_bfqq->waker_bfqq &&
2931            bfqq->waker_bfqq != new_bfqq) {
2932                new_bfqq->waker_bfqq = bfqq->waker_bfqq;
2933                new_bfqq->tentative_waker_bfqq = NULL;
2934
2935                /*
2936                 * If the waker queue disappears, then
2937                 * new_bfqq->waker_bfqq must be reset. So insert
2938                 * new_bfqq into the woken_list of the waker. See
2939                 * bfq_check_waker for details.
2940                 */
2941                hlist_add_head(&new_bfqq->woken_list_node,
2942                               &new_bfqq->waker_bfqq->woken_list);
2943
2944        }
2945
2946        /*
2947         * If bfqq is weight-raised, then let new_bfqq inherit
2948         * weight-raising. To reduce false positives, neglect the case
2949         * where bfqq has just been created, but has not yet made it
2950         * to be weight-raised (which may happen because EQM may merge
2951         * bfqq even before bfq_add_request is executed for the first
2952         * time for bfqq). Handling this case would however be very
2953         * easy, thanks to the flag just_created.
2954         */
2955        if (new_bfqq->wr_coeff == 1 && bfqq->wr_coeff > 1) {
2956                new_bfqq->wr_coeff = bfqq->wr_coeff;
2957                new_bfqq->wr_cur_max_time = bfqq->wr_cur_max_time;
2958                new_bfqq->last_wr_start_finish = bfqq->last_wr_start_finish;
2959                new_bfqq->wr_start_at_switch_to_srt =
2960                        bfqq->wr_start_at_switch_to_srt;
2961                if (bfq_bfqq_busy(new_bfqq))
2962                        bfqd->wr_busy_queues++;
2963                new_bfqq->entity.prio_changed = 1;
2964        }
2965
2966        if (bfqq->wr_coeff > 1) { /* bfqq has given its wr to new_bfqq */
2967                bfqq->wr_coeff = 1;
2968                bfqq->entity.prio_changed = 1;
2969                if (bfq_bfqq_busy(bfqq))
2970                        bfqd->wr_busy_queues--;
2971        }
2972
2973        bfq_log_bfqq(bfqd, new_bfqq, "merge_bfqqs: wr_busy %d",
2974                     bfqd->wr_busy_queues);
2975
2976        /*
2977         * Merge queues (that is, let bic redirect its requests to new_bfqq)
2978         */
2979        bic_set_bfqq(bic, new_bfqq, 1);
2980        bfq_mark_bfqq_coop(new_bfqq);
2981        /*
2982         * new_bfqq now belongs to at least two bics (it is a shared queue):
2983         * set new_bfqq->bic to NULL. bfqq either:
2984         * - does not belong to any bic any more, and hence bfqq->bic must
2985         *   be set to NULL, or
2986         * - is a queue whose owning bics have already been redirected to a
2987         *   different queue, hence the queue is destined to not belong to
2988         *   any bic soon and bfqq->bic is already NULL (therefore the next
2989         *   assignment causes no harm).
2990         */
2991        new_bfqq->bic = NULL;
2992        /*
2993         * If the queue is shared, the pid is the pid of one of the associated
2994         * processes. Which pid depends on the exact sequence of merge events
2995         * the queue underwent. So printing such a pid is useless and confusing
2996         * because it reports a random pid between those of the associated
2997         * processes.
2998         * We mark such a queue with a pid -1, and then print SHARED instead of
2999         * a pid in logging messages.
3000         */
3001        new_bfqq->pid = -1;
3002        bfqq->bic = NULL;
3003
3004        bfq_reassign_last_bfqq(bfqq, new_bfqq);
3005
3006        bfq_release_process_ref(bfqd, bfqq);
3007}
3008
3009static bool bfq_allow_bio_merge(struct request_queue *q, struct request *rq,
3010                                struct bio *bio)
3011{
3012        struct bfq_data *bfqd = q->elevator->elevator_data;
3013        bool is_sync = op_is_sync(bio->bi_opf);
3014        struct bfq_queue *bfqq = bfqd->bio_bfqq, *new_bfqq;
3015
3016        /*
3017         * Disallow merge of a sync bio into an async request.
3018         */
3019        if (is_sync && !rq_is_sync(rq))
3020                return false;
3021
3022        /*
3023         * Lookup the bfqq that this bio will be queued with. Allow
3024         * merge only if rq is queued there.
3025         */
3026        if (!bfqq)
3027                return false;
3028
3029        /*
3030         * We take advantage of this function to perform an early merge
3031         * of the queues of possible cooperating processes.
3032         */
3033        new_bfqq = bfq_setup_cooperator(bfqd, bfqq, bio, false, bfqd->bio_bic);
3034        if (new_bfqq) {
3035                /*
3036                 * bic still points to bfqq, then it has not yet been
3037                 * redirected to some other bfq_queue, and a queue
3038                 * merge between bfqq and new_bfqq can be safely
3039                 * fulfilled, i.e., bic can be redirected to new_bfqq
3040                 * and bfqq can be put.
3041                 */
3042                bfq_merge_bfqqs(bfqd, bfqd->bio_bic, bfqq,
3043                                new_bfqq);
3044                /*
3045                 * If we get here, bio will be queued into new_queue,
3046                 * so use new_bfqq to decide whether bio and rq can be
3047                 * merged.
3048                 */
3049                bfqq = new_bfqq;
3050
3051                /*
3052                 * Change also bqfd->bio_bfqq, as
3053                 * bfqd->bio_bic now points to new_bfqq, and
3054                 * this function may be invoked again (and then may
3055                 * use again bqfd->bio_bfqq).
3056                 */
3057                bfqd->bio_bfqq = bfqq;
3058        }
3059
3060        return bfqq == RQ_BFQQ(rq);
3061}
3062
3063/*
3064 * Set the maximum time for the in-service queue to consume its
3065 * budget. This prevents seeky processes from lowering the throughput.
3066 * In practice, a time-slice service scheme is used with seeky
3067 * processes.
3068 */
3069static void bfq_set_budget_timeout(struct bfq_data *bfqd,
3070                                   struct bfq_queue *bfqq)
3071{
3072        unsigned int timeout_coeff;
3073
3074        if (bfqq->wr_cur_max_time == bfqd->bfq_wr_rt_max_time)
3075                timeout_coeff = 1;
3076        else
3077                timeout_coeff = bfqq->entity.weight / bfqq->entity.orig_weight;
3078
3079        bfqd->last_budget_start = ktime_get();
3080
3081        bfqq->budget_timeout = jiffies +
3082                bfqd->bfq_timeout * timeout_coeff;
3083}
3084
3085static void __bfq_set_in_service_queue(struct bfq_data *bfqd,
3086                                       struct bfq_queue *bfqq)
3087{
3088        if (bfqq) {
3089                bfq_clear_bfqq_fifo_expire(bfqq);
3090
3091                bfqd->budgets_assigned = (bfqd->budgets_assigned * 7 + 256) / 8;
3092
3093                if (time_is_before_jiffies(bfqq->last_wr_start_finish) &&
3094                    bfqq->wr_coeff > 1 &&
3095                    bfqq->wr_cur_max_time == bfqd->bfq_wr_rt_max_time &&
3096                    time_is_before_jiffies(bfqq->budget_timeout)) {
3097                        /*
3098                         * For soft real-time queues, move the start
3099                         * of the weight-raising period forward by the
3100                         * time the queue has not received any
3101                         * service. Otherwise, a relatively long
3102                         * service delay is likely to cause the
3103                         * weight-raising period of the queue to end,
3104                         * because of the short duration of the
3105                         * weight-raising period of a soft real-time
3106                         * queue.  It is worth noting that this move
3107                         * is not so dangerous for the other queues,
3108                         * because soft real-time queues are not
3109                         * greedy.
3110                         *
3111                         * To not add a further variable, we use the
3112                         * overloaded field budget_timeout to
3113                         * determine for how long the queue has not
3114                         * received service, i.e., how much time has
3115                         * elapsed since the queue expired. However,
3116                         * this is a little imprecise, because
3117                         * budget_timeout is set to jiffies if bfqq
3118                         * not only expires, but also remains with no
3119                         * request.
3120                         */
3121                        if (time_after(bfqq->budget_timeout,
3122                                       bfqq->last_wr_start_finish))
3123                                bfqq->last_wr_start_finish +=
3124                                        jiffies - bfqq->budget_timeout;
3125                        else
3126                                bfqq->last_wr_start_finish = jiffies;
3127                }
3128
3129                bfq_set_budget_timeout(bfqd, bfqq);
3130                bfq_log_bfqq(bfqd, bfqq,
3131                             "set_in_service_queue, cur-budget = %d",
3132                             bfqq->entity.budget);
3133        }
3134
3135        bfqd->in_service_queue = bfqq;
3136        bfqd->in_serv_last_pos = 0;
3137}
3138
3139/*
3140 * Get and set a new queue for service.
3141 */
3142static struct bfq_queue *bfq_set_in_service_queue(struct bfq_data *bfqd)
3143{
3144        struct bfq_queue *bfqq = bfq_get_next_queue(bfqd);
3145
3146        __bfq_set_in_service_queue(bfqd, bfqq);
3147        return bfqq;
3148}
3149
3150static void bfq_arm_slice_timer(struct bfq_data *bfqd)
3151{
3152        struct bfq_queue *bfqq = bfqd->in_service_queue;
3153        u32 sl;
3154
3155        bfq_mark_bfqq_wait_request(bfqq);
3156
3157        /*
3158         * We don't want to idle for seeks, but we do want to allow
3159         * fair distribution of slice time for a process doing back-to-back
3160         * seeks. So allow a little bit of time for him to submit a new rq.
3161         */
3162        sl = bfqd->bfq_slice_idle;
3163        /*
3164         * Unless the queue is being weight-raised or the scenario is
3165         * asymmetric, grant only minimum idle time if the queue
3166         * is seeky. A long idling is preserved for a weight-raised
3167         * queue, or, more in general, in an asymmetric scenario,
3168         * because a long idling is needed for guaranteeing to a queue
3169         * its reserved share of the throughput (in particular, it is
3170         * needed if the queue has a higher weight than some other
3171         * queue).
3172         */
3173        if (BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1 &&
3174            !bfq_asymmetric_scenario(bfqd, bfqq))
3175                sl = min_t(u64, sl, BFQ_MIN_TT);
3176        else if (bfqq->wr_coeff > 1)
3177                sl = max_t(u32, sl, 20ULL * NSEC_PER_MSEC);
3178
3179        bfqd->last_idling_start = ktime_get();
3180        bfqd->last_idling_start_jiffies = jiffies;
3181
3182        hrtimer_start(&bfqd->idle_slice_timer, ns_to_ktime(sl),
3183                      HRTIMER_MODE_REL);
3184        bfqg_stats_set_start_idle_time(bfqq_group(bfqq));
3185}
3186
3187/*
3188 * In autotuning mode, max_budget is dynamically recomputed as the
3189 * amount of sectors transferred in timeout at the estimated peak
3190 * rate. This enables BFQ to utilize a full timeslice with a full
3191 * budget, even if the in-service queue is served at peak rate. And
3192 * this maximises throughput with sequential workloads.
3193 */
3194static unsigned long bfq_calc_max_budget(struct bfq_data *bfqd)
3195{
3196        return (u64)bfqd->peak_rate * USEC_PER_MSEC *
3197                jiffies_to_msecs(bfqd->bfq_timeout)>>BFQ_RATE_SHIFT;
3198}
3199
3200/*
3201 * Update parameters related to throughput and responsiveness, as a
3202 * function of the estimated peak rate. See comments on
3203 * bfq_calc_max_budget(), and on the ref_wr_duration array.
3204 */
3205static void update_thr_responsiveness_params(struct bfq_data *bfqd)
3206{
3207        if (bfqd->bfq_user_max_budget == 0) {
3208                bfqd->bfq_max_budget =
3209                        bfq_calc_max_budget(bfqd);
3210                bfq_log(bfqd, "new max_budget = %d", bfqd->bfq_max_budget);
3211        }
3212}
3213
3214static void bfq_reset_rate_computation(struct bfq_data *bfqd,
3215                                       struct request *rq)
3216{
3217        if (rq != NULL) { /* new rq dispatch now, reset accordingly */
3218                bfqd->last_dispatch = bfqd->first_dispatch = ktime_get_ns();
3219                bfqd->peak_rate_samples = 1;
3220                bfqd->sequential_samples = 0;
3221                bfqd->tot_sectors_dispatched = bfqd->last_rq_max_size =
3222                        blk_rq_sectors(rq);
3223        } else /* no new rq dispatched, just reset the number of samples */
3224                bfqd->peak_rate_samples = 0; /* full re-init on next disp. */
3225
3226        bfq_log(bfqd,
3227                "reset_rate_computation at end, sample %u/%u tot_sects %llu",
3228                bfqd->peak_rate_samples, bfqd->sequential_samples,
3229                bfqd->tot_sectors_dispatched);
3230}
3231
3232static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq)
3233{
3234        u32 rate, weight, divisor;
3235
3236        /*
3237         * For the convergence property to hold (see comments on
3238         * bfq_update_peak_rate()) and for the assessment to be
3239         * reliable, a minimum number of samples must be present, and
3240         * a minimum amount of time must have elapsed. If not so, do
3241         * not compute new rate. Just reset parameters, to get ready
3242         * for a new evaluation attempt.
3243         */
3244        if (bfqd->peak_rate_samples < BFQ_RATE_MIN_SAMPLES ||
3245            bfqd->delta_from_first < BFQ_RATE_MIN_INTERVAL)
3246                goto reset_computation;
3247
3248        /*
3249         * If a new request completion has occurred after last
3250         * dispatch, then, to approximate the rate at which requests
3251         * have been served by the device, it is more precise to
3252         * extend the observation interval to the last completion.
3253         */
3254        bfqd->delta_from_first =
3255                max_t(u64, bfqd->delta_from_first,
3256                      bfqd->last_completion - bfqd->first_dispatch);
3257
3258        /*
3259         * Rate computed in sects/usec, and not sects/nsec, for
3260         * precision issues.
3261         */
3262        rate = div64_ul(bfqd->tot_sectors_dispatched<<BFQ_RATE_SHIFT,
3263                        div_u64(bfqd->delta_from_first, NSEC_PER_USEC));
3264
3265        /*
3266         * Peak rate not updated if:
3267         * - the percentage of sequential dispatches is below 3/4 of the
3268         *   total, and rate is below the current estimated peak rate
3269         * - rate is unreasonably high (> 20M sectors/sec)
3270         */
3271        if ((bfqd->sequential_samples < (3 * bfqd->peak_rate_samples)>>2 &&
3272             rate <= bfqd->peak_rate) ||
3273                rate > 20<<BFQ_RATE_SHIFT)
3274                goto reset_computation;
3275
3276        /*
3277         * We have to update the peak rate, at last! To this purpose,
3278         * we use a low-pass filter. We compute the smoothing constant
3279         * of the filter as a function of the 'weight' of the new
3280         * measured rate.
3281         *
3282         * As can be seen in next formulas, we define this weight as a
3283         * quantity proportional to how sequential the workload is,
3284         * and to how long the observation time interval is.
3285         *
3286         * The weight runs from 0 to 8. The maximum value of the
3287         * weight, 8, yields the minimum value for the smoothing
3288         * constant. At this minimum value for the smoothing constant,
3289         * the measured rate contributes for half of the next value of
3290         * the estimated peak rate.
3291         *
3292         * So, the first step is to compute the weight as a function
3293         * of how sequential the workload is. Note that the weight
3294         * cannot reach 9, because bfqd->sequential_samples cannot
3295         * become equal to bfqd->peak_rate_samples, which, in its
3296         * turn, holds true because bfqd->sequential_samples is not
3297         * incremented for the first sample.
3298         */
3299        weight = (9 * bfqd->sequential_samples) / bfqd->peak_rate_samples;
3300
3301        /*
3302         * Second step: further refine the weight as a function of the
3303         * duration of the observation interval.
3304         */
3305        weight = min_t(u32, 8,
3306                       div_u64(weight * bfqd->delta_from_first,
3307                               BFQ_RATE_REF_INTERVAL));
3308
3309        /*
3310         * Divisor ranging from 10, for minimum weight, to 2, for
3311         * maximum weight.
3312         */
3313        divisor = 10 - weight;
3314
3315        /*
3316         * Finally, update peak rate:
3317         *
3318         * peak_rate = peak_rate * (divisor-1) / divisor  +  rate / divisor
3319         */
3320        bfqd->peak_rate *= divisor-1;
3321        bfqd->peak_rate /= divisor;
3322        rate /= divisor; /* smoothing constant alpha = 1/divisor */
3323
3324        bfqd->peak_rate += rate;
3325
3326        /*
3327         * For a very slow device, bfqd->peak_rate can reach 0 (see
3328         * the minimum representable values reported in the comments
3329         * on BFQ_RATE_SHIFT). Push to 1 if this happens, to avoid
3330         * divisions by zero where bfqd->peak_rate is used as a
3331         * divisor.
3332         */
3333        bfqd->peak_rate = max_t(u32, 1, bfqd->peak_rate);
3334
3335        update_thr_responsiveness_params(bfqd);
3336
3337reset_computation:
3338        bfq_reset_rate_computation(bfqd, rq);
3339}
3340
3341/*
3342 * Update the read/write peak rate (the main quantity used for
3343 * auto-tuning, see update_thr_responsiveness_params()).
3344 *
3345 * It is not trivial to estimate the peak rate (correctly): because of
3346 * the presence of sw and hw queues between the scheduler and the
3347 * device components that finally serve I/O requests, it is hard to
3348 * say exactly when a given dispatched request is served inside the
3349 * device, and for how long. As a consequence, it is hard to know
3350 * precisely at what rate a given set of requests is actually served
3351 * by the device.
3352 *
3353 * On the opposite end, the dispatch time of any request is trivially
3354 * available, and, from this piece of information, the "dispatch rate"
3355 * of requests can be immediately computed. So, the idea in the next
3356 * function is to use what is known, namely request dispatch times
3357 * (plus, when useful, request completion times), to estimate what is
3358 * unknown, namely in-device request service rate.
3359 *
3360 * The main issue is that, because of the above facts, the rate at
3361 * which a certain set of requests is dispatched over a certain time
3362 * interval can vary greatly with respect to the rate at which the
3363 * same requests are then served. But, since the size of any
3364 * intermediate queue is limited, and the service scheme is lossless
3365 * (no request is silently dropped), the following obvious convergence
3366 * property holds: the number of requests dispatched MUST become
3367 * closer and closer to the number of requests completed as the
3368 * observation interval grows. This is the key property used in
3369 * the next function to estimate the peak service rate as a function
3370 * of the observed dispatch rate. The function assumes to be invoked
3371 * on every request dispatch.
3372 */
3373static void bfq_update_peak_rate(struct bfq_data *bfqd, struct request *rq)
3374{
3375        u64 now_ns = ktime_get_ns();
3376
3377        if (bfqd->peak_rate_samples == 0) { /* first dispatch */
3378                bfq_log(bfqd, "update_peak_rate: goto reset, samples %d",
3379                        bfqd->peak_rate_samples);
3380                bfq_reset_rate_computation(bfqd, rq);
3381                goto update_last_values; /* will add one sample */
3382        }
3383
3384        /*
3385         * Device idle for very long: the observation interval lasting
3386         * up to this dispatch cannot be a valid observation interval
3387         * for computing a new peak rate (similarly to the late-
3388         * completion event in bfq_completed_request()). Go to
3389         * update_rate_and_reset to have the following three steps
3390         * taken:
3391         * - close the observation interval at the last (previous)
3392         *   request dispatch or completion
3393         * - compute rate, if possible, for that observation interval
3394         * - start a new observation interval with this dispatch
3395         */
3396        if (now_ns - bfqd->last_dispatch > 100*NSEC_PER_MSEC &&
3397            bfqd->rq_in_driver == 0)
3398                goto update_rate_and_reset;
3399
3400        /* Update sampling information */
3401        bfqd->peak_rate_samples++;
3402
3403        if ((bfqd->rq_in_driver > 0 ||
3404                now_ns - bfqd->last_completion < BFQ_MIN_TT)
3405            && !BFQ_RQ_SEEKY(bfqd, bfqd->last_position, rq))
3406                bfqd->sequential_samples++;
3407
3408        bfqd->tot_sectors_dispatched += blk_rq_sectors(rq);
3409
3410        /* Reset max observed rq size every 32 dispatches */
3411        if (likely(bfqd->peak_rate_samples % 32))
3412                bfqd->last_rq_max_size =
3413                        max_t(u32, blk_rq_sectors(rq), bfqd->last_rq_max_size);
3414        else
3415                bfqd->last_rq_max_size = blk_rq_sectors(rq);
3416
3417        bfqd->delta_from_first = now_ns - bfqd->first_dispatch;
3418
3419        /* Target observation interval not yet reached, go on sampling */
3420        if (bfqd->delta_from_first < BFQ_RATE_REF_INTERVAL)
3421                goto update_last_values;
3422
3423update_rate_and_reset:
3424        bfq_update_rate_reset(bfqd, rq);
3425update_last_values:
3426        bfqd->last_position = blk_rq_pos(rq) + blk_rq_sectors(rq);
3427        if (RQ_BFQQ(rq) == bfqd->in_service_queue)
3428                bfqd->in_serv_last_pos = bfqd->last_position;
3429        bfqd->last_dispatch = now_ns;
3430}
3431
3432/*
3433 * Remove request from internal lists.
3434 */
3435static void bfq_dispatch_remove(struct request_queue *q, struct request *rq)
3436{
3437        struct bfq_queue *bfqq = RQ_BFQQ(rq);
3438
3439        /*
3440         * For consistency, the next instruction should have been
3441         * executed after removing the request from the queue and
3442         * dispatching it.  We execute instead this instruction before
3443         * bfq_remove_request() (and hence introduce a temporary
3444         * inconsistency), for efficiency.  In fact, should this
3445         * dispatch occur for a non in-service bfqq, this anticipated
3446         * increment prevents two counters related to bfqq->dispatched
3447         * from risking to be, first, uselessly decremented, and then
3448         * incremented again when the (new) value of bfqq->dispatched
3449         * happens to be taken into account.
3450         */
3451        bfqq->dispatched++;
3452        bfq_update_peak_rate(q->elevator->elevator_data, rq);
3453
3454        bfq_remove_request(q, rq);
3455}
3456
3457/*
3458 * There is a case where idling does not have to be performed for
3459 * throughput concerns, but to preserve the throughput share of
3460 * the process associated with bfqq.
3461 *
3462 * To introduce this case, we can note that allowing the drive
3463 * to enqueue more than one request at a time, and hence
3464 * delegating de facto final scheduling decisions to the
3465 * drive's internal scheduler, entails loss of control on the
3466 * actual request service order. In particular, the critical
3467 * situation is when requests from different processes happen
3468 * to be present, at the same time, in the internal queue(s)
3469 * of the drive. In such a situation, the drive, by deciding
3470 * the service order of the internally-queued requests, does
3471 * determine also the actual throughput distribution among
3472 * these processes. But the drive typically has no notion or
3473 * concern about per-process throughput distribution, and
3474 * makes its decisions only on a per-request basis. Therefore,
3475 * the service distribution enforced by the drive's internal
3476 * scheduler is likely to coincide with the desired throughput
3477 * distribution only in a completely symmetric, or favorably
3478 * skewed scenario where:
3479 * (i-a) each of these processes must get the same throughput as
3480 *       the others,
3481 * (i-b) in case (i-a) does not hold, it holds that the process
3482 *       associated with bfqq must receive a lower or equal
3483 *       throughput than any of the other processes;
3484 * (ii)  the I/O of each process has the same properties, in
3485 *       terms of locality (sequential or random), direction
3486 *       (reads or writes), request sizes, greediness
3487 *       (from I/O-bound to sporadic), and so on;
3488
3489 * In fact, in such a scenario, the drive tends to treat the requests
3490 * of each process in about the same way as the requests of the
3491 * others, and thus to provide each of these processes with about the
3492 * same throughput.  This is exactly the desired throughput
3493 * distribution if (i-a) holds, or, if (i-b) holds instead, this is an
3494 * even more convenient distribution for (the process associated with)
3495 * bfqq.
3496 *
3497 * In contrast, in any asymmetric or unfavorable scenario, device
3498 * idling (I/O-dispatch plugging) is certainly needed to guarantee
3499 * that bfqq receives its assigned fraction of the device throughput
3500 * (see [1] for details).
3501 *
3502 * The problem is that idling may significantly reduce throughput with
3503 * certain combinations of types of I/O and devices. An important
3504 * example is sync random I/O on flash storage with command
3505 * queueing. So, unless bfqq falls in cases where idling also boosts
3506 * throughput, it is important to check conditions (i-a), i(-b) and
3507 * (ii) accurately, so as to avoid idling when not strictly needed for
3508 * service guarantees.
3509 *
3510 * Unfortunately, it is extremely difficult to thoroughly check
3511 * condition (ii). And, in case there are active groups, it becomes
3512 * very difficult to check conditions (i-a) and (i-b) too.  In fact,
3513 * if there are active groups, then, for conditions (i-a) or (i-b) to
3514 * become false 'indirectly', it is enough that an active group
3515 * contains more active processes or sub-groups than some other active
3516 * group. More precisely, for conditions (i-a) or (i-b) to become
3517 * false because of such a group, it is not even necessary that the
3518 * group is (still) active: it is sufficient that, even if the group
3519 * has become inactive, some of its descendant processes still have
3520 * some request already dispatched but still waiting for
3521 * completion. In fact, requests have still to be guaranteed their
3522 * share of the throughput even after being dispatched. In this
3523 * respect, it is easy to show that, if a group frequently becomes
3524 * inactive while still having in-flight requests, and if, when this
3525 * happens, the group is not considered in the calculation of whether
3526 * the scenario is asymmetric, then the group may fail to be
3527 * guaranteed its fair share of the throughput (basically because
3528 * idling may not be performed for the descendant processes of the
3529 * group, but it had to be).  We address this issue with the following
3530 * bi-modal behavior, implemented in the function
3531 * bfq_asymmetric_scenario().
3532 *
3533 * If there are groups with requests waiting for completion
3534 * (as commented above, some of these groups may even be
3535 * already inactive), then the scenario is tagged as
3536 * asymmetric, conservatively, without checking any of the
3537 * conditions (i-a), (i-b) or (ii). So the device is idled for bfqq.
3538 * This behavior matches also the fact that groups are created
3539 * exactly if controlling I/O is a primary concern (to
3540 * preserve bandwidth and latency guarantees).
3541 *
3542 * On the opposite end, if there are no groups with requests waiting
3543 * for completion, then only conditions (i-a) and (i-b) are actually
3544 * controlled, i.e., provided that conditions (i-a) or (i-b) holds,
3545 * idling is not performed, regardless of whether condition (ii)
3546 * holds.  In other words, only if conditions (i-a) and (i-b) do not
3547 * hold, then idling is allowed, and the device tends to be prevented
3548 * from queueing many requests, possibly of several processes. Since
3549 * there are no groups with requests waiting for completion, then, to
3550 * control conditions (i-a) and (i-b) it is enough to check just
3551 * whether all the queues with requests waiting for completion also
3552 * have the same weight.
3553 *
3554 * Not checking condition (ii) evidently exposes bfqq to the
3555 * risk of getting less throughput than its fair share.
3556 * However, for queues with the same weight, a further
3557 * mechanism, preemption, mitigates or even eliminates this
3558 * problem. And it does so without consequences on overall
3559 * throughput. This mechanism and its benefits are explained
3560 * in the next three paragraphs.
3561 *
3562 * Even if a queue, say Q, is expired when it remains idle, Q
3563 * can still preempt the new in-service queue if the next
3564 * request of Q arrives soon (see the comments on
3565 * bfq_bfqq_update_budg_for_activation). If all queues and
3566 * groups have the same weight, this form of preemption,
3567 * combined with the hole-recovery heuristic described in the
3568 * comments on function bfq_bfqq_update_budg_for_activation,
3569 * are enough to preserve a correct bandwidth distribution in
3570 * the mid term, even without idling. In fact, even if not
3571 * idling allows the internal queues of the device to contain
3572 * many requests, and thus to reorder requests, we can rather
3573 * safely assume that the internal scheduler still preserves a
3574 * minimum of mid-term fairness.
3575 *
3576 * More precisely, this preemption-based, idleless approach
3577 * provides fairness in terms of IOPS, and not sectors per
3578 * second. This can be seen with a simple example. Suppose
3579 * that there are two queues with the same weight, but that
3580 * the first queue receives requests of 8 sectors, while the
3581 * second queue receives requests of 1024 sectors. In
3582 * addition, suppose that each of the two queues contains at
3583 * most one request at a time, which implies that each queue
3584 * always remains idle after it is served. Finally, after
3585 * remaining idle, each queue receives very quickly a new
3586 * request. It follows that the two queues are served
3587 * alternatively, preempting each other if needed. This
3588 * implies that, although both queues have the same weight,
3589 * the queue with large requests receives a service that is
3590 * 1024/8 times as high as the service received by the other
3591 * queue.
3592 *
3593 * The motivation for using preemption instead of idling (for
3594 * queues with the same weight) is that, by not idling,
3595 * service guarantees are preserved (completely or at least in
3596 * part) without minimally sacrificing throughput. And, if
3597 * there is no active group, then the primary expectation for
3598 * this device is probably a high throughput.
3599 *
3600 * We are now left only with explaining the two sub-conditions in the
3601 * additional compound condition that is checked below for deciding
3602 * whether the scenario is asymmetric. To explain the first
3603 * sub-condition, we need to add that the function
3604 * bfq_asymmetric_scenario checks the weights of only
3605 * non-weight-raised queues, for efficiency reasons (see comments on
3606 * bfq_weights_tree_add()). Then the fact that bfqq is weight-raised
3607 * is checked explicitly here. More precisely, the compound condition
3608 * below takes into account also the fact that, even if bfqq is being
3609 * weight-raised, the scenario is still symmetric if all queues with
3610 * requests waiting for completion happen to be
3611 * weight-raised. Actually, we should be even more precise here, and
3612 * differentiate between interactive weight raising and soft real-time
3613 * weight raising.
3614 *
3615 * The second sub-condition checked in the compound condition is
3616 * whether there is a fair amount of already in-flight I/O not
3617 * belonging to bfqq. If so, I/O dispatching is to be plugged, for the
3618 * following reason. The drive may decide to serve in-flight
3619 * non-bfqq's I/O requests before bfqq's ones, thereby delaying the
3620 * arrival of new I/O requests for bfqq (recall that bfqq is sync). If
3621 * I/O-dispatching is not plugged, then, while bfqq remains empty, a
3622 * basically uncontrolled amount of I/O from other queues may be
3623 * dispatched too, possibly causing the service of bfqq's I/O to be
3624 * delayed even longer in the drive. This problem gets more and more
3625 * serious as the speed and the queue depth of the drive grow,
3626 * because, as these two quantities grow, the probability to find no
3627 * queue busy but many requests in flight grows too. By contrast,
3628 * plugging I/O dispatching minimizes the delay induced by already
3629 * in-flight I/O, and enables bfqq to recover the bandwidth it may
3630 * lose because of this delay.
3631 *
3632 * As a side note, it is worth considering that the above
3633 * device-idling countermeasures may however fail in the following
3634 * unlucky scenario: if I/O-dispatch plugging is (correctly) disabled
3635 * in a time period during which all symmetry sub-conditions hold, and
3636 * therefore the device is allowed to enqueue many requests, but at
3637 * some later point in time some sub-condition stops to hold, then it
3638 * may become impossible to make requests be served in the desired
3639 * order until all the requests already queued in the device have been
3640 * served. The last sub-condition commented above somewhat mitigates
3641 * this problem for weight-raised queues.
3642 *
3643 * However, as an additional mitigation for this problem, we preserve
3644 * plugging for a special symmetric case that may suddenly turn into
3645 * asymmetric: the case where only bfqq is busy. In this case, not
3646 * expiring bfqq does not cause any harm to any other queues in terms
3647 * of service guarantees. In contrast, it avoids the following unlucky
3648 * sequence of events: (1) bfqq is expired, (2) a new queue with a
3649 * lower weight than bfqq becomes busy (or more queues), (3) the new
3650 * queue is served until a new request arrives for bfqq, (4) when bfqq
3651 * is finally served, there are so many requests of the new queue in
3652 * the drive that the pending requests for bfqq take a lot of time to
3653 * be served. In particular, event (2) may case even already
3654 * dispatched requests of bfqq to be delayed, inside the drive. So, to
3655 * avoid this series of events, the scenario is preventively declared
3656 * as asymmetric also if bfqq is the only busy queues
3657 */
3658static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
3659                                                 struct bfq_queue *bfqq)
3660{
3661        int tot_busy_queues = bfq_tot_busy_queues(bfqd);
3662
3663        /* No point in idling for bfqq if it won't get requests any longer */
3664        if (unlikely(!bfqq_process_refs(bfqq)))
3665                return false;
3666
3667        return (bfqq->wr_coeff > 1 &&
3668                (bfqd->wr_busy_queues <
3669                 tot_busy_queues ||
3670                 bfqd->rq_in_driver >=
3671                 bfqq->dispatched + 4)) ||
3672                bfq_asymmetric_scenario(bfqd, bfqq) ||
3673                tot_busy_queues == 1;
3674}
3675
3676static bool __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq,
3677                              enum bfqq_expiration reason)
3678{
3679        /*
3680         * If this bfqq is shared between multiple processes, check
3681         * to make sure that those processes are still issuing I/Os
3682         * within the mean seek distance. If not, it may be time to
3683         * break the queues apart again.
3684         */
3685        if (bfq_bfqq_coop(bfqq) && BFQQ_SEEKY(bfqq))
3686                bfq_mark_bfqq_split_coop(bfqq);
3687
3688        /*
3689         * Consider queues with a higher finish virtual time than
3690         * bfqq. If idling_needed_for_service_guarantees(bfqq) returns
3691         * true, then bfqq's bandwidth would be violated if an
3692         * uncontrolled amount of I/O from these queues were
3693         * dispatched while bfqq is waiting for its new I/O to
3694         * arrive. This is exactly what may happen if this is a forced
3695         * expiration caused by a preemption attempt, and if bfqq is
3696         * not re-scheduled. To prevent this from happening, re-queue
3697         * bfqq if it needs I/O-dispatch plugging, even if it is
3698         * empty. By doing so, bfqq is granted to be served before the
3699         * above queues (provided that bfqq is of course eligible).
3700         */
3701        if (RB_EMPTY_ROOT(&bfqq->sort_list) &&
3702            !(reason == BFQQE_PREEMPTED &&
3703              idling_needed_for_service_guarantees(bfqd, bfqq))) {
3704                if (bfqq->dispatched == 0)
3705                        /*
3706                         * Overloading budget_timeout field to store
3707                         * the time at which the queue remains with no
3708                         * backlog and no outstanding request; used by
3709                         * the weight-raising mechanism.
3710                         */
3711                        bfqq->budget_timeout = jiffies;
3712
3713                bfq_del_bfqq_busy(bfqd, bfqq, true);
3714        } else {
3715                bfq_requeue_bfqq(bfqd, bfqq, true);
3716                /*
3717                 * Resort priority tree of potential close cooperators.
3718                 * See comments on bfq_pos_tree_add_move() for the unlikely().
3719                 */
3720                if (unlikely(!bfqd->nonrot_with_queueing &&
3721                             !RB_EMPTY_ROOT(&bfqq->sort_list)))
3722                        bfq_pos_tree_add_move(bfqd, bfqq);
3723        }
3724
3725        /*
3726         * All in-service entities must have been properly deactivated
3727         * or requeued before executing the next function, which
3728         * resets all in-service entities as no more in service. This
3729         * may cause bfqq to be freed. If this happens, the next
3730         * function returns true.
3731         */
3732        return __bfq_bfqd_reset_in_service(bfqd);
3733}
3734
3735/**
3736 * __bfq_bfqq_recalc_budget - try to adapt the budget to the @bfqq behavior.
3737 * @bfqd: device data.
3738 * @bfqq: queue to update.
3739 * @reason: reason for expiration.
3740 *
3741 * Handle the feedback on @bfqq budget at queue expiration.
3742 * See the body for detailed comments.
3743 */
3744static void __bfq_bfqq_recalc_budget(struct bfq_data *bfqd,
3745                                     struct bfq_queue *bfqq,
3746                                     enum bfqq_expiration reason)
3747{
3748        struct request *next_rq;
3749        int budget, min_budget;
3750
3751        min_budget = bfq_min_budget(bfqd);
3752
3753        if (bfqq->wr_coeff == 1)
3754                budget = bfqq->max_budget;
3755        else /*
3756              * Use a constant, low budget for weight-raised queues,
3757              * to help achieve a low latency. Keep it slightly higher
3758              * than the minimum possible budget, to cause a little
3759              * bit fewer expirations.
3760              */
3761                budget = 2 * min_budget;
3762
3763        bfq_log_bfqq(bfqd, bfqq, "recalc_budg: last budg %d, budg left %d",
3764                bfqq->entity.budget, bfq_bfqq_budget_left(bfqq));
3765        bfq_log_bfqq(bfqd, bfqq, "recalc_budg: last max_budg %d, min budg %d",
3766                budget, bfq_min_budget(bfqd));
3767        bfq_log_bfqq(bfqd, bfqq, "recalc_budg: sync %d, seeky %d",
3768                bfq_bfqq_sync(bfqq), BFQQ_SEEKY(bfqd->in_service_queue));
3769
3770        if (bfq_bfqq_sync(bfqq) && bfqq->wr_coeff == 1) {
3771                switch (reason) {
3772                /*
3773                 * Caveat: in all the following cases we trade latency
3774                 * for throughput.
3775                 */
3776                case BFQQE_TOO_IDLE:
3777                        /*
3778                         * This is the only case where we may reduce
3779                         * the budget: if there is no request of the
3780                         * process still waiting for completion, then
3781                         * we assume (tentatively) that the timer has
3782                         * expired because the batch of requests of
3783                         * the process could have been served with a
3784                         * smaller budget.  Hence, betting that
3785                         * process will behave in the same way when it
3786                         * becomes backlogged again, we reduce its
3787                         * next budget.  As long as we guess right,
3788                         * this budget cut reduces the latency
3789                         * experienced by the process.
3790                         *
3791                         * However, if there are still outstanding
3792                         * requests, then the process may have not yet
3793                         * issued its next request just because it is
3794                         * still waiting for the completion of some of
3795                         * the still outstanding ones.  So in this
3796                         * subcase we do not reduce its budget, on the
3797                         * contrary we increase it to possibly boost
3798                         * the throughput, as discussed in the
3799                         * comments to the BUDGET_TIMEOUT case.
3800                         */
3801                        if (bfqq->dispatched > 0) /* still outstanding reqs */
3802                                budget = min(budget * 2, bfqd->bfq_max_budget);
3803                        else {
3804                                if (budget > 5 * min_budget)
3805                                        budget -= 4 * min_budget;
3806                                else
3807                                        budget = min_budget;
3808                        }
3809                        break;
3810                case BFQQE_BUDGET_TIMEOUT:
3811                        /*
3812                         * We double the budget here because it gives
3813                         * the chance to boost the throughput if this
3814                         * is not a seeky process (and has bumped into
3815                         * this timeout because of, e.g., ZBR).
3816                         */
3817                        budget = min(budget * 2, bfqd->bfq_max_budget);
3818                        break;
3819                case BFQQE_BUDGET_EXHAUSTED:
3820                        /*
3821                         * The process still has backlog, and did not
3822                         * let either the budget timeout or the disk
3823                         * idling timeout expire. Hence it is not
3824                         * seeky, has a short thinktime and may be
3825                         * happy with a higher budget too. So
3826                         * definitely increase the budget of this good
3827                         * candidate to boost the disk throughput.
3828                         */
3829                        budget = min(budget * 4, bfqd->bfq_max_budget);
3830                        break;
3831                case BFQQE_NO_MORE_REQUESTS:
3832                        /*
3833                         * For queues that expire for this reason, it
3834                         * is particularly important to keep the
3835                         * budget close to the actual service they
3836                         * need. Doing so reduces the timestamp
3837                         * misalignment problem described in the
3838                         * comments in the body of
3839                         * __bfq_activate_entity. In fact, suppose
3840                         * that a queue systematically expires for
3841                         * BFQQE_NO_MORE_REQUESTS and presents a
3842                         * new request in time to enjoy timestamp
3843                         * back-shifting. The larger the budget of the
3844                         * queue is with respect to the service the
3845                         * queue actually requests in each service
3846                         * slot, the more times the queue can be
3847                         * reactivated with the same virtual finish
3848                         * time. It follows that, even if this finish
3849                         * time is pushed to the system virtual time
3850                         * to reduce the consequent timestamp
3851                         * misalignment, the queue unjustly enjoys for
3852                         * many re-activations a lower finish time
3853                         * than all newly activated queues.
3854                         *
3855                         * The service needed by bfqq is measured
3856                         * quite precisely by bfqq->entity.service.
3857                         * Since bfqq does not enjoy device idling,
3858                         * bfqq->entity.service is equal to the number
3859                         * of sectors that the process associated with
3860                         * bfqq requested to read/write before waiting
3861                         * for request completions, or blocking for
3862                         * other reasons.
3863                         */
3864                        budget = max_t(int, bfqq->entity.service, min_budget);
3865                        break;
3866                default:
3867                        return;
3868                }
3869        } else if (!bfq_bfqq_sync(bfqq)) {
3870                /*
3871                 * Async queues get always the maximum possible
3872                 * budget, as for them we do not care about latency
3873                 * (in addition, their ability to dispatch is limited
3874                 * by the charging factor).
3875                 */
3876                budget = bfqd->bfq_max_budget;
3877        }
3878
3879        bfqq->max_budget = budget;
3880
3881        if (bfqd->budgets_assigned >= bfq_stats_min_budgets &&
3882            !bfqd->bfq_user_max_budget)
3883                bfqq->max_budget = min(bfqq->max_budget, bfqd->bfq_max_budget);
3884
3885        /*
3886         * If there is still backlog, then assign a new budget, making
3887         * sure that it is large enough for the next request.  Since
3888         * the finish time of bfqq must be kept in sync with the
3889         * budget, be sure to call __bfq_bfqq_expire() *after* this
3890         * update.
3891         *
3892         * If there is no backlog, then no need to update the budget;
3893         * it will be updated on the arrival of a new request.
3894         */
3895        next_rq = bfqq->next_rq;
3896        if (next_rq)
3897                bfqq->entity.budget = max_t(unsigned long, bfqq->max_budget,
3898                                            bfq_serv_to_charge(next_rq, bfqq));
3899
3900        bfq_log_bfqq(bfqd, bfqq, "head sect: %u, new budget %d",
3901                        next_rq ? blk_rq_sectors(next_rq) : 0,
3902                        bfqq->entity.budget);
3903}
3904
3905/*
3906 * Return true if the process associated with bfqq is "slow". The slow
3907 * flag is used, in addition to the budget timeout, to reduce the
3908 * amount of service provided to seeky processes, and thus reduce
3909 * their chances to lower the throughput. More details in the comments
3910 * on the function bfq_bfqq_expire().
3911 *
3912 * An important observation is in order: as discussed in the comments
3913 * on the function bfq_update_peak_rate(), with devices with internal
3914 * queues, it is hard if ever possible to know when and for how long
3915 * an I/O request is processed by the device (apart from the trivial
3916 * I/O pattern where a new request is dispatched only after the
3917 * previous one has been completed). This makes it hard to evaluate
3918 * the real rate at which the I/O requests of each bfq_queue are
3919 * served.  In fact, for an I/O scheduler like BFQ, serving a
3920 * bfq_queue means just dispatching its requests during its service
3921 * slot (i.e., until the budget of the queue is exhausted, or the
3922 * queue remains idle, or, finally, a timeout fires). But, during the
3923 * service slot of a bfq_queue, around 100 ms at most, the device may
3924 * be even still processing requests of bfq_queues served in previous
3925 * service slots. On the opposite end, the requests of the in-service
3926 * bfq_queue may be completed after the service slot of the queue
3927 * finishes.
3928 *
3929 * Anyway, unless more sophisticated solutions are used
3930 * (where possible), the sum of the sizes of the requests dispatched
3931 * during the service slot of a bfq_queue is probably the only
3932 * approximation available for the service received by the bfq_queue
3933 * during its service slot. And this sum is the quantity used in this
3934 * function to evaluate the I/O speed of a process.
3935 */
3936static bool bfq_bfqq_is_slow(struct bfq_data *bfqd, struct bfq_queue *bfqq,
3937                                 bool compensate, enum bfqq_expiration reason,
3938                                 unsigned long *delta_ms)
3939{
3940        ktime_t delta_ktime;
3941        u32 delta_usecs;
3942        bool slow = BFQQ_SEEKY(bfqq); /* if delta too short, use seekyness */
3943
3944        if (!bfq_bfqq_sync(bfqq))
3945                return false;
3946
3947        if (compensate)
3948                delta_ktime = bfqd->last_idling_start;
3949        else
3950                delta_ktime = ktime_get();
3951        delta_ktime = ktime_sub(delta_ktime, bfqd->last_budget_start);
3952        delta_usecs = ktime_to_us(delta_ktime);
3953
3954        /* don't use too short time intervals */
3955        if (delta_usecs < 1000) {
3956                if (blk_queue_nonrot(bfqd->queue))
3957                         /*
3958                          * give same worst-case guarantees as idling
3959                          * for seeky
3960                          */
3961                        *delta_ms = BFQ_MIN_TT / NSEC_PER_MSEC;
3962                else /* charge at least one seek */
3963                        *delta_ms = bfq_slice_idle / NSEC_PER_MSEC;
3964
3965                return slow;
3966        }
3967
3968        *delta_ms = delta_usecs / USEC_PER_MSEC;
3969
3970        /*
3971         * Use only long (> 20ms) intervals to filter out excessive
3972         * spikes in service rate estimation.
3973         */
3974        if (delta_usecs > 20000) {
3975                /*
3976                 * Caveat for rotational devices: processes doing I/O
3977                 * in the slower disk zones tend to be slow(er) even
3978                 * if not seeky. In this respect, the estimated peak
3979                 * rate is likely to be an average over the disk
3980                 * surface. Accordingly, to not be too harsh with
3981                 * unlucky processes, a process is deemed slow only if
3982                 * its rate has been lower than half of the estimated
3983                 * peak rate.
3984                 */
3985                slow = bfqq->entity.service < bfqd->bfq_max_budget / 2;
3986        }
3987
3988        bfq_log_bfqq(bfqd, bfqq, "bfq_bfqq_is_slow: slow %d", slow);
3989
3990        return slow;
3991}
3992
3993/*
3994 * To be deemed as soft real-time, an application must meet two
3995 * requirements. First, the application must not require an average
3996 * bandwidth higher than the approximate bandwidth required to playback or
3997 * record a compressed high-definition video.
3998 * The next function is invoked on the completion of the last request of a
3999 * batch, to compute the next-start time instant, soft_rt_next_start, such
4000 * that, if the next request of the application does not arrive before
4001 * soft_rt_next_start, then the above requirement on the bandwidth is met.
4002 *
4003 * The second requirement is that the request pattern of the application is
4004 * isochronous, i.e., that, after issuing a request or a batch of requests,
4005 * the application stops issuing new requests until all its pending requests
4006 * have been completed. After that, the application may issue a new batch,
4007 * and so on.
4008 * For this reason the next function is invoked to compute
4009 * soft_rt_next_start only for applications that meet this requirement,
4010 * whereas soft_rt_next_start is set to infinity for applications that do
4011 * not.
4012 *
4013 * Unfortunately, even a greedy (i.e., I/O-bound) application may
4014 * happen to meet, occasionally or systematically, both the above
4015 * bandwidth and isochrony requirements. This may happen at least in
4016 * the following circumstances. First, if the CPU load is high. The
4017 * application may stop issuing requests while the CPUs are busy
4018 * serving other processes, then restart, then stop again for a while,
4019 * and so on. The other circumstances are related to the storage
4020 * device: the storage device is highly loaded or reaches a low-enough
4021 * throughput with the I/O of the application (e.g., because the I/O
4022 * is random and/or the device is slow). In all these cases, the
4023 * I/O of the application may be simply slowed down enough to meet
4024 * the bandwidth and isochrony requirements. To reduce the probability
4025 * that greedy applications are deemed as soft real-time in these
4026 * corner cases, a further rule is used in the computation of
4027 * soft_rt_next_start: the return value of this function is forced to
4028 * be higher than the maximum between the following two quantities.
4029 *
4030 * (a) Current time plus: (1) the maximum time for which the arrival
4031 *     of a request is waited for when a sync queue becomes idle,
4032 *     namely bfqd->bfq_slice_idle, and (2) a few extra jiffies. We
4033 *     postpone for a moment the reason for adding a few extra
4034 *     jiffies; we get back to it after next item (b).  Lower-bounding
4035 *     the return value of this function with the current time plus
4036 *     bfqd->bfq_slice_idle tends to filter out greedy applications,
4037 *     because the latter issue their next request as soon as possible
4038 *     after the last one has been completed. In contrast, a soft
4039 *     real-time application spends some time processing data, after a
4040 *     batch of its requests has been completed.
4041 *
4042 * (b) Current value of bfqq->soft_rt_next_start. As pointed out
4043 *     above, greedy applications may happen to meet both the
4044 *     bandwidth and isochrony requirements under heavy CPU or
4045 *     storage-device load. In more detail, in these scenarios, these
4046 *     applications happen, only for limited time periods, to do I/O
4047 *     slowly enough to meet all the requirements described so far,
4048 *     including the filtering in above item (a). These slow-speed
4049 *     time intervals are usually interspersed between other time
4050 *     intervals during which these applications do I/O at a very high
4051 *     speed. Fortunately, exactly because of the high speed of the
4052 *     I/O in the high-speed intervals, the values returned by this
4053 *     function happen to be so high, near the end of any such
4054 *     high-speed interval, to be likely to fall *after* the end of
4055 *     the low-speed time interval that follows. These high values are
4056 *     stored in bfqq->soft_rt_next_start after each invocation of
4057 *     this function. As a consequence, if the last value of
4058 *     bfqq->soft_rt_next_start is constantly used to lower-bound the
4059 *     next value that this function may return, then, from the very
4060 *     beginning of a low-speed interval, bfqq->soft_rt_next_start is
4061 *     likely to be constantly kept so high that any I/O request
4062 *     issued during the low-speed interval is considered as arriving
4063 *     to soon for the application to be deemed as soft
4064 *     real-time. Then, in the high-speed interval that follows, the
4065 *     application will not be deemed as soft real-time, just because
4066 *     it will do I/O at a high speed. And so on.
4067 *
4068 * Getting back to the filtering in item (a), in the following two
4069 * cases this filtering might be easily passed by a greedy
4070 * application, if the reference quantity was just
4071 * bfqd->bfq_slice_idle:
4072 * 1) HZ is so low that the duration of a jiffy is comparable to or
4073 *    higher than bfqd->bfq_slice_idle. This happens, e.g., on slow
4074 *    devices with HZ=100. The time granularity may be so coarse
4075 *    that the approximation, in jiffies, of bfqd->bfq_slice_idle
4076 *    is rather lower than the exact value.
4077 * 2) jiffies, instead of increasing at a constant rate, may stop increasing
4078 *    for a while, then suddenly 'jump' by several units to recover the lost
4079 *    increments. This seems to happen, e.g., inside virtual machines.
4080 * To address this issue, in the filtering in (a) we do not use as a
4081 * reference time interval just bfqd->bfq_slice_idle, but
4082 * bfqd->bfq_slice_idle plus a few jiffies. In particular, we add the
4083 * minimum number of jiffies for which the filter seems to be quite
4084 * precise also in embedded systems and KVM/QEMU virtual machines.
4085 */
4086static unsigned long bfq_bfqq_softrt_next_start(struct bfq_data *bfqd,
4087                                                struct bfq_queue *bfqq)
4088{
4089        return max3(bfqq->soft_rt_next_start,
4090                    bfqq->last_idle_bklogged +
4091                    HZ * bfqq->service_from_backlogged /
4092                    bfqd->bfq_wr_max_softrt_rate,
4093                    jiffies + nsecs_to_jiffies(bfqq->bfqd->bfq_slice_idle) + 4);
4094}
4095
4096/**
4097 * bfq_bfqq_expire - expire a queue.
4098 * @bfqd: device owning the queue.
4099 * @bfqq: the queue to expire.
4100 * @compensate: if true, compensate for the time spent idling.
4101 * @reason: the reason causing the expiration.
4102 *
4103 * If the process associated with bfqq does slow I/O (e.g., because it
4104 * issues random requests), we charge bfqq with the time it has been
4105 * in service instead of the service it has received (see
4106 * bfq_bfqq_charge_time for details on how this goal is achieved). As
4107 * a consequence, bfqq will typically get higher timestamps upon
4108 * reactivation, and hence it will be rescheduled as if it had
4109 * received more service than what it has actually received. In the
4110 * end, bfqq receives less service in proportion to how slowly its
4111 * associated process consumes its budgets (and hence how seriously it
4112 * tends to lower the throughput). In addition, this time-charging
4113 * strategy guarantees time fairness among slow processes. In
4114 * contrast, if the process associated with bfqq is not slow, we
4115 * charge bfqq exactly with the service it has received.
4116 *
4117 * Charging time to the first type of queues and the exact service to
4118 * the other has the effect of using the WF2Q+ policy to schedule the
4119 * former on a timeslice basis, without violating service domain
4120 * guarantees among the latter.
4121 */
4122void bfq_bfqq_expire(struct bfq_data *bfqd,
4123                     struct bfq_queue *bfqq,
4124                     bool compensate,
4125                     enum bfqq_expiration reason)
4126{
4127        bool slow;
4128        unsigned long delta = 0;
4129        struct bfq_entity *entity = &bfqq->entity;
4130
4131        /*
4132         * Check whether the process is slow (see bfq_bfqq_is_slow).
4133         */
4134        slow = bfq_bfqq_is_slow(bfqd, bfqq, compensate, reason, &delta);
4135
4136        /*
4137         * As above explained, charge slow (typically seeky) and
4138         * timed-out queues with the time and not the service
4139         * received, to favor sequential workloads.
4140         *
4141         * Processes doing I/O in the slower disk zones will tend to
4142         * be slow(er) even if not seeky. Therefore, since the
4143         * estimated peak rate is actually an average over the disk
4144         * surface, these processes may timeout just for bad luck. To
4145         * avoid punishing them, do not charge time to processes that
4146         * succeeded in consuming at least 2/3 of their budget. This
4147         * allows BFQ to preserve enough elasticity to still perform
4148         * bandwidth, and not time, distribution with little unlucky
4149         * or quasi-sequential processes.
4150         */
4151        if (bfqq->wr_coeff == 1 &&
4152            (slow ||
4153             (reason == BFQQE_BUDGET_TIMEOUT &&
4154              bfq_bfqq_budget_left(bfqq) >=  entity->budget / 3)))
4155                bfq_bfqq_charge_time(bfqd, bfqq, delta);
4156
4157        if (bfqd->low_latency && bfqq->wr_coeff == 1)
4158                bfqq->last_wr_start_finish = jiffies;
4159
4160        if (bfqd->low_latency && bfqd->bfq_wr_max_softrt_rate > 0 &&
4161            RB_EMPTY_ROOT(&bfqq->sort_list)) {
4162                /*
4163                 * If we get here, and there are no outstanding
4164                 * requests, then the request pattern is isochronous
4165                 * (see the comments on the function
4166                 * bfq_bfqq_softrt_next_start()). Therefore we can
4167                 * compute soft_rt_next_start.
4168                 *
4169                 * If, instead, the queue still has outstanding
4170                 * requests, then we have to wait for the completion
4171                 * of all the outstanding requests to discover whether
4172                 * the request pattern is actually isochronous.
4173                 */
4174                if (bfqq->dispatched == 0)
4175                        bfqq->soft_rt_next_start =
4176                                bfq_bfqq_softrt_next_start(bfqd, bfqq);
4177                else if (bfqq->dispatched > 0) {
4178                        /*
4179                         * Schedule an update of soft_rt_next_start to when
4180                         * the task may be discovered to be isochronous.
4181                         */
4182                        bfq_mark_bfqq_softrt_update(bfqq);
4183                }
4184        }
4185
4186        bfq_log_bfqq(bfqd, bfqq,
4187                "expire (%d, slow %d, num_disp %d, short_ttime %d)", reason,
4188                slow, bfqq->dispatched, bfq_bfqq_has_short_ttime(bfqq));
4189
4190        /*
4191         * bfqq expired, so no total service time needs to be computed
4192         * any longer: reset state machine for measuring total service
4193         * times.
4194         */
4195        bfqd->rqs_injected = bfqd->wait_dispatch = false;
4196        bfqd->waited_rq = NULL;
4197
4198        /*
4199         * Increase, decrease or leave budget unchanged according to
4200         * reason.
4201         */
4202        __bfq_bfqq_recalc_budget(bfqd, bfqq, reason);
4203        if (__bfq_bfqq_expire(bfqd, bfqq, reason))
4204                /* bfqq is gone, no more actions on it */
4205                return;
4206
4207        /* mark bfqq as waiting a request only if a bic still points to it */
4208        if (!bfq_bfqq_busy(bfqq) &&
4209            reason != BFQQE_BUDGET_TIMEOUT &&
4210            reason != BFQQE_BUDGET_EXHAUSTED) {
4211                bfq_mark_bfqq_non_blocking_wait_rq(bfqq);
4212                /*
4213                 * Not setting service to 0, because, if the next rq
4214                 * arrives in time, the queue will go on receiving
4215                 * service with this same budget (as if it never expired)
4216                 */
4217        } else
4218                entity->service = 0;
4219
4220        /*
4221         * Reset the received-service counter for every parent entity.
4222         * Differently from what happens with bfqq->entity.service,
4223         * the resetting of this counter never needs to be postponed
4224         * for parent entities. In fact, in case bfqq may have a
4225         * chance to go on being served using the last, partially
4226         * consumed budget, bfqq->entity.service needs to be kept,
4227         * because if bfqq then actually goes on being served using
4228         * the same budget, the last value of bfqq->entity.service is
4229         * needed to properly decrement bfqq->entity.budget by the
4230         * portion already consumed. In contrast, it is not necessary
4231         * to keep entity->service for parent entities too, because
4232         * the bubble up of the new value of bfqq->entity.budget will
4233         * make sure that the budgets of parent entities are correct,
4234         * even in case bfqq and thus parent entities go on receiving
4235         * service with the same budget.
4236         */
4237        entity = entity->parent;
4238        for_each_entity(entity)
4239                entity->service = 0;
4240}
4241
4242/*
4243 * Budget timeout is not implemented through a dedicated timer, but
4244 * just checked on request arrivals and completions, as well as on
4245 * idle timer expirations.
4246 */
4247static bool bfq_bfqq_budget_timeout(struct bfq_queue *bfqq)
4248{
4249        return time_is_before_eq_jiffies(bfqq->budget_timeout);
4250}
4251
4252/*
4253 * If we expire a queue that is actively waiting (i.e., with the
4254 * device idled) for the arrival of a new request, then we may incur
4255 * the timestamp misalignment problem described in the body of the
4256 * function __bfq_activate_entity. Hence we return true only if this
4257 * condition does not hold, or if the queue is slow enough to deserve
4258 * only to be kicked off for preserving a high throughput.
4259 */
4260static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq)
4261{
4262        bfq_log_bfqq(bfqq->bfqd, bfqq,
4263                "may_budget_timeout: wait_request %d left %d timeout %d",
4264                bfq_bfqq_wait_request(bfqq),
4265                        bfq_bfqq_budget_left(bfqq) >=  bfqq->entity.budget / 3,
4266                bfq_bfqq_budget_timeout(bfqq));
4267
4268        return (!bfq_bfqq_wait_request(bfqq) ||
4269                bfq_bfqq_budget_left(bfqq) >=  bfqq->entity.budget / 3)
4270                &&
4271                bfq_bfqq_budget_timeout(bfqq);
4272}
4273
4274static bool idling_boosts_thr_without_issues(struct bfq_data *bfqd,
4275                                             struct bfq_queue *bfqq)
4276{
4277        bool rot_without_queueing =
4278                !blk_queue_nonrot(bfqd->queue) && !bfqd->hw_tag,
4279                bfqq_sequential_and_IO_bound,
4280                idling_boosts_thr;
4281
4282        /* No point in idling for bfqq if it won't get requests any longer */
4283        if (unlikely(!bfqq_process_refs(bfqq)))
4284                return false;
4285
4286        bfqq_sequential_and_IO_bound = !BFQQ_SEEKY(bfqq) &&
4287                bfq_bfqq_IO_bound(bfqq) && bfq_bfqq_has_short_ttime(bfqq);
4288
4289        /*
4290         * The next variable takes into account the cases where idling
4291         * boosts the throughput.
4292         *
4293         * The value of the variable is computed considering, first, that
4294         * idling is virtually always beneficial for the throughput if:
4295         * (a) the device is not NCQ-capable and rotational, or
4296         * (b) regardless of the presence of NCQ, the device is rotational and
4297         *     the request pattern for bfqq is I/O-bound and sequential, or
4298         * (c) regardless of whether it is rotational, the device is
4299         *     not NCQ-capable and the request pattern for bfqq is
4300         *     I/O-bound and sequential.
4301         *
4302         * Secondly, and in contrast to the above item (b), idling an
4303         * NCQ-capable flash-based device would not boost the
4304         * throughput even with sequential I/O; rather it would lower
4305         * the throughput in proportion to how fast the device
4306         * is. Accordingly, the next variable is true if any of the
4307         * above conditions (a), (b) or (c) is true, and, in
4308         * particular, happens to be false if bfqd is an NCQ-capable
4309         * flash-based device.
4310         */
4311        idling_boosts_thr = rot_without_queueing ||
4312                ((!blk_queue_nonrot(bfqd->queue) || !bfqd->hw_tag) &&
4313                 bfqq_sequential_and_IO_bound);
4314
4315        /*
4316         * The return value of this function is equal to that of
4317         * idling_boosts_thr, unless a special case holds. In this
4318         * special case, described below, idling may cause problems to
4319         * weight-raised queues.
4320         *
4321         * When the request pool is saturated (e.g., in the presence
4322         * of write hogs), if the processes associated with
4323         * non-weight-raised queues ask for requests at a lower rate,
4324         * then processes associated with weight-raised queues have a
4325         * higher probability to get a request from the pool
4326         * immediately (or at least soon) when they need one. Thus
4327         * they have a higher probability to actually get a fraction
4328         * of the device throughput proportional to their high
4329         * weight. This is especially true with NCQ-capable drives,
4330         * which enqueue several requests in advance, and further
4331         * reorder internally-queued requests.
4332         *
4333         * For this reason, we force to false the return value if
4334         * there are weight-raised busy queues. In this case, and if
4335         * bfqq is not weight-raised, this guarantees that the device
4336         * is not idled for bfqq (if, instead, bfqq is weight-raised,
4337         * then idling will be guaranteed by another variable, see
4338         * below). Combined with the timestamping rules of BFQ (see
4339         * [1] for details), this behavior causes bfqq, and hence any
4340         * sync non-weight-raised queue, to get a lower number of
4341         * requests served, and thus to ask for a lower number of
4342         * requests from the request pool, before the busy
4343         * weight-raised queues get served again. This often mitigates
4344         * starvation problems in the presence of heavy write
4345         * workloads and NCQ, thereby guaranteeing a higher
4346         * application and system responsiveness in these hostile
4347         * scenarios.
4348         */
4349        return idling_boosts_thr &&
4350                bfqd->wr_busy_queues == 0;
4351}
4352
4353/*
4354 * For a queue that becomes empty, device idling is allowed only if
4355 * this function returns true for that queue. As a consequence, since
4356 * device idling plays a critical role for both throughput boosting
4357 * and service guarantees, the return value of this function plays a
4358 * critical role as well.
4359 *
4360 * In a nutshell, this function returns true only if idling is
4361 * beneficial for throughput or, even if detrimental for throughput,
4362 * idling is however necessary to preserve service guarantees (low
4363 * latency, desired throughput distribution, ...). In particular, on
4364 * NCQ-capable devices, this function tries to return false, so as to
4365 * help keep the drives' internal queues full, whenever this helps the
4366 * device boost the throughput without causing any service-guarantee
4367 * issue.
4368 *
4369 * Most of the issues taken into account to get the return value of
4370 * this function are not trivial. We discuss these issues in the two
4371 * functions providing the main pieces of information needed by this
4372 * function.
4373 */
4374static bool bfq_better_to_idle(struct bfq_queue *bfqq)
4375{
4376        struct bfq_data *bfqd = bfqq->bfqd;
4377        bool idling_boosts_thr_with_no_issue, idling_needed_for_service_guar;
4378
4379        /* No point in idling for bfqq if it won't get requests any longer */
4380        if (unlikely(!bfqq_process_refs(bfqq)))
4381                return false;
4382
4383        if (unlikely(bfqd->strict_guarantees))
4384                return true;
4385
4386        /*
4387         * Idling is performed only if slice_idle > 0. In addition, we
4388         * do not idle if
4389         * (a) bfqq is async
4390         * (b) bfqq is in the idle io prio class: in this case we do
4391         * not idle because we want to minimize the bandwidth that
4392         * queues in this class can steal to higher-priority queues
4393         */
4394        if (bfqd->bfq_slice_idle == 0 || !bfq_bfqq_sync(bfqq) ||
4395           bfq_class_idle(bfqq))
4396                return false;
4397
4398        idling_boosts_thr_with_no_issue =
4399                idling_boosts_thr_without_issues(bfqd, bfqq);
4400
4401        idling_needed_for_service_guar =
4402                idling_needed_for_service_guarantees(bfqd, bfqq);
4403
4404        /*
4405         * We have now the two components we need to compute the
4406         * return value of the function, which is true only if idling
4407         * either boosts the throughput (without issues), or is
4408         * necessary to preserve service guarantees.
4409         */
4410        return idling_boosts_thr_with_no_issue ||
4411                idling_needed_for_service_guar;
4412}
4413
4414/*
4415 * If the in-service queue is empty but the function bfq_better_to_idle
4416 * returns true, then:
4417 * 1) the queue must remain in service and cannot be expired, and
4418 * 2) the device must be idled to wait for the possible arrival of a new
4419 *    request for the queue.
4420 * See the comments on the function bfq_better_to_idle for the reasons
4421 * why performing device idling is the best choice to boost the throughput
4422 * and preserve service guarantees when bfq_better_to_idle itself
4423 * returns true.
4424 */
4425static bool bfq_bfqq_must_idle(struct bfq_queue *bfqq)
4426{
4427        return RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_better_to_idle(bfqq);
4428}
4429
4430/*
4431 * This function chooses the queue from which to pick the next extra
4432 * I/O request to inject, if it finds a compatible queue. See the
4433 * comments on bfq_update_inject_limit() for details on the injection
4434 * mechanism, and for the definitions of the quantities mentioned
4435 * below.
4436 */
4437static struct bfq_queue *
4438bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
4439{
4440        struct bfq_queue *bfqq, *in_serv_bfqq = bfqd->in_service_queue;
4441        unsigned int limit = in_serv_bfqq->inject_limit;
4442        /*
4443         * If
4444         * - bfqq is not weight-raised and therefore does not carry
4445         *   time-critical I/O,
4446         * or
4447         * - regardless of whether bfqq is weight-raised, bfqq has
4448         *   however a long think time, during which it can absorb the
4449         *   effect of an appropriate number of extra I/O requests
4450         *   from other queues (see bfq_update_inject_limit for
4451         *   details on the computation of this number);
4452         * then injection can be performed without restrictions.
4453         */
4454        bool in_serv_always_inject = in_serv_bfqq->wr_coeff == 1 ||
4455                !bfq_bfqq_has_short_ttime(in_serv_bfqq);
4456
4457        /*
4458         * If
4459         * - the baseline total service time could not be sampled yet,
4460         *   so the inject limit happens to be still 0, and
4461         * - a lot of time has elapsed since the plugging of I/O
4462         *   dispatching started, so drive speed is being wasted
4463         *   significantly;
4464         * then temporarily raise inject limit to one request.
4465         */
4466        if (limit == 0 && in_serv_bfqq->last_serv_time_ns == 0 &&
4467            bfq_bfqq_wait_request(in_serv_bfqq) &&
4468            time_is_before_eq_jiffies(bfqd->last_idling_start_jiffies +
4469                                      bfqd->bfq_slice_idle)
4470                )
4471                limit = 1;
4472
4473        if (bfqd->rq_in_driver >= limit)
4474                return NULL;
4475
4476        /*
4477         * Linear search of the source queue for injection; but, with
4478         * a high probability, very few steps are needed to find a
4479         * candidate queue, i.e., a queue with enough budget left for
4480         * its next request. In fact:
4481         * - BFQ dynamically updates the budget of every queue so as
4482         *   to accommodate the expected backlog of the queue;
4483         * - if a queue gets all its requests dispatched as injected
4484         *   service, then the queue is removed from the active list
4485         *   (and re-added only if it gets new requests, but then it
4486         *   is assigned again enough budget for its new backlog).
4487         */
4488        list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list)
4489                if (!RB_EMPTY_ROOT(&bfqq->sort_list) &&
4490                    (in_serv_always_inject || bfqq->wr_coeff > 1) &&
4491                    bfq_serv_to_charge(bfqq->next_rq, bfqq) <=
4492                    bfq_bfqq_budget_left(bfqq)) {
4493                        /*
4494                         * Allow for only one large in-flight request
4495                         * on non-rotational devices, for the
4496                         * following reason. On non-rotationl drives,
4497                         * large requests take much longer than
4498                         * smaller requests to be served. In addition,
4499                         * the drive prefers to serve large requests
4500                         * w.r.t. to small ones, if it can choose. So,
4501                         * having more than one large requests queued
4502                         * in the drive may easily make the next first
4503                         * request of the in-service queue wait for so
4504                         * long to break bfqq's service guarantees. On
4505                         * the bright side, large requests let the
4506                         * drive reach a very high throughput, even if
4507                         * there is only one in-flight large request
4508                         * at a time.
4509                         */
4510                        if (blk_queue_nonrot(bfqd->queue) &&
4511                            blk_rq_sectors(bfqq->next_rq) >=
4512                            BFQQ_SECT_THR_NONROT)
4513                                limit = min_t(unsigned int, 1, limit);
4514                        else
4515                                limit = in_serv_bfqq->inject_limit;
4516
4517                        if (bfqd->rq_in_driver < limit) {
4518                                bfqd->rqs_injected = true;
4519                                return bfqq;
4520                        }
4521                }
4522
4523        return NULL;
4524}
4525
4526/*
4527 * Select a queue for service.  If we have a current queue in service,
4528 * check whether to continue servicing it, or retrieve and set a new one.
4529 */
4530static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
4531{
4532        struct bfq_queue *bfqq;
4533        struct request *next_rq;
4534        enum bfqq_expiration reason = BFQQE_BUDGET_TIMEOUT;
4535
4536        bfqq = bfqd->in_service_queue;
4537        if (!bfqq)
4538                goto new_queue;
4539
4540        bfq_log_bfqq(bfqd, bfqq, "select_queue: already in-service queue");
4541
4542        /*
4543         * Do not expire bfqq for budget timeout if bfqq may be about
4544         * to enjoy device idling. The reason why, in this case, we
4545         * prevent bfqq from expiring is the same as in the comments
4546         * on the case where bfq_bfqq_must_idle() returns true, in
4547         * bfq_completed_request().
4548         */
4549        if (bfq_may_expire_for_budg_timeout(bfqq) &&
4550            !bfq_bfqq_must_idle(bfqq))
4551                goto expire;
4552
4553check_queue:
4554        /*
4555         * This loop is rarely executed more than once. Even when it
4556         * happens, it is much more convenient to re-execute this loop
4557         * than to return NULL and trigger a new dispatch to get a
4558         * request served.
4559         */
4560        next_rq = bfqq->next_rq;
4561        /*
4562         * If bfqq has requests queued and it has enough budget left to
4563         * serve them, keep the queue, otherwise expire it.
4564         */
4565        if (next_rq) {
4566                if (bfq_serv_to_charge(next_rq, bfqq) >
4567                        bfq_bfqq_budget_left(bfqq)) {
4568                        /*
4569                         * Expire the queue for budget exhaustion,
4570                         * which makes sure that the next budget is
4571                         * enough to serve the next request, even if
4572                         * it comes from the fifo expired path.
4573                         */
4574                        reason = BFQQE_BUDGET_EXHAUSTED;
4575                        goto expire;
4576                } else {
4577                        /*
4578                         * The idle timer may be pending because we may
4579                         * not disable disk idling even when a new request
4580                         * arrives.
4581                         */
4582                        if (bfq_bfqq_wait_request(bfqq)) {
4583                                /*
4584                                 * If we get here: 1) at least a new request
4585                                 * has arrived but we have not disabled the
4586                                 * timer because the request was too small,
4587                                 * 2) then the block layer has unplugged
4588                                 * the device, causing the dispatch to be
4589                                 * invoked.
4590                                 *
4591                                 * Since the device is unplugged, now the
4592                                 * requests are probably large enough to
4593                                 * provide a reasonable throughput.
4594                                 * So we disable idling.
4595                                 */
4596                                bfq_clear_bfqq_wait_request(bfqq);
4597                                hrtimer_try_to_cancel(