// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Budget Fair Queueing (BFQ) I/O scheduler.
 *
 * Based on ideas and code from CFQ:
 * Copyright (C) 2003 Jens Axboe <axboe@kernel.dk>
 *
 * Copyright (C) 2008 Fabio Checconi <fabio@gandalf.sssup.it>
 *                    Paolo Valente <paolo.valente@unimore.it>
 *
 * Copyright (C) 2010 Paolo Valente <paolo.valente@unimore.it>
 *                    Arianna Avanzini <avanzini@google.com>
 *
 * Copyright (C) 2017 Paolo Valente <paolo.valente@linaro.org>
 *
 * BFQ is a proportional-share I/O scheduler, with some extra
 * low-latency capabilities. BFQ also supports full hierarchical
 * scheduling through cgroups. Next paragraphs provide an introduction
 * on BFQ inner workings. Details on BFQ benefits, usage and
 * limitations can be found in Documentation/block/bfq-iosched.rst.
 *
 * BFQ is a proportional-share storage-I/O scheduling algorithm based
 * on the slice-by-slice service scheme of CFQ. But BFQ assigns
 * budgets, measured in number of sectors, to processes instead of
 * time slices. The device is not granted to the in-service process
 * for a given time slice, but until it has exhausted its assigned
 * budget. This change from the time to the service domain enables BFQ
 * to distribute the device throughput among processes as desired,
 * without any distortion due to throughput fluctuations, or to device
 * internal queueing. BFQ uses an ad hoc internal scheduler, called
 * B-WF2Q+, to schedule processes according to their budgets. More
 * precisely, BFQ schedules queues associated with processes. Each
 * process/queue is assigned a user-configurable weight, and B-WF2Q+
 * guarantees that each queue receives a fraction of the throughput
 * proportional to its weight. Thanks to the accurate policy of
 * B-WF2Q+, BFQ can afford to assign high budgets to I/O-bound
 * processes issuing sequential requests (to boost the throughput),
 * and yet guarantee a low latency to interactive and soft real-time
 * applications.
 *
 * In particular, to provide these low-latency guarantees, BFQ
 * explicitly privileges the I/O of two classes of time-sensitive
 * applications: interactive and soft real-time. In more detail, BFQ
 * behaves this way if the low_latency parameter is set (default
 * configuration). This feature enables BFQ to provide applications in
 * these classes with a very low latency.
 *
 * To implement this feature, BFQ constantly tries to detect whether
 * the I/O requests in a bfq_queue come from an interactive or a soft
 * real-time application. For brevity, in these cases, the queue is
 * said to be interactive or soft real-time. In both cases, BFQ
 * privileges the service of the queue, over that of non-interactive
 * and non-soft-real-time queues. This privileging is performed,
 * mainly, by raising the weight of the queue. So, for brevity, we
 * call just weight-raising periods the time periods during which a
 * queue is privileged, because deemed interactive or soft real-time.
 *
 * The detection of soft real-time queues/applications is described in
 * detail in the comments on the function
 * bfq_bfqq_softrt_next_start. On the other hand, the detection of an
 * interactive queue works as follows: a queue is deemed interactive
 * if it is constantly non empty only for a limited time interval,
 * after which it does become empty. The queue may be deemed
 * interactive again (for a limited time), if it restarts being
 * constantly non empty, provided that this happens only after the
 * queue has remained empty for a given minimum idle time.
 *
 * By default, BFQ computes automatically the above maximum time
 * interval, i.e., the time interval after which a constantly
 * non-empty queue stops being deemed interactive. Since a queue is
 * weight-raised while it is deemed interactive, this maximum time
 * interval happens to coincide with the (maximum) duration of the
 * weight-raising for interactive queues.
 *
 * Finally, BFQ also features additional heuristics for
 * preserving both a low latency and a high throughput on NCQ-capable,
 * rotational or flash-based devices, and to get the job done quickly
 * for applications consisting in many I/O-bound processes.
 *
 * NOTE: if the main or only goal, with a given device, is to achieve
 * the maximum-possible throughput at all times, then do switch off
 * all low-latency heuristics for that device, by setting low_latency
 * to 0.
 *
 * BFQ is described in [1], where also a reference to the initial,
 * more theoretical paper on BFQ can be found. The interested reader
 * can find in the latter paper full details on the main algorithm, as
 * well as formulas of the guarantees and formal proofs of all the
 * properties.  With respect to the version of BFQ presented in these
 * papers, this implementation adds a few more heuristics, such as the
 * ones that guarantee a low latency to interactive and soft real-time
 * applications, and a hierarchical extension based on H-WF2Q+.
 *
 * B-WF2Q+ is based on WF2Q+, which is described in [2], together with
 * H-WF2Q+, while the augmented tree used here to implement B-WF2Q+
 * with O(log N) complexity derives from the one introduced with EEVDF
 * in [3].
 *
 * [1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
 *     Scheduler", Proceedings of the First Workshop on Mobile System
 *     Technologies (MST-2015), May 2015.
 *     http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf
 *
 * [2] Jon C.R. Bennett and H. Zhang, "Hierarchical Packet Fair Queueing
 *     Algorithms", IEEE/ACM Transactions on Networking, 5(5):675-689,
 *     Oct 1997.
 *
 * http://www.cs.cmu.edu/~hzhang/papers/TON-97-Oct.ps.gz
 *
 * [3] I. Stoica and H. Abdel-Wahab, "Earliest Eligible Virtual Deadline
 *     First: A Flexible and Accurate Mechanism for Proportional Share
 *     Resource Allocation", technical report.
 *
 * http://www.cs.berkeley.edu/~istoica/papers/eevdf-tr-95.pdf
 */
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/cgroup.h>
#include <linux/ktime.h>
#include <linux/rbtree.h>
#include <linux/ioprio.h>
#include <linux/sbitmap.h>
#include <linux/delay.h>
#include <linux/backing-dev.h>

#include <trace/events/block.h>

#include "elevator.h"
#include "blk.h"
#include "blk-mq.h"
#include "blk-mq-sched.h"
#include "bfq-iosched.h"
#include "blk-wbt.h"

#define BFQ_BFQQ_FNS(name)                                              \
void bfq_mark_bfqq_##name(struct bfq_queue *bfqq)                       \
{                                                                       \
        __set_bit(BFQQF_##name, &(bfqq)->flags);                        \
}                                                                       \
void bfq_clear_bfqq_##name(struct bfq_queue *bfqq)                      \
{                                                                       \
        __clear_bit(BFQQF_##name, &(bfqq)->flags);              \
}                                                                       \
int bfq_bfqq_##name(const struct bfq_queue *bfqq)                       \
{                                                                       \
        return test_bit(BFQQF_##name, &(bfqq)->flags);          \
}

BFQ_BFQQ_FNS(just_created);
BFQ_BFQQ_FNS(busy);
BFQ_BFQQ_FNS(wait_request);
BFQ_BFQQ_FNS(non_blocking_wait_rq);
BFQ_BFQQ_FNS(fifo_expire);
BFQ_BFQQ_FNS(has_short_ttime);
BFQ_BFQQ_FNS(sync);
BFQ_BFQQ_FNS(IO_bound);
BFQ_BFQQ_FNS(in_large_burst);
BFQ_BFQQ_FNS(coop);
BFQ_BFQQ_FNS(split_coop);
BFQ_BFQQ_FNS(softrt_update);
#undef BFQ_BFQQ_FNS                                             \

/* Expiration time of async (0) and sync (1) requests, in ns. */
static const u64 bfq_fifo_expire[2] = { NSEC_PER_SEC / 4, NSEC_PER_SEC / 8 };

/* Maximum backwards seek (magic number lifted from CFQ), in KiB. */
static const int bfq_back_max = 16 * 1024;

/* Penalty of a backwards seek, in number of sectors. */
static const int bfq_back_penalty = 2;

/* Idling period duration, in ns. */
static u64 bfq_slice_idle = NSEC_PER_SEC / 125;

/* Minimum number of assigned budgets for which stats are safe to compute. */
static const int bfq_stats_min_budgets = 194;

/* Default maximum budget values, in sectors and number of requests. */
static const int bfq_default_max_budget = 16 * 1024;

/*
 * When a sync request is dispatched, the queue that contains that
 * request, and all the ancestor entities of that queue, are charged
 * with the number of sectors of the request. In contrast, if the
 * request is async, then the queue and its ancestor entities are
 * charged with the number of sectors of the request, multiplied by
 * the factor below. This throttles the bandwidth for async I/O,
 * w.r.t. to sync I/O, and it is done to counter the tendency of async
 * writes to steal I/O throughput to reads.
 *
 * The current value of this parameter is the result of a tuning with
 * several hardware and software configurations. We tried to find the
 * lowest value for which writes do not cause noticeable problems to
 * reads. In fact, the lower this parameter, the stabler I/O control,
 * in the following respect.  The lower this parameter is, the less
 * the bandwidth enjoyed by a group decreases
 * - when the group does writes, w.r.t. to when it does reads;
 * - when other groups do reads, w.r.t. to when they do writes.
 */
static const int bfq_async_charge_factor = 3;

/* Default timeout values, in jiffies, approximating CFQ defaults. */
const int bfq_timeout = HZ / 8;

/*
 * Time limit for merging (see comments in bfq_setup_cooperator). Set
 * to the slowest value that, in our tests, proved to be effective in
 * removing false positives, while not causing true positives to miss
 * queue merging.
 *
 * As can be deduced from the low time limit below, queue merging, if
 * successful, happens at the very beginning of the I/O of the involved
 * cooperating processes, as a consequence of the arrival of the very
 * first requests from each cooperator.  After that, there is very
 * little chance to find cooperators.
 */
static const unsigned long bfq_merge_time_limit = HZ/10;

static struct kmem_cache *bfq_pool;

/* Below this threshold (in ns), we consider thinktime immediate. */
#define BFQ_MIN_TT              (2 * NSEC_PER_MSEC)

/* hw_tag detection: parallel requests threshold and min samples needed. */
#define BFQ_HW_QUEUE_THRESHOLD  3
#define BFQ_HW_QUEUE_SAMPLES    32

#define BFQQ_SEEK_THR           (sector_t)(8 * 100)
#define BFQQ_SECT_THR_NONROT    (sector_t)(2 * 32)
#define BFQ_RQ_SEEKY(bfqd, last_pos, rq) \
        (get_sdist(last_pos, rq) >                      \
         BFQQ_SEEK_THR &&                               \
         (!blk_queue_nonrot(bfqd->queue) ||             \
          blk_rq_sectors(rq) < BFQQ_SECT_THR_NONROT))
#define BFQQ_CLOSE_THR          (sector_t)(8 * 1024)
#define BFQQ_SEEKY(bfqq)        (hweight32(bfqq->seek_history) > 19)
/*
 * Sync random I/O is likely to be confused with soft real-time I/O,
 * because it is characterized by limited throughput and apparently
 * isochronous arrival pattern. To avoid false positives, queues
 * containing only random (seeky) I/O are prevented from being tagged
 * as soft real-time.
 */
#define BFQQ_TOTALLY_SEEKY(bfqq)        (bfqq->seek_history == -1)

/* Min number of samples required to perform peak-rate update */
#define BFQ_RATE_MIN_SAMPLES    32
/* Min observation time interval required to perform a peak-rate update (ns) */
#define BFQ_RATE_MIN_INTERVAL   (300*NSEC_PER_MSEC)
/* Target observation time interval for a peak-rate update (ns) */
#define BFQ_RATE_REF_INTERVAL   NSEC_PER_SEC

/*
 * Shift used for peak-rate fixed precision calculations.
 * With
 * - the current shift: 16 positions
 * - the current type used to store rate: u32
 * - the current unit of measure for rate: [sectors/usec], or, more precisely,
 *   [(sectors/usec) / 2^BFQ_RATE_SHIFT] to take into account the shift,
 * the range of rates that can be stored is
 * [1 / 2^BFQ_RATE_SHIFT, 2^(32 - BFQ_RATE_SHIFT)] sectors/usec =
 * [1 / 2^16, 2^16] sectors/usec = [15e-6, 65536] sectors/usec =
 * [15, 65G] sectors/sec
 * Which, assuming a sector size of 512B, corresponds to a range of
 * [7.5K, 33T] B/sec
 */
#define BFQ_RATE_SHIFT          16

/*
 * When configured for computing the duration of the weight-raising
 * for interactive queues automatically (see the comments at the
 * beginning of this file), BFQ does it using the following formula:
 * duration = (ref_rate / r) * ref_wr_duration,
 * where r is the peak rate of the device, and ref_rate and
 * ref_wr_duration are two reference parameters.  In particular,
 * ref_rate is the peak rate of the reference storage device (see
 * below), and ref_wr_duration is about the maximum time needed, with
 * BFQ and while reading two files in parallel, to load typical large
 * applications on the reference device (see the comments on
 * max_service_from_wr below, for more details on how ref_wr_duration
 * is obtained).  In practice, the slower/faster the device at hand
 * is, the more/less it takes to load applications with respect to the
 * reference device.  Accordingly, the longer/shorter BFQ grants
 * weight raising to interactive applications.
 *
 * BFQ uses two different reference pairs (ref_rate, ref_wr_duration),
 * depending on whether the device is rotational or non-rotational.
 *
 * In the following definitions, ref_rate[0] and ref_wr_duration[0]
 * are the reference values for a rotational device, whereas
 * ref_rate[1] and ref_wr_duration[1] are the reference values for a
 * non-rotational device. The reference rates are not the actual peak
 * rates of the devices used as a reference, but slightly lower
 * values. The reason for using slightly lower values is that the
 * peak-rate estimator tends to yield slightly lower values than the
 * actual peak rate (it can yield the actual peak rate only if there
 * is only one process doing I/O, and the process does sequential
 * I/O).
 *
 * The reference peak rates are measured in sectors/usec, left-shifted
 * by BFQ_RATE_SHIFT.
 */
static int ref_rate[2] = {14000, 33000};
/*
 * To improve readability, a conversion function is used to initialize
 * the following array, which entails that the array can be
 * initialized only in a function.
 */
static int ref_wr_duration[2];

/*
 * BFQ uses the above-detailed, time-based weight-raising mechanism to
 * privilege interactive tasks. This mechanism is vulnerable to the
 * following false positives: I/O-bound applications that will go on
 * doing I/O for much longer than the duration of weight
 * raising. These applications have basically no benefit from being
 * weight-raised at the beginning of their I/O. On the opposite end,
 * while being weight-raised, these applications
 * a) unjustly steal throughput to applications that may actually need
 * low latency;
 * b) make BFQ uselessly perform device idling; device idling results
 * in loss of device throughput with most flash-based storage, and may
 * increase latencies when used purposelessly.
 *
 * BFQ tries to reduce these problems, by adopting the following
 * countermeasure. To introduce this countermeasure, we need first to
 * finish explaining how the duration of weight-raising for
 * interactive tasks is computed.
 *
 * For a bfq_queue deemed as interactive, the duration of weight
 * raising is dynamically adjusted, as a function of the estimated
 * peak rate of the device, so as to be equal to the time needed to
 * execute the 'largest' interactive task we benchmarked so far. By
 * largest task, we mean the task for which each involved process has
 * to do more I/O than for any of the other tasks we benchmarked. This
 * reference interactive task is the start-up of LibreOffice Writer,
 * and in this task each process/bfq_queue needs to have at most ~110K
 * sectors transferred.
 *
 * This last piece of information enables BFQ to reduce the actual
 * duration of weight-raising for at least one class of I/O-bound
 * applications: those doing sequential or quasi-sequential I/O. An
 * example is file copy. In fact, once started, the main I/O-bound
 * processes of these applications usually consume the above 110K
 * sectors in much less time than the processes of an application that
 * is starting, because these I/O-bound processes will greedily devote
 * almost all their CPU cycles only to their target,
 * throughput-friendly I/O operations. This is even more true if BFQ
 * happens to be underestimating the device peak rate, and thus
 * overestimating the duration of weight raising. But, according to
 * our measurements, once transferred 110K sectors, these processes
 * have no right to be weight-raised any longer.
 *
 * Basing on the last consideration, BFQ ends weight-raising for a
 * bfq_queue if the latter happens to have received an amount of
 * service at least equal to the following constant. The constant is
 * set to slightly more than 110K, to have a minimum safety margin.
 *
 * This early ending of weight-raising reduces the amount of time
 * during which interactive false positives cause the two problems
 * described at the beginning of these comments.
 */
static const unsigned long max_service_from_wr = 120000;

/*
 * Maximum time between the creation of two queues, for stable merge
 * to be activated (in ms)
 */
static const unsigned long bfq_activation_stable_merging = 600;
/*
 * Minimum time to be waited before evaluating delayed stable merge (in ms)
 */
static const unsigned long bfq_late_stable_merging = 600;

#define RQ_BIC(rq)              ((struct bfq_io_cq *)((rq)->elv.priv[0]))
#define RQ_BFQQ(rq)             ((rq)->elv.priv[1])

struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync,
                              unsigned int actuator_idx)
{
        if (is_sync)
                return bic->bfqq[1][actuator_idx];

        return bic->bfqq[0][actuator_idx];
}

static void bfq_put_stable_ref(struct bfq_queue *bfqq);

void bic_set_bfqq(struct bfq_io_cq *bic,
                  struct bfq_queue *bfqq,
                  bool is_sync,
                  unsigned int actuator_idx)
{
        struct bfq_queue *old_bfqq = bic->bfqq[is_sync][actuator_idx];

        /*
         * If bfqq != NULL, then a non-stable queue merge between
         * bic->bfqq and bfqq is happening here. This causes troubles
         * in the following case: bic->bfqq has also been scheduled
         * for a possible stable merge with bic->stable_merge_bfqq,
         * and bic->stable_merge_bfqq == bfqq happens to
         * hold. Troubles occur because bfqq may then undergo a split,
         * thereby becoming eligible for a stable merge. Yet, if
         * bic->stable_merge_bfqq points exactly to bfqq, then bfqq
         * would be stably merged with itself. To avoid this anomaly,
         * we cancel the stable merge if
         * bic->stable_merge_bfqq == bfqq.
         */
        struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[actuator_idx];

        /* Clear bic pointer if bfqq is detached from this bic */
        if (old_bfqq && old_bfqq->bic == bic)
                old_bfqq->bic = NULL;

        if (is_sync)
                bic->bfqq[1][actuator_idx] = bfqq;
        else
                bic->bfqq[0][actuator_idx] = bfqq;

        if (bfqq && bfqq_data->stable_merge_bfqq == bfqq) {
                /*
                 * Actually, these same instructions are executed also
                 * in bfq_setup_cooperator, in case of abort or actual
                 * execution of a stable merge. We could avoid
                 * repeating these instructions there too, but if we
                 * did so, we would nest even more complexity in this
                 * function.
                 */
                bfq_put_stable_ref(bfqq_data->stable_merge_bfqq);

                bfqq_data->stable_merge_bfqq = NULL;
        }
}

struct bfq_data *bic_to_bfqd(struct bfq_io_cq *bic)
{
        return bic->icq.q->elevator->elevator_data;
}

/**
 * icq_to_bic - convert iocontext queue structure to bfq_io_cq.
 * @icq: the iocontext queue.
 */
static struct bfq_io_cq *icq_to_bic(struct io_cq *icq)
{
        /* bic->icq is the first member, %NULL will convert to %NULL */
        return container_of(icq, struct bfq_io_cq, icq);
}

/**
 * bfq_bic_lookup - search into @ioc a bic associated to @bfqd.
 * @q: the request queue.
 */
static struct bfq_io_cq *bfq_bic_lookup(struct request_queue *q)
{
        struct bfq_io_cq *icq;
        unsigned long flags;

        if (!current->io_context)
                return NULL;

        spin_lock_irqsave(&q->queue_lock, flags);
        icq = icq_to_bic(ioc_lookup_icq(q));
        spin_unlock_irqrestore(&q->queue_lock, flags);

        return icq;
}

/*
 * Scheduler run of queue, if there are requests pending and no one in the
 * driver that will restart queueing.
 */
void bfq_schedule_dispatch(struct bfq_data *bfqd)
{
        lockdep_assert_held(&bfqd->lock);

        if (bfqd->queued != 0) {
                bfq_log(bfqd, "schedule dispatch");
                blk_mq_run_hw_queues(bfqd->queue, true);
        }
}

#define bfq_class_idle(bfqq)    ((bfqq)->ioprio_class == IOPRIO_CLASS_IDLE)

#define bfq_sample_valid(samples)       ((samples) > 80)

/*
 * Lifted from AS - choose which of rq1 and rq2 that is best served now.
 * We choose the request that is closer to the head right now.  Distance
 * behind the head is penalized and only allowed to a certain extent.
 */
static struct request *bfq_choose_req(struct bfq_data *bfqd,
                                      struct request *rq1,
                                      struct request *rq2,
                                      sector_t last)
{
        sector_t s1, s2, d1 = 0, d2 = 0;
        unsigned long back_max;
#define BFQ_RQ1_WRAP    0x01 /* request 1 wraps */
#define BFQ_RQ2_WRAP    0x02 /* request 2 wraps */
        unsigned int wrap = 0; /* bit mask: requests behind the disk head? */

        if (!rq1 || rq1 == rq2)
                return rq2;
        if (!rq2)
                return rq1;

        if (rq_is_sync(rq1) && !rq_is_sync(rq2))
                return rq1;
        else if (rq_is_sync(rq2) && !rq_is_sync(rq1))
                return rq2;
        if ((rq1->cmd_flags & REQ_META) && !(rq2->cmd_flags & REQ_META))
                return rq1;
        else if ((rq2->cmd_flags & REQ_META) && !(rq1->cmd_flags & REQ_META))
                return rq2;

        s1 = blk_rq_pos(rq1);
        s2 = blk_rq_pos(rq2);

        /*
         * By definition, 1KiB is 2 sectors.
         */
        back_max = bfqd->bfq_back_max * 2;

        /*
         * Strict one way elevator _except_ in the case where we allow
         * short backward seeks which are biased as twice the cost of a
         * similar forward seek.
         */
        if (s1 >= last)
                d1 = s1 - last;
        else if (s1 + back_max >= last)
                d1 = (last - s1) * bfqd->bfq_back_penalty;
        else
                wrap |= BFQ_RQ1_WRAP;

        if (s2 >= last)
                d2 = s2 - last;
        else if (s2 + back_max >= last)
                d2 = (last - s2) * bfqd->bfq_back_penalty;
        else
                wrap |= BFQ_RQ2_WRAP;

        /* Found required data */

        /*
         * By doing switch() on the bit mask "wrap" we avoid having to
         * check two variables for all permutations: --> faster!
         */
        switch (wrap) {
        case 0: /* common case for CFQ: rq1 and rq2 not wrapped */
                if (d1 < d2)
                        return rq1;
                else if (d2 < d1)
                        return rq2;

                if (s1 >= s2)
                        return rq1;
                else
                        return rq2;

        case BFQ_RQ2_WRAP:
                return rq1;
        case BFQ_RQ1_WRAP:
                return rq2;
        case BFQ_RQ1_WRAP|BFQ_RQ2_WRAP: /* both rqs wrapped */
        default:
                /*
                 * Since both rqs are wrapped,
                 * start with the one that's further behind head
                 * (--> only *one* back seek required),
                 * since back seek takes more time than forward.
                 */
                if (s1 <= s2)
                        return rq1;
                else
                        return rq2;
        }
}

#define BFQ_LIMIT_INLINE_DEPTH 16

#ifdef CONFIG_BFQ_GROUP_IOSCHED
static bool bfqq_request_over_limit(struct bfq_queue *bfqq, int limit)
{
        struct bfq_data *bfqd = bfqq->bfqd;
        struct bfq_entity *entity = &bfqq->entity;
        struct bfq_entity *inline_entities[BFQ_LIMIT_INLINE_DEPTH];
        struct bfq_entity **entities = inline_entities;
        int depth, level, alloc_depth = BFQ_LIMIT_INLINE_DEPTH;
        int class_idx = bfqq->ioprio_class - 1;
        struct bfq_sched_data *sched_data;
        unsigned long wsum;
        bool ret = false;

        if (!entity->on_st_or_in_serv)
                return false;

retry:
        spin_lock_irq(&bfqd->lock);
        /* +1 for bfqq entity, root cgroup not included */
        depth = bfqg_to_blkg(bfqq_group(bfqq))->blkcg->css.cgroup->level + 1;
        if (depth > alloc_depth) {
                spin_unlock_irq(&bfqd->lock);
                if (entities != inline_entities)
                        kfree(entities);
                entities = kmalloc_array(depth, sizeof(*entities), GFP_NOIO);
                if (!entities)
                        return false;
                alloc_depth = depth;
                goto retry;
        }

        sched_data = entity->sched_data;
        /* Gather our ancestors as we need to traverse them in reverse order */
        level = 0;
        for_each_entity(entity) {
                /*
                 * If at some level entity is not even active, allow request
                 * queueing so that BFQ knows there's work to do and activate
                 * entities.
                 */
                if (!entity->on_st_or_in_serv)
                        goto out;
                /* Uh, more parents than cgroup subsystem thinks? */
                if (WARN_ON_ONCE(level >= depth))
                        break;
                entities[level++] = entity;
        }
        WARN_ON_ONCE(level != depth);
        for (level--; level >= 0; level--) {
                entity = entities[level];
                if (level > 0) {
                        wsum = bfq_entity_service_tree(entity)->wsum;
                } else {
                        int i;
                        /*
                         * For bfqq itself we take into account service trees
                         * of all higher priority classes and multiply their
                         * weights so that low prio queue from higher class
                         * gets more requests than high prio queue from lower
                         * class.
                         */
                        wsum = 0;
                        for (i = 0; i <= class_idx; i++) {
                                wsum = wsum * IOPRIO_BE_NR +
                                        sched_data->service_tree[i].wsum;
                        }
                }
                if (!wsum)
                        continue;
                limit = DIV_ROUND_CLOSEST(limit * entity->weight, wsum);
                if (entity->allocated >= limit) {
                        bfq_log_bfqq(bfqq->bfqd, bfqq,
                                "too many requests: allocated %d limit %d level %d",
                                entity->allocated, limit, level);
                        ret = true;
                        break;
                }
        }
out:
        spin_unlock_irq(&bfqd->lock);
        if (entities != inline_entities)
                kfree(entities);
        return ret;
}
#else
static bool bfqq_request_over_limit(struct bfq_queue *bfqq, int limit)
{
        return false;
}
#endif

/*
 * Async I/O can easily starve sync I/O (both sync reads and sync
 * writes), by consuming all tags. Similarly, storms of sync writes,
 * such as those that sync(2) may trigger, can starve sync reads.
 * Limit depths of async I/O and sync writes so as to counter both
 * problems.
 *
 * Also if a bfq queue or its parent cgroup consume more tags than would be
 * appropriate for their weight, we trim the available tag depth to 1. This
 * avoids a situation where one cgroup can starve another cgroup from tags and
 * thus block service differentiation among cgroups. Note that because the
 * queue / cgroup already has many requests allocated and queued, this does not
 * significantly affect service guarantees coming from the BFQ scheduling
 * algorithm.
 */
static void bfq_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
{
        struct bfq_data *bfqd = data->q->elevator->elevator_data;
        struct bfq_io_cq *bic = bfq_bic_lookup(data->q);
        int depth;
        unsigned limit = data->q->nr_requests;
        unsigned int act_idx;

        /* Sync reads have full depth available */
        if (op_is_sync(opf) && !op_is_write(opf)) {
                depth = 0;
        } else {
                depth = bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(opf)];
                limit = (limit * depth) >> bfqd->full_depth_shift;
        }

        for (act_idx = 0; bic && act_idx < bfqd->num_actuators; act_idx++) {
                struct bfq_queue *bfqq =
                        bic_to_bfqq(bic, op_is_sync(opf), act_idx);

                /*
                 * Does queue (or any parent entity) exceed number of
                 * requests that should be available to it? Heavily
                 * limit depth so that it cannot consume more
                 * available requests and thus starve other entities.
                 */
                if (bfqq && bfqq_request_over_limit(bfqq, limit)) {
                        depth = 1;
                        break;
                }
        }
        bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
                __func__, bfqd->wr_busy_queues, op_is_sync(opf), depth);
        if (depth)
                data->shallow_depth = depth;
}

static struct bfq_queue *
bfq_rq_pos_tree_lookup(struct bfq_data *bfqd, struct rb_root *root,
                     sector_t sector, struct rb_node **ret_parent,
                     struct rb_node ***rb_link)
{
        struct rb_node **p, *parent;
        struct bfq_queue *bfqq = NULL;

        parent = NULL;
        p = &root->rb_node;
        while (*p) {
                struct rb_node **n;

                parent = *p;
                bfqq = rb_entry(parent, struct bfq_queue, pos_node);

                /*
                 * Sort strictly based on sector. Smallest to the left,
                 * largest to the right.
                 */
                if (sector > blk_rq_pos(bfqq->next_rq))
                        n = &(*p)->rb_right;
                else if (sector < blk_rq_pos(bfqq->next_rq))
                        n = &(*p)->rb_left;
                else
                        break;
                p = n;
                bfqq = NULL;
        }

        *ret_parent = parent;
        if (rb_link)
                *rb_link = p;

        bfq_log(bfqd, "rq_pos_tree_lookup %llu: returning %d",
                (unsigned long long)sector,
                bfqq ? bfqq->pid : 0);

        return bfqq;
}

static bool bfq_too_late_for_merging(struct bfq_queue *bfqq)
{
        return bfqq->service_from_backlogged > 0 &&
                time_is_before_jiffies(bfqq->first_IO_time +
                                       bfq_merge_time_limit);
}

/*
 * The following function is not marked as __cold because it is
 * actually cold, but for the same performance goal described in the
 * comments on the likely() at the beginning of
 * bfq_setup_cooperator(). Unexpectedly, to reach an even lower
 * execution time for the case where this function is not invoked, we
 * had to add an unlikely() in each involved if().
 */
void __cold
bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        struct rb_node **p, *parent;
        struct bfq_queue *__bfqq;

        if (bfqq->pos_root) {
                rb_erase(&bfqq->pos_node, bfqq->pos_root);
                bfqq->pos_root = NULL;
        }

        /* oom_bfqq does not participate in queue merging */
        if (bfqq == &bfqd->oom_bfqq)
                return;

        /*
         * bfqq cannot be merged any longer (see comments in
         * bfq_setup_cooperator): no point in adding bfqq into the
         * position tree.
         */
        if (bfq_too_late_for_merging(bfqq))
                return;

        if (bfq_class_idle(bfqq))
                return;
        if (!bfqq->next_rq)
                return;

        bfqq->pos_root = &bfqq_group(bfqq)->rq_pos_tree;
        __bfqq = bfq_rq_pos_tree_lookup(bfqd, bfqq->pos_root,
                        blk_rq_pos(bfqq->next_rq), &parent, &p);
        if (!__bfqq) {
                rb_link_node(&bfqq->pos_node, parent, p);
                rb_insert_color(&bfqq->pos_node, bfqq->pos_root);
        } else
                bfqq->pos_root = NULL;
}

/*
 * The following function returns false either if every active queue
 * must receive the same share of the throughput (symmetric scenario),
 * or, as a special case, if bfqq must receive a share of the
 * throughput lower than or equal to the share that every other active
 * queue must receive.  If bfqq does sync I/O, then these are the only
 * two cases where bfqq happens to be guaranteed its share of the
 * throughput even if I/O dispatching is not plugged when bfqq remains
 * temporarily empty (for more details, see the comments in the
 * function bfq_better_to_idle()). For this reason, the return value
 * of this function is used to check whether I/O-dispatch plugging can
 * be avoided.
 *
 * The above first case (symmetric scenario) occurs when:
 * 1) all active queues have the same weight,
 * 2) all active queues belong to the same I/O-priority class,
 * 3) all active groups at the same level in the groups tree have the same
 *    weight,
 * 4) all active groups at the same level in the groups tree have the same
 *    number of children.
 *
 * Unfortunately, keeping the necessary state for evaluating exactly
 * the last two symmetry sub-conditions above would be quite complex
 * and time consuming. Therefore this function evaluates, instead,
 * only the following stronger three sub-conditions, for which it is
 * much easier to maintain the needed state:
 * 1) all active queues have the same weight,
 * 2) all active queues belong to the same I/O-priority class,
 * 3) there is at most one active group.
 * In particular, the last condition is always true if hierarchical
 * support or the cgroups interface are not enabled, thus no state
 * needs to be maintained in this case.
 */
static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
                                   struct bfq_queue *bfqq)
{
        bool smallest_weight = bfqq &&
                bfqq->weight_counter &&
                bfqq->weight_counter ==
                container_of(
                        rb_first_cached(&bfqd->queue_weights_tree),
                        struct bfq_weight_counter,
                        weights_node);

        /*
         * For queue weights to differ, queue_weights_tree must contain
         * at least two nodes.
         */
        bool varied_queue_weights = !smallest_weight &&
                !RB_EMPTY_ROOT(&bfqd->queue_weights_tree.rb_root) &&
                (bfqd->queue_weights_tree.rb_root.rb_node->rb_left ||
                 bfqd->queue_weights_tree.rb_root.rb_node->rb_right);

        bool multiple_classes_busy =
                (bfqd->busy_queues[0] && bfqd->busy_queues[1]) ||
                (bfqd->busy_queues[0] && bfqd->busy_queues[2]) ||
                (bfqd->busy_queues[1] && bfqd->busy_queues[2]);

        return varied_queue_weights || multiple_classes_busy
#ifdef CONFIG_BFQ_GROUP_IOSCHED
               || bfqd->num_groups_with_pending_reqs > 1
#endif
                ;
}

/*
 * If the weight-counter tree passed as input contains no counter for
 * the weight of the input queue, then add that counter; otherwise just
 * increment the existing counter.
 *
 * Note that weight-counter trees contain few nodes in mostly symmetric
 * scenarios. For example, if all queues have the same weight, then the
 * weight-counter tree for the queues may contain at most one node.
 * This holds even if low_latency is on, because weight-raised queues
 * are not inserted in the tree.
 * In most scenarios, the rate at which nodes are created/destroyed
 * should be low too.
 */
void bfq_weights_tree_add(struct bfq_queue *bfqq)
{
        struct rb_root_cached *root = &bfqq->bfqd->queue_weights_tree;
        struct bfq_entity *entity = &bfqq->entity;
        struct rb_node **new = &(root->rb_root.rb_node), *parent = NULL;
        bool leftmost = true;

        /*
         * Do not insert if the queue is already associated with a
         * counter, which happens if:
         *   1) a request arrival has caused the queue to become both
         *      non-weight-raised, and hence change its weight, and
         *      backlogged; in this respect, each of the two events
         *      causes an invocation of this function,
         *   2) this is the invocation of this function caused by the
         *      second event. This second invocation is actually useless,
         *      and we handle this fact by exiting immediately. More
         *      efficient or clearer solutions might possibly be adopted.
         */
        if (bfqq->weight_counter)
                return;

        while (*new) {
                struct bfq_weight_counter *__counter = container_of(*new,
                                                struct bfq_weight_counter,
                                                weights_node);
                parent = *new;

                if (entity->weight == __counter->weight) {
                        bfqq->weight_counter = __counter;
                        goto inc_counter;
                }
                if (entity->weight < __counter->weight)
                        new = &((*new)->rb_left);
                else {
                        new = &((*new)->rb_right);
                        leftmost = false;
                }
        }

        bfqq->weight_counter = kzalloc(sizeof(struct bfq_weight_counter),
                                       GFP_ATOMIC);

        /*
         * In the unlucky event of an allocation failure, we just
         * exit. This will cause the weight of queue to not be
         * considered in bfq_asymmetric_scenario, which, in its turn,
         * causes the scenario to be deemed wrongly symmetric in case
         * bfqq's weight would have been the only weight making the
         * scenario asymmetric.  On the bright side, no unbalance will
         * however occur when bfqq becomes inactive again (the
         * invocation of this function is triggered by an activation
         * of queue).  In fact, bfq_weights_tree_remove does nothing
         * if !bfqq->weight_counter.
         */
        if (unlikely(!bfqq->weight_counter))
                return;

        bfqq->weight_counter->weight = entity->weight;
        rb_link_node(&bfqq->weight_counter->weights_node, parent, new);
        rb_insert_color_cached(&bfqq->weight_counter->weights_node, root,
                                leftmost);

inc_counter:
        bfqq->weight_counter->num_active++;
        bfqq->ref++;
}

/*
 * Decrement the weight counter associated with the queue, and, if the
 * counter reaches 0, remove the counter from the tree.
 * See the comments to the function bfq_weights_tree_add() for considerations
 * about overhead.
 */
void bfq_weights_tree_remove(struct bfq_queue *bfqq)
{
        struct rb_root_cached *root;

        if (!bfqq->weight_counter)
                return;

        root = &bfqq->bfqd->queue_weights_tree;
        bfqq->weight_counter->num_active--;
        if (bfqq->weight_counter->num_active > 0)
                goto reset_entity_pointer;

        rb_erase_cached(&bfqq->weight_counter->weights_node, root);
        kfree(bfqq->weight_counter);

reset_entity_pointer:
        bfqq->weight_counter = NULL;
        bfq_put_queue(bfqq);
}

/*
 * Return expired entry, or NULL to just start from scratch in rbtree.
 */
static struct request *bfq_check_fifo(struct bfq_queue *bfqq,
                                      struct request *last)
{
        struct request *rq;

        if (bfq_bfqq_fifo_expire(bfqq))
                return NULL;

        bfq_mark_bfqq_fifo_expire(bfqq);

        rq = rq_entry_fifo(bfqq->fifo.next);

        if (rq == last || ktime_get_ns() < rq->fifo_time)
                return NULL;

        bfq_log_bfqq(bfqq->bfqd, bfqq, "check_fifo: returned %p", rq);
        return rq;
}

static struct request *bfq_find_next_rq(struct bfq_data *bfqd,
                                        struct bfq_queue *bfqq,
                                        struct request *last)
{
        struct rb_node *rbnext = rb_next(&last->rb_node);
        struct rb_node *rbprev = rb_prev(&last->rb_node);
        struct request *next, *prev = NULL;

        /* Follow expired path, else get first next available. */
        next = bfq_check_fifo(bfqq, last);
        if (next)
                return next;

        if (rbprev)
                prev = rb_entry_rq(rbprev);

        if (rbnext)
                next = rb_entry_rq(rbnext);
        else {
                rbnext = rb_first(&bfqq->sort_list);
                if (rbnext && rbnext != &last->rb_node)
                        next = rb_entry_rq(rbnext);
        }

        return bfq_choose_req(bfqd, next, prev, blk_rq_pos(last));
}

/* see the definition of bfq_async_charge_factor for details */
static unsigned long bfq_serv_to_charge(struct request *rq,
                                        struct bfq_queue *bfqq)
{
        if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1 ||
            bfq_asymmetric_scenario(bfqq->bfqd, bfqq))
                return blk_rq_sectors(rq);

        return blk_rq_sectors(rq) * bfq_async_charge_factor;
}

/**
 * bfq_updated_next_req - update the queue after a new next_rq selection.
 * @bfqd: the device data the queue belongs to.
 * @bfqq: the queue to update.
 *
 * If the first request of a queue changes we make sure that the queue
 * has enough budget to serve at least its first request (if the
 * request has grown).  We do this because if the queue has not enough
 * budget for its first request, it has to go through two dispatch
 * rounds to actually get it dispatched.
 */
static void bfq_updated_next_req(struct bfq_data *bfqd,
                                 struct bfq_queue *bfqq)
{
        struct bfq_entity *entity = &bfqq->entity;
        struct request *next_rq = bfqq->next_rq;
        unsigned long new_budget;

        if (!next_rq)
                return;

        if (bfqq == bfqd->in_service_queue)
                /*
                 * In order not to break guarantees, budgets cannot be
                 * changed after an entity has been selected.
                 */
                return;

        new_budget = max_t(unsigned long,
                           max_t(unsigned long, bfqq->max_budget,
                                 bfq_serv_to_charge(next_rq, bfqq)),
                           entity->service);
        if (entity->budget != new_budget) {
                entity->budget = new_budget;
                bfq_log_bfqq(bfqd, bfqq, "updated next rq: new budget %lu",
                                         new_budget);
                bfq_requeue_bfqq(bfqd, bfqq, false);
        }
}

static unsigned int bfq_wr_duration(struct bfq_data *bfqd)
{
        u64 dur;

        dur = bfqd->rate_dur_prod;
        do_div(dur, bfqd->peak_rate);

        /*
         * Limit duration between 3 and 25 seconds. The upper limit
         * has been conservatively set after the following worst case:
         * on a QEMU/KVM virtual machine
         * - running in a slow PC
         * - with a virtual disk stacked on a slow low-end 5400rpm HDD
         * - serving a heavy I/O workload, such as the sequential reading
         *   of several files
         * mplayer took 23 seconds to start, if constantly weight-raised.
         *
         * As for higher values than that accommodating the above bad
         * scenario, tests show that higher values would often yield
         * the opposite of the desired result, i.e., would worsen
         * responsiveness by allowing non-interactive applications to
         * preserve weight raising for too long.
         *
         * On the other end, lower values than 3 seconds make it
         * difficult for most interactive tasks to complete their jobs
         * before weight-raising finishes.
         */
        return clamp_val(dur, msecs_to_jiffies(3000), msecs_to_jiffies(25000));
}

/* switch back from soft real-time to interactive weight raising */
static void switch_back_to_interactive_wr(struct bfq_queue *bfqq,
                                          struct bfq_data *bfqd)
{
        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
        bfqq->last_wr_start_finish = bfqq->wr_start_at_switch_to_srt;
}

static void
bfq_bfqq_resume_state(struct bfq_queue *bfqq, struct bfq_data *bfqd,
                      struct bfq_io_cq *bic, bool bfq_already_existing)
{
        unsigned int old_wr_coeff = 1;
        bool busy = bfq_already_existing && bfq_bfqq_busy(bfqq);
        unsigned int a_idx = bfqq->actuator_idx;
        struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[a_idx];

        if (bfqq_data->saved_has_short_ttime)
                bfq_mark_bfqq_has_short_ttime(bfqq);
        else
                bfq_clear_bfqq_has_short_ttime(bfqq);

        if (bfqq_data->saved_IO_bound)
                bfq_mark_bfqq_IO_bound(bfqq);
        else
                bfq_clear_bfqq_IO_bound(bfqq);

        bfqq->last_serv_time_ns = bfqq_data->saved_last_serv_time_ns;
        bfqq->inject_limit = bfqq_data->saved_inject_limit;
        bfqq->decrease_time_jif = bfqq_data->saved_decrease_time_jif;

        bfqq->entity.new_weight = bfqq_data->saved_weight;
        bfqq->ttime = bfqq_data->saved_ttime;
        bfqq->io_start_time = bfqq_data->saved_io_start_time;
        bfqq->tot_idle_time = bfqq_data->saved_tot_idle_time;
        /*
         * Restore weight coefficient only if low_latency is on
         */
        if (bfqd->low_latency) {
                old_wr_coeff = bfqq->wr_coeff;
                bfqq->wr_coeff = bfqq_data->saved_wr_coeff;
        }
        bfqq->service_from_wr = bfqq_data->saved_service_from_wr;
        bfqq->wr_start_at_switch_to_srt =
                bfqq_data->saved_wr_start_at_switch_to_srt;
        bfqq->last_wr_start_finish = bfqq_data->saved_last_wr_start_finish;
        bfqq->wr_cur_max_time = bfqq_data->saved_wr_cur_max_time;

        if (bfqq->wr_coeff > 1 && (bfq_bfqq_in_large_burst(bfqq) ||
            time_is_before_jiffies(bfqq->last_wr_start_finish +
                                   bfqq->wr_cur_max_time))) {
                if (bfqq->wr_cur_max_time == bfqd->bfq_wr_rt_max_time &&
                    !bfq_bfqq_in_large_burst(bfqq) &&
                    time_is_after_eq_jiffies(bfqq->wr_start_at_switch_to_srt +
                                             bfq_wr_duration(bfqd))) {
                        switch_back_to_interactive_wr(bfqq, bfqd);
                } else {
                        bfqq->wr_coeff = 1;
                        bfq_log_bfqq(bfqq->bfqd, bfqq,
                                     "resume state: switching off wr");
                }
        }

        /* make sure weight will be updated, however we got here */
        bfqq->entity.prio_changed = 1;

        if (likely(!busy))
                return;

        if (old_wr_coeff == 1 && bfqq->wr_coeff > 1)
                bfqd->wr_busy_queues++;
        else if (old_wr_coeff > 1 && bfqq->wr_coeff == 1)
                bfqd->wr_busy_queues--;
}

static int bfqq_process_refs(struct bfq_queue *bfqq)
{
        return bfqq->ref - bfqq->entity.allocated -
                bfqq->entity.on_st_or_in_serv -
                (bfqq->weight_counter != NULL) - bfqq->stable_ref;
}

/* Empty burst list and add just bfqq (see comments on bfq_handle_burst) */
static void bfq_reset_burst_list(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        struct bfq_queue *item;
        struct hlist_node *n;

        hlist_for_each_entry_safe(item, n, &bfqd->burst_list, burst_list_node)
                hlist_del_init(&item->burst_list_node);

        /*
         * Start the creation of a new burst list only if there is no
         * active queue. See comments on the conditional invocation of
         * bfq_handle_burst().
         */
        if (bfq_tot_busy_queues(bfqd) == 0) {
                hlist_add_head(&bfqq->burst_list_node, &bfqd->burst_list);
                bfqd->burst_size = 1;
        } else
                bfqd->burst_size = 0;

        bfqd->burst_parent_entity = bfqq->entity.parent;
}

/* Add bfqq to the list of queues in current burst (see bfq_handle_burst) */
static void bfq_add_to_burst(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        /* Increment burst size to take into account also bfqq */
        bfqd->burst_size++;

        if (bfqd->burst_size == bfqd->bfq_large_burst_thresh) {
                struct bfq_queue *pos, *bfqq_item;
                struct hlist_node *n;

                /*
                 * Enough queues have been activated shortly after each
                 * other to consider this burst as large.
                 */
                bfqd->large_burst = true;

                /*
                 * We can now mark all queues in the burst list as
                 * belonging to a large burst.
                 */
                hlist_for_each_entry(bfqq_item, &bfqd->burst_list,
                                     burst_list_node)
                        bfq_mark_bfqq_in_large_burst(bfqq_item);
                bfq_mark_bfqq_in_large_burst(bfqq);

                /*
                 * From now on, and until the current burst finishes, any
                 * new queue being activated shortly after the last queue
                 * was inserted in the burst can be immediately marked as
                 * belonging to a large burst. So the burst list is not
                 * needed any more. Remove it.
                 */
                hlist_for_each_entry_safe(pos, n, &bfqd->burst_list,
                                          burst_list_node)
                        hlist_del_init(&pos->burst_list_node);
        } else /*
                * Burst not yet large: add bfqq to the burst list. Do
                * not increment the ref counter for bfqq, because bfqq
                * is removed from the burst list before freeing bfqq
                * in put_queue.
                */
                hlist_add_head(&bfqq->burst_list_node, &bfqd->burst_list);
}

/*
 * If many queues belonging to the same group happen to be created
 * shortly after each other, then the processes associated with these
 * queues have typically a common goal. In particular, bursts of queue
 * creations are usually caused by services or applications that spawn
 * many parallel threads/processes. Examples are systemd during boot,
 * or git grep. To help these processes get their job done as soon as
 * possible, it is usually better to not grant either weight-raising
 * or device idling to their queues, unless these queues must be
 * protected from the I/O flowing through other active queues.
 *
 * In this comment we describe, firstly, the reasons why this fact
 * holds, and, secondly, the next function, which implements the main
 * steps needed to properly mark these queues so that they can then be
 * treated in a different way.
 *
 * The above services or applications benefit mostly from a high
 * throughput: the quicker the requests of the activated queues are
 * cumulatively served, the sooner the target job of these queues gets
 * completed. As a consequence, weight-raising any of these queues,
 * which also implies idling the device for it, is almost always
 * counterproductive, unless there are other active queues to isolate
 * these new queues from. If there no other active queues, then
 * weight-raising these new queues just lowers throughput in most
 * cases.
 *
 * On the other hand, a burst of queue creations may be caused also by
 * the start of an application that does not consist of a lot of
 * parallel I/O-bound threads. In fact, with a complex application,
 * several short processes may need to be executed to start-up the
 * application. In this respect, to start an application as quickly as
 * possible, the best thing to do is in any case to privilege the I/O
 * related to the application with respect to all other
 * I/O. Therefore, the best strategy to start as quickly as possible
 * an application that causes a burst of queue creations is to
 * weight-raise all the queues created during the burst. This is the
 * exact opposite of the best strategy for the other type of bursts.
 *
 * In the end, to take the best action for each of the two cases, the
 * two types of bursts need to be distinguished. Fortunately, this
 * seems relatively easy, by looking at the sizes of the bursts. In
 * particular, we found a threshold such that only bursts with a
 * larger size than that threshold are apparently caused by
 * services or commands such as systemd or git grep. For brevity,
 * hereafter we call just 'large' these bursts. BFQ *does not*
 * weight-raise queues whose creation occurs in a large burst. In
 * addition, for each of these queues BFQ performs or does not perform
 * idling depending on which choice boosts the throughput more. The
 * exact choice depends on the device and request pattern at
 * hand.
 *
 * Unfortunately, false positives may occur while an interactive task
 * is starting (e.g., an application is being started). The
 * consequence is that the queues associated with the task do not
 * enjoy weight raising as expected. Fortunately these false positives
 * are very rare. They typically occur if some service happens to
 * start doing I/O exactly when the interactive task starts.
 *
 * Turning back to the next function, it is invoked only if there are
 * no active queues (apart from active queues that would belong to the
 * same, possible burst bfqq would belong to), and it implements all
 * the steps needed to detect the occurrence of a large burst and to
 * properly mark all the queues belonging to it (so that they can then
 * be treated in a different way). This goal is achieved by
 * maintaining a "burst list" that holds, temporarily, the queues that
 * belong to the burst in progress. The list is then used to mark
 * these queues as belonging to a large burst if the burst does become
 * large. The main steps are the following.
 *
 * . when the very first queue is created, the queue is inserted into the
 *   list (as it could be the first queue in a possible burst)
 *
 * . if the current burst has not yet become large, and a queue Q that does
 *   not yet belong to the burst is activated shortly after the last time
 *   at which a new queue entered the burst list, then the function appends
 *   Q to the burst list
 *
 * . if, as a consequence of the previous step, the burst size reaches
 *   the large-burst threshold, then
 *
 *     . all the queues in the burst list are marked as belonging to a
 *       large burst
 *
 *     . the burst list is deleted; in fact, the burst list already served
 *       its purpose (keeping temporarily track of the queues in a burst,
 *       so as to be able to mark them as belonging to a large burst in the
 *       previous sub-step), and now is not needed any more
 *
 *     . the device enters a large-burst mode
 *
 * . if a queue Q that does not belong to the burst is created while
 *   the device is in large-burst mode and shortly after the last time
 *   at which a queue either entered the burst list or was marked as
 *   belonging to the current large burst, then Q is immediately marked
 *   as belonging to a large burst.
 *
 * . if a queue Q that does not belong to the burst is created a while
 *   later, i.e., not shortly after, than the last time at which a queue
 *   either entered the burst list or was marked as belonging to the
 *   current large burst, then the current burst is deemed as finished and:
 *
 *        . the large-burst mode is reset if set
 *
 *        . the burst list is emptied
 *
 *        . Q is inserted in the burst list, as Q may be the first queue
 *          in a possible new burst (then the burst list contains just Q
 *          after this step).
 */
static void bfq_handle_burst(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        /*
         * If bfqq is already in the burst list or is part of a large
         * burst, or finally has just been split, then there is
         * nothing else to do.
         */
        if (!hlist_unhashed(&bfqq->burst_list_node) ||
            bfq_bfqq_in_large_burst(bfqq) ||
            time_is_after_eq_jiffies(bfqq->split_time +
                                     msecs_to_jiffies(10)))
                return;

        /*
         * If bfqq's creation happens late enough, or bfqq belongs to
         * a different group than the burst group, then the current
         * burst is finished, and related data structures must be
         * reset.
         *
         * In this respect, consider the special case where bfqq is
         * the very first queue created after BFQ is selected for this
         * device. In this case, last_ins_in_burst and
         * burst_parent_entity are not yet significant when we get
         * here. But it is easy to verify that, whether or not the
         * following condition is true, bfqq will end up being
         * inserted into the burst list. In particular the list will
         * happen to contain only bfqq. And this is exactly what has
         * to happen, as bfqq may be the first queue of the first
         * burst.
         */
        if (time_is_before_jiffies(bfqd->last_ins_in_burst +
            bfqd->bfq_burst_interval) ||
            bfqq->entity.parent != bfqd->burst_parent_entity) {
                bfqd->large_burst = false;
                bfq_reset_burst_list(bfqd, bfqq);
                goto end;
        }

        /*
         * If we get here, then bfqq is being activated shortly after the
         * last queue. So, if the current burst is also large, we can mark
         * bfqq as belonging to this large burst immediately.
         */
        if (bfqd->large_burst) {
                bfq_mark_bfqq_in_large_burst(bfqq);
                goto end;
        }

        /*
         * If we get here, then a large-burst state has not yet been
         * reached, but bfqq is being activated shortly after the last
         * queue. Then we add bfqq to the burst.
         */
        bfq_add_to_burst(bfqd, bfqq);
end:
        /*
         * At this point, bfqq either has been added to the current
         * burst or has caused the current burst to terminate and a
         * possible new burst to start. In particular, in the second
         * case, bfqq has become the first queue in the possible new
         * burst.  In both cases last_ins_in_burst needs to be moved
         * forward.
         */
        bfqd->last_ins_in_burst = jiffies;
}

static int bfq_bfqq_budget_left(struct bfq_queue *bfqq)
{
        struct bfq_entity *entity = &bfqq->entity;

        return entity->budget - entity->service;
}

/*
 * If enough samples have been computed, return the current max budget
 * stored in bfqd, which is dynamically updated according to the
 * estimated disk peak rate; otherwise return the default max budget
 */
static int bfq_max_budget(struct bfq_data *bfqd)
{
        if (bfqd->budgets_assigned < bfq_stats_min_budgets)
                return bfq_default_max_budget;
        else
                return bfqd->bfq_max_budget;
}

/*
 * Return min budget, which is a fraction of the current or default
 * max budget (trying with 1/32)
 */
static int bfq_min_budget(struct bfq_data *bfqd)
{
        if (bfqd->budgets_assigned < bfq_stats_min_budgets)
                return bfq_default_max_budget / 32;
        else
                return bfqd->bfq_max_budget / 32;
}

/*
 * The next function, invoked after the input queue bfqq switches from
 * idle to busy, updates the budget of bfqq. The function also tells
 * whether the in-service queue should be expired, by returning
 * true. The purpose of expiring the in-service queue is to give bfqq
 * the chance to possibly preempt the in-service queue, and the reason
 * for preempting the in-service queue is to achieve one of the two
 * goals below.
 *
 * 1. Guarantee to bfqq its reserved bandwidth even if bfqq has
 * expired because it has remained idle. In particular, bfqq may have
 * expired for one of the following two reasons:
 *
 * - BFQQE_NO_MORE_REQUESTS bfqq did not enjoy any device idling
 *   and did not make it to issue a new request before its last
 *   request was served;
 *
 * - BFQQE_TOO_IDLE bfqq did enjoy device idling, but did not issue
 *   a new request before the expiration of the idling-time.
 *
 * Even if bfqq has expired for one of the above reasons, the process
 * associated with the queue may be however issuing requests greedily,
 * and thus be sensitive to the bandwidth it receives (bfqq may have
 * remained idle for other reasons: CPU high load, bfqq not enjoying
 * idling, I/O throttling somewhere in the path from the process to
 * the I/O scheduler, ...). But if, after every expiration for one of
 * the above two reasons, bfqq has to wait for the service of at least
 * one full budget of another queue before being served again, then
 * bfqq is likely to get a much lower bandwidth or resource time than
 * its reserved ones. To address this issue, two countermeasures need
 * to be taken.
 *
 * First, the budget and the timestamps of bfqq need to be updated in
 * a special way on bfqq reactivation: they need to be updated as if
 * bfqq did not remain idle and did not expire. In fact, if they are
 * computed as if bfqq expired and remained idle until reactivation,
 * then the process associated with bfqq is treated as if, instead of
 * being greedy, it stopped issuing requests when bfqq remained idle,
 * and restarts issuing requests only on this reactivation. In other
 * words, the scheduler does not help the process recover the "service
 * hole" between bfqq expiration and reactivation. As a consequence,
 * the process receives a lower bandwidth than its reserved one. In
 * contrast, to recover this hole, the budget must be updated as if
 * bfqq was not expired at all before this reactivation, i.e., it must
 * be set to the value of the remaining budget when bfqq was
 * expired. Along the same line, timestamps need to be assigned the
 * value they had the last time bfqq was selected for service, i.e.,
 * before last expiration. Thus timestamps need to be back-shifted
 * with respect to their normal computation (see [1] for more details
 * on this tricky aspect).
 *
 * Secondly, to allow the process to recover the hole, the in-service
 * queue must be expired too, to give bfqq the chance to preempt it
 * immediately. In fact, if bfqq has to wait for a full budget of the
 * in-service queue to be completed, then it may become impossible to
 * let the process recover the hole, even if the back-shifted
 * timestamps of bfqq are lower than those of the in-service queue. If
 * this happens for most or all of the holes, then the process may not
 * receive its reserved bandwidth. In this respect, it is worth noting
 * that, being the service of outstanding requests unpreemptible, a
 * little fraction of the holes may however be unrecoverable, thereby
 * causing a little loss of bandwidth.
 *
 * The last important point is detecting whether bfqq does need this
 * bandwidth recovery. In this respect, the next function deems the
 * process associated with bfqq greedy, and thus allows it to recover
 * the hole, if: 1) the process is waiting for the arrival of a new
 * request (which implies that bfqq expired for one of the above two
 * reasons), and 2) such a request has arrived soon. The first
 * condition is controlled through the flag non_blocking_wait_rq,
 * while the second through the flag arrived_in_time. If both
 * conditions hold, then the function computes the budget in the
 * above-described special way, and signals that the in-service queue
 * should be expired. Timestamp back-shifting is done later in
 * __bfq_activate_entity.
 *
 * 2. Reduce latency. Even if timestamps are not backshifted to let
 * the process associated with bfqq recover a service hole, bfqq may
 * however happen to have, after being (re)activated, a lower finish
 * timestamp than the in-service queue.  That is, the next budget of
 * bfqq may have to be completed before the one of the in-service
 * queue. If this is the case, then preempting the in-service queue
 * allows this goal to be achieved, apart from the unpreemptible,
 * outstanding requests mentioned above.
 *
 * Unfortunately, regardless of which of the above two goals one wants
 * to achieve, service trees need first to be updated to know whether
 * the in-service queue must be preempted. To have service trees
 * correctly updated, the in-service queue must be expired and
 * rescheduled, and bfqq must be scheduled too. This is one of the
 * most costly operations (in future versions, the scheduling
 * mechanism may be re-designed in such a way to make it possible to
 * know whether preemption is needed without needing to update service
 * trees). In addition, queue preemptions almost always cause random
 * I/O, which may in turn cause loss of throughput. Finally, there may
 * even be no in-service queue when the next function is invoked (so,
 * no queue to compare timestamps with). Because of these facts, the
 * next function adopts the following simple scheme to avoid costly
 * operations, too frequent preemptions and too many dependencies on
 * the state of the scheduler: it requests the expiration of the
 * in-service queue (unconditionally) only for queues that need to
 * recover a hole. Then it delegates to other parts of the code the
 * responsibility of handling the above case 2.
 */
static bool bfq_bfqq_update_budg_for_activation(struct bfq_data *bfqd,
                                                struct bfq_queue *bfqq,
                                                bool arrived_in_time)
{
        struct bfq_entity *entity = &bfqq->entity;

        /*
         * In the next compound condition, we check also whether there
         * is some budget left, because otherwise there is no point in
         * trying to go on serving bfqq with this same budget: bfqq
         * would be expired immediately after being selected for
         * service. This would only cause useless overhead.
         */
        if (bfq_bfqq_non_blocking_wait_rq(bfqq) && arrived_in_time &&
            bfq_bfqq_budget_left(bfqq) > 0) {
                /*
                 * We do not clear the flag non_blocking_wait_rq here, as
                 * the latter is used in bfq_activate_bfqq to signal
                 * that timestamps need to be back-shifted (and is
                 * cleared right after).
                 */

                /*
                 * In next assignment we rely on that either
                 * entity->service or entity->budget are not updated
                 * on expiration if bfqq is empty (see
                 * __bfq_bfqq_recalc_budget). Thus both quantities
                 * remain unchanged after such an expiration, and the
                 * following statement therefore assigns to
                 * entity->budget the remaining budget on such an
                 * expiration.
                 */
                entity->budget = min_t(unsigned long,
                                       bfq_bfqq_budget_left(bfqq),
                                       bfqq->max_budget);

                /*
                 * At this point, we have used entity->service to get
                 * the budget left (needed for updating
                 * entity->budget). Thus we finally can, and have to,
                 * reset entity->service. The latter must be reset
                 * because bfqq would otherwise be charged again for
                 * the service it has received during its previous
                 * service slot(s).
                 */
                entity->service = 0;

                return true;
        }

        /*
         * We can finally complete expiration, by setting service to 0.
         */
        entity->service = 0;
        entity->budget = max_t(unsigned long, bfqq->max_budget,
                               bfq_serv_to_charge(bfqq->next_rq, bfqq));
        bfq_clear_bfqq_non_blocking_wait_rq(bfqq);
        return false;
}

/*
 * Return the farthest past time instant according to jiffies
 * macros.
 */
static unsigned long bfq_smallest_from_now(void)
{
        return jiffies - MAX_JIFFY_OFFSET;
}

static void bfq_update_bfqq_wr_on_rq_arrival(struct bfq_data *bfqd,
                                             struct bfq_queue *bfqq,
                                             unsigned int old_wr_coeff,
                                             bool wr_or_deserves_wr,
                                             bool interactive,
                                             bool in_burst,
                                             bool soft_rt)
{
        if (old_wr_coeff == 1 && wr_or_deserves_wr) {
                /* start a weight-raising period */
                if (interactive) {
                        bfqq->service_from_wr = 0;
                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
                } else {
                        /*
                         * No interactive weight raising in progress
                         * here: assign minus infinity to
                         * wr_start_at_switch_to_srt, to make sure
                         * that, at the end of the soft-real-time
                         * weight raising periods that is starting
                         * now, no interactive weight-raising period
                         * may be wrongly considered as still in
                         * progress (and thus actually started by
                         * mistake).
                         */
                        bfqq->wr_start_at_switch_to_srt =
                                bfq_smallest_from_now();
                        bfqq->wr_coeff = bfqd->bfq_wr_coeff *
                                BFQ_SOFTRT_WEIGHT_FACTOR;
                        bfqq->wr_cur_max_time =
                                bfqd->bfq_wr_rt_max_time;
                }

                /*
                 * If needed, further reduce budget to make sure it is
                 * close to bfqq's backlog, so as to reduce the
                 * scheduling-error component due to a too large
                 * budget. Do not care about throughput consequences,
                 * but only about latency. Finally, do not assign a
                 * too small budget either, to avoid increasing
                 * latency by causing too frequent expirations.
                 */
                bfqq->entity.budget = min_t(unsigned long,
                                            bfqq->entity.budget,
                                            2 * bfq_min_budget(bfqd));
        } else if (old_wr_coeff > 1) {
                if (interactive) { /* update wr coeff and duration */
                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
                } else if (in_burst)
                        bfqq->wr_coeff = 1;
                else if (soft_rt) {
                        /*
                         * The application is now or still meeting the
                         * requirements for being deemed soft rt.  We
                         * can then correctly and safely (re)charge
                         * the weight-raising duration for the
                         * application with the weight-raising
                         * duration for soft rt applications.
                         *
                         * In particular, doing this recharge now, i.e.,
                         * before the weight-raising period for the
                         * application finishes, reduces the probability
                         * of the following negative scenario:
                         * 1) the weight of a soft rt application is
                         *    raised at startup (as for any newly
                         *    created application),
                         * 2) since the application is not interactive,
                         *    at a certain time weight-raising is
                         *    stopped for the application,
                         * 3) at that time the application happens to
                         *    still have pending requests, and hence
                         *    is destined to not have a chance to be
                         *    deemed soft rt before these requests are
                         *    completed (see the comments to the
                         *    function bfq_bfqq_softrt_next_start()
                         *    for details on soft rt detection),
                         * 4) these pending requests experience a high
                         *    latency because the application is not
                         *    weight-raised while they are pending.
                         */
                        if (bfqq->wr_cur_max_time !=
                                bfqd->bfq_wr_rt_max_time) {
                                bfqq->wr_start_at_switch_to_srt =
                                        bfqq->last_wr_start_finish;

                                bfqq->wr_cur_max_time =
                                        bfqd->bfq_wr_rt_max_time;
                                bfqq->wr_coeff = bfqd->bfq_wr_coeff *
                                        BFQ_SOFTRT_WEIGHT_FACTOR;
                        }
                        bfqq->last_wr_start_finish = jiffies;
                }
        }
}

static bool bfq_bfqq_idle_for_long_time(struct bfq_data *bfqd,
                                        struct bfq_queue *bfqq)
{
        return bfqq->dispatched == 0 &&
                time_is_before_jiffies(
                        bfqq->budget_timeout +
                        bfqd->bfq_wr_min_idle_time);
}


/*
 * Return true if bfqq is in a higher priority class, or has a higher
 * weight than the in-service queue.
 */
static bool bfq_bfqq_higher_class_or_weight(struct bfq_queue *bfqq,
                                            struct bfq_queue *in_serv_bfqq)
{
        int bfqq_weight, in_serv_weight;

        if (bfqq->ioprio_class < in_serv_bfqq->ioprio_class)
                return true;

        if (in_serv_bfqq->entity.parent == bfqq->entity.parent) {
                bfqq_weight = bfqq->entity.weight;
                in_serv_weight = in_serv_bfqq->entity.weight;
        } else {
                if (bfqq->entity.parent)
                        bfqq_weight = bfqq->entity.parent->weight;
                else
                        bfqq_weight = bfqq->entity.weight;
                if (in_serv_bfqq->entity.parent)
                        in_serv_weight = in_serv_bfqq->entity.parent->weight;
                else
                        in_serv_weight = in_serv_bfqq->entity.weight;
        }

        return bfqq_weight > in_serv_weight;
}

/*
 * Get the index of the actuator that will serve bio.
 */
static unsigned int bfq_actuator_index(struct bfq_data *bfqd, struct bio *bio)
{
        unsigned int i;
        sector_t end;

        /* no search needed if one or zero ranges present */
        if (bfqd->num_actuators == 1)
                return 0;

        /* bio_end_sector(bio) gives the sector after the last one */
        end = bio_end_sector(bio) - 1;

        for (i = 0; i < bfqd->num_actuators; i++) {
                if (end >= bfqd->sector[i] &&
                    end < bfqd->sector[i] + bfqd->nr_sectors[i])
                        return i;
        }

        WARN_ONCE(true,
                  "bfq_actuator_index: bio sector out of ranges: end=%llu\n",
                  end);
        return 0;
}

static bool bfq_better_to_idle(struct bfq_queue *bfqq);

static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
                                             struct bfq_queue *bfqq,
                                             int old_wr_coeff,
                                             struct request *rq,
                                             bool *interactive)
{
        bool soft_rt, in_burst, wr_or_deserves_wr,
                bfqq_wants_to_preempt,
                idle_for_long_time = bfq_bfqq_idle_for_long_time(bfqd, bfqq),
                /*
                 * See the comments on
                 * bfq_bfqq_update_budg_for_activation for
                 * details on the usage of the next variable.
                 */
                arrived_in_time =  ktime_get_ns() <=
                        bfqq->ttime.last_end_request +
                        bfqd->bfq_slice_idle * 3;
        unsigned int act_idx = bfq_actuator_index(bfqd, rq->bio);
        bool bfqq_non_merged_or_stably_merged =
                bfqq->bic || RQ_BIC(rq)->bfqq_data[act_idx].stably_merged;

        /*
         * bfqq deserves to be weight-raised if:
         * - it is sync,
         * - it does not belong to a large burst,
         * - it has been idle for enough time or is soft real-time,
         * - is linked to a bfq_io_cq (it is not shared in any sense),
         * - has a default weight (otherwise we assume the user wanted
         *   to control its weight explicitly)
         */
        in_burst = bfq_bfqq_in_large_burst(bfqq);
        soft_rt = bfqd->bfq_wr_max_softrt_rate > 0 &&
                !BFQQ_TOTALLY_SEEKY(bfqq) &&
                !in_burst &&
                time_is_before_jiffies(bfqq->soft_rt_next_start) &&
                bfqq->dispatched == 0 &&
                bfqq->entity.new_weight == 40;
        *interactive = !in_burst && idle_for_long_time &&
                bfqq->entity.new_weight == 40;
        /*
         * Merged bfq_queues are kept out of weight-raising
         * (low-latency) mechanisms. The reason is that these queues
         * are usually created for non-interactive and
         * non-soft-real-time tasks. Yet this is not the case for
         * stably-merged queues. These queues are merged just because
         * they are created shortly after each other. So they may
         * easily serve the I/O of an interactive or soft-real time
         * application, if the application happens to spawn multiple
         * processes. So let also stably-merged queued enjoy weight
         * raising.
         */
        wr_or_deserves_wr = bfqd->low_latency &&
                (bfqq->wr_coeff > 1 ||
                 (bfq_bfqq_sync(bfqq) && bfqq_non_merged_or_stably_merged &&
                  (*interactive || soft_rt)));

        /*
         * Using the last flag, update budget and check whether bfqq
         * may want to preempt the in-service queue.
         */
        bfqq_wants_to_preempt =
                bfq_bfqq_update_budg_for_activation(bfqd, bfqq,
                                                    arrived_in_time);

        /*
         * If bfqq happened to be activated in a burst, but has been
         * idle for much more than an interactive queue, then we
         * assume that, in the overall I/O initiated in the burst, the
         * I/O associated with bfqq is finished. So bfqq does not need
         * to be treated as a queue belonging to a burst
         * anymore. Accordingly, we reset bfqq's in_large_burst flag
         * if set, and remove bfqq from the burst list if it's
         * there. We do not decrement burst_size, because the fact
         * that bfqq does not need to belong to the burst list any
         * more does not invalidate the fact that bfqq was created in
         * a burst.
         */
        if (likely(!bfq_bfqq_just_created(bfqq)) &&
            idle_for_long_time &&
            time_is_before_jiffies(
                    bfqq->budget_timeout +
                    msecs_to_jiffies(10000))) {
                hlist_del_init(&bfqq->burst_list_node);
                bfq_clear_bfqq_in_large_burst(bfqq);
        }

        bfq_clear_bfqq_just_created(bfqq);

        if (bfqd->low_latency) {
                if (unlikely(time_is_after_jiffies(bfqq->split_time)))
                        /* wraparound */
                        bfqq->split_time =
                                jiffies - bfqd->bfq_wr_min_idle_time - 1;

                if (time_is_before_jiffies(bfqq->split_time +
                                           bfqd->bfq_wr_min_idle_time)) {
                        bfq_update_bfqq_wr_on_rq_arrival(bfqd, bfqq,
                                                         old_wr_coeff,
                                                         wr_or_deserves_wr,
                                                         *interactive,
                                                         in_burst,
                                                         soft_rt);

                        if (old_wr_coeff != bfqq->wr_coeff)
                                bfqq->entity.prio_changed = 1;
                }
        }

        bfqq->last_idle_bklogged = jiffies;
        bfqq->service_from_backlogged = 0;
        bfq_clear_bfqq_softrt_update(bfqq);

        bfq_add_bfqq_busy(bfqq);

        /*
         * Expire in-service queue if preemption may be needed for
         * guarantees or throughput. As for guarantees, we care
         * explicitly about two cases. The first is that bfqq has to
         * recover a service hole, as explained in the comments on
         * bfq_bfqq_update_budg_for_activation(), i.e., that
         * bfqq_wants_to_preempt is true. However, if bfqq does not
         * carry time-critical I/O, then bfqq's bandwidth is less
         * important than that of queues that carry time-critical I/O.
         * So, as a further constraint, we consider this case only if
         * bfqq is at least as weight-raised, i.e., at least as time
         * critical, as the in-service queue.
         *
         * The second case is that bfqq is in a higher priority class,
         * or has a higher weight than the in-service queue. If this
         * condition does not hold, we don't care because, even if
         * bfqq does not start to be served immediately, the resulting
         * delay for bfqq's I/O is however lower or much lower than
         * the ideal completion time to be guaranteed to bfqq's I/O.
         *
         * In both cases, preemption is needed only if, according to
         * the timestamps of both bfqq and of the in-service queue,
         * bfqq actually is the next queue to serve. So, to reduce
         * useless preemptions, the return value of
         * next_queue_may_preempt() is considered in the next compound
         * condition too. Yet next_queue_may_preempt() just checks a
         * simple, necessary condition for bfqq to be the next queue
         * to serve. In fact, to evaluate a sufficient condition, the
         * timestamps of the in-service queue would need to be
         * updated, and this operation is quite costly (see the
         * comments on bfq_bfqq_update_budg_for_activation()).
         *
         * As for throughput, we ask bfq_better_to_idle() whether we
         * still need to plug I/O dispatching. If bfq_better_to_idle()
         * says no, then plugging is not needed any longer, either to
         * boost throughput or to perserve service guarantees. Then
         * the best option is to stop plugging I/O, as not doing so
         * would certainly lower throughput. We may end up in this
         * case if: (1) upon a dispatch attempt, we detected that it
         * was better to plug I/O dispatch, and to wait for a new
         * request to arrive for the currently in-service queue, but
         * (2) this switch of bfqq to busy changes the scenario.
         */
        if (bfqd->in_service_queue &&
            ((bfqq_wants_to_preempt &&
              bfqq->wr_coeff >= bfqd->in_service_queue->wr_coeff) ||
             bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue) ||
             !bfq_better_to_idle(bfqd->in_service_queue)) &&
            next_queue_may_preempt(bfqd))
                bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
                                false, BFQQE_PREEMPTED);
}

static void bfq_reset_inject_limit(struct bfq_data *bfqd,
                                   struct bfq_queue *bfqq)
{
        /* invalidate baseline total service time */
        bfqq->last_serv_time_ns = 0;

        /*
         * Reset pointer in case we are waiting for
         * some request completion.
         */
        bfqd->waited_rq = NULL;

        /*
         * If bfqq has a short think time, then start by setting the
         * inject limit to 0 prudentially, because the service time of
         * an injected I/O request may be higher than the think time