linux/net/sched/sch_pie.c
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   1// SPDX-License-Identifier: GPL-2.0-only
   2/* Copyright (C) 2013 Cisco Systems, Inc, 2013.
   3 *
   4 * Author: Vijay Subramanian <vijaynsu@cisco.com>
   5 * Author: Mythili Prabhu <mysuryan@cisco.com>
   6 *
   7 * ECN support is added by Naeem Khademi <naeemk@ifi.uio.no>
   8 * University of Oslo, Norway.
   9 *
  10 * References:
  11 * RFC 8033: https://tools.ietf.org/html/rfc8033
  12 */
  13
  14#include <linux/module.h>
  15#include <linux/slab.h>
  16#include <linux/types.h>
  17#include <linux/kernel.h>
  18#include <linux/errno.h>
  19#include <linux/skbuff.h>
  20#include <net/pkt_sched.h>
  21#include <net/inet_ecn.h>
  22#include <net/pie.h>
  23
  24/* private data for the Qdisc */
  25struct pie_sched_data {
  26        struct pie_vars vars;
  27        struct pie_params params;
  28        struct pie_stats stats;
  29        struct timer_list adapt_timer;
  30        struct Qdisc *sch;
  31};
  32
  33bool pie_drop_early(struct Qdisc *sch, struct pie_params *params,
  34                    struct pie_vars *vars, u32 backlog, u32 packet_size)
  35{
  36        u64 rnd;
  37        u64 local_prob = vars->prob;
  38        u32 mtu = psched_mtu(qdisc_dev(sch));
  39
  40        /* If there is still burst allowance left skip random early drop */
  41        if (vars->burst_time > 0)
  42                return false;
  43
  44        /* If current delay is less than half of target, and
  45         * if drop prob is low already, disable early_drop
  46         */
  47        if ((vars->qdelay < params->target / 2) &&
  48            (vars->prob < MAX_PROB / 5))
  49                return false;
  50
  51        /* If we have fewer than 2 mtu-sized packets, disable pie_drop_early,
  52         * similar to min_th in RED
  53         */
  54        if (backlog < 2 * mtu)
  55                return false;
  56
  57        /* If bytemode is turned on, use packet size to compute new
  58         * probablity. Smaller packets will have lower drop prob in this case
  59         */
  60        if (params->bytemode && packet_size <= mtu)
  61                local_prob = (u64)packet_size * div_u64(local_prob, mtu);
  62        else
  63                local_prob = vars->prob;
  64
  65        if (local_prob == 0)
  66                vars->accu_prob = 0;
  67        else
  68                vars->accu_prob += local_prob;
  69
  70        if (vars->accu_prob < (MAX_PROB / 100) * 85)
  71                return false;
  72        if (vars->accu_prob >= (MAX_PROB / 2) * 17)
  73                return true;
  74
  75        prandom_bytes(&rnd, 8);
  76        if ((rnd >> BITS_PER_BYTE) < local_prob) {
  77                vars->accu_prob = 0;
  78                return true;
  79        }
  80
  81        return false;
  82}
  83EXPORT_SYMBOL_GPL(pie_drop_early);
  84
  85static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
  86                             struct sk_buff **to_free)
  87{
  88        struct pie_sched_data *q = qdisc_priv(sch);
  89        bool enqueue = false;
  90
  91        if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
  92                q->stats.overlimit++;
  93                goto out;
  94        }
  95
  96        if (!pie_drop_early(sch, &q->params, &q->vars, sch->qstats.backlog,
  97                            skb->len)) {
  98                enqueue = true;
  99        } else if (q->params.ecn && (q->vars.prob <= MAX_PROB / 10) &&
 100                   INET_ECN_set_ce(skb)) {
 101                /* If packet is ecn capable, mark it if drop probability
 102                 * is lower than 10%, else drop it.
 103                 */
 104                q->stats.ecn_mark++;
 105                enqueue = true;
 106        }
 107
 108        /* we can enqueue the packet */
 109        if (enqueue) {
 110                /* Set enqueue time only when dq_rate_estimator is disabled. */
 111                if (!q->params.dq_rate_estimator)
 112                        pie_set_enqueue_time(skb);
 113
 114                q->stats.packets_in++;
 115                if (qdisc_qlen(sch) > q->stats.maxq)
 116                        q->stats.maxq = qdisc_qlen(sch);
 117
 118                return qdisc_enqueue_tail(skb, sch);
 119        }
 120
 121out:
 122        q->stats.dropped++;
 123        q->vars.accu_prob = 0;
 124        return qdisc_drop(skb, sch, to_free);
 125}
 126
 127static const struct nla_policy pie_policy[TCA_PIE_MAX + 1] = {
 128        [TCA_PIE_TARGET]                = {.type = NLA_U32},
 129        [TCA_PIE_LIMIT]                 = {.type = NLA_U32},
 130        [TCA_PIE_TUPDATE]               = {.type = NLA_U32},
 131        [TCA_PIE_ALPHA]                 = {.type = NLA_U32},
 132        [TCA_PIE_BETA]                  = {.type = NLA_U32},
 133        [TCA_PIE_ECN]                   = {.type = NLA_U32},
 134        [TCA_PIE_BYTEMODE]              = {.type = NLA_U32},
 135        [TCA_PIE_DQ_RATE_ESTIMATOR]     = {.type = NLA_U32},
 136};
 137
 138static int pie_change(struct Qdisc *sch, struct nlattr *opt,
 139                      struct netlink_ext_ack *extack)
 140{
 141        struct pie_sched_data *q = qdisc_priv(sch);
 142        struct nlattr *tb[TCA_PIE_MAX + 1];
 143        unsigned int qlen, dropped = 0;
 144        int err;
 145
 146        if (!opt)
 147                return -EINVAL;
 148
 149        err = nla_parse_nested_deprecated(tb, TCA_PIE_MAX, opt, pie_policy,
 150                                          NULL);
 151        if (err < 0)
 152                return err;
 153
 154        sch_tree_lock(sch);
 155
 156        /* convert from microseconds to pschedtime */
 157        if (tb[TCA_PIE_TARGET]) {
 158                /* target is in us */
 159                u32 target = nla_get_u32(tb[TCA_PIE_TARGET]);
 160
 161                /* convert to pschedtime */
 162                q->params.target = PSCHED_NS2TICKS((u64)target * NSEC_PER_USEC);
 163        }
 164
 165        /* tupdate is in jiffies */
 166        if (tb[TCA_PIE_TUPDATE])
 167                q->params.tupdate =
 168                        usecs_to_jiffies(nla_get_u32(tb[TCA_PIE_TUPDATE]));
 169
 170        if (tb[TCA_PIE_LIMIT]) {
 171                u32 limit = nla_get_u32(tb[TCA_PIE_LIMIT]);
 172
 173                q->params.limit = limit;
 174                sch->limit = limit;
 175        }
 176
 177        if (tb[TCA_PIE_ALPHA])
 178                q->params.alpha = nla_get_u32(tb[TCA_PIE_ALPHA]);
 179
 180        if (tb[TCA_PIE_BETA])
 181                q->params.beta = nla_get_u32(tb[TCA_PIE_BETA]);
 182
 183        if (tb[TCA_PIE_ECN])
 184                q->params.ecn = nla_get_u32(tb[TCA_PIE_ECN]);
 185
 186        if (tb[TCA_PIE_BYTEMODE])
 187                q->params.bytemode = nla_get_u32(tb[TCA_PIE_BYTEMODE]);
 188
 189        if (tb[TCA_PIE_DQ_RATE_ESTIMATOR])
 190                q->params.dq_rate_estimator =
 191                                nla_get_u32(tb[TCA_PIE_DQ_RATE_ESTIMATOR]);
 192
 193        /* Drop excess packets if new limit is lower */
 194        qlen = sch->q.qlen;
 195        while (sch->q.qlen > sch->limit) {
 196                struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);
 197
 198                dropped += qdisc_pkt_len(skb);
 199                qdisc_qstats_backlog_dec(sch, skb);
 200                rtnl_qdisc_drop(skb, sch);
 201        }
 202        qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, dropped);
 203
 204        sch_tree_unlock(sch);
 205        return 0;
 206}
 207
 208void pie_process_dequeue(struct sk_buff *skb, struct pie_params *params,
 209                         struct pie_vars *vars, u32 backlog)
 210{
 211        psched_time_t now = psched_get_time();
 212        u32 dtime = 0;
 213
 214        /* If dq_rate_estimator is disabled, calculate qdelay using the
 215         * packet timestamp.
 216         */
 217        if (!params->dq_rate_estimator) {
 218                vars->qdelay = now - pie_get_enqueue_time(skb);
 219
 220                if (vars->dq_tstamp != DTIME_INVALID)
 221                        dtime = now - vars->dq_tstamp;
 222
 223                vars->dq_tstamp = now;
 224
 225                if (backlog == 0)
 226                        vars->qdelay = 0;
 227
 228                if (dtime == 0)
 229                        return;
 230
 231                goto burst_allowance_reduction;
 232        }
 233
 234        /* If current queue is about 10 packets or more and dq_count is unset
 235         * we have enough packets to calculate the drain rate. Save
 236         * current time as dq_tstamp and start measurement cycle.
 237         */
 238        if (backlog >= QUEUE_THRESHOLD && vars->dq_count == DQCOUNT_INVALID) {
 239                vars->dq_tstamp = psched_get_time();
 240                vars->dq_count = 0;
 241        }
 242
 243        /* Calculate the average drain rate from this value. If queue length
 244         * has receded to a small value viz., <= QUEUE_THRESHOLD bytes, reset
 245         * the dq_count to -1 as we don't have enough packets to calculate the
 246         * drain rate anymore. The following if block is entered only when we
 247         * have a substantial queue built up (QUEUE_THRESHOLD bytes or more)
 248         * and we calculate the drain rate for the threshold here.  dq_count is
 249         * in bytes, time difference in psched_time, hence rate is in
 250         * bytes/psched_time.
 251         */
 252        if (vars->dq_count != DQCOUNT_INVALID) {
 253                vars->dq_count += skb->len;
 254
 255                if (vars->dq_count >= QUEUE_THRESHOLD) {
 256                        u32 count = vars->dq_count << PIE_SCALE;
 257
 258                        dtime = now - vars->dq_tstamp;
 259
 260                        if (dtime == 0)
 261                                return;
 262
 263                        count = count / dtime;
 264
 265                        if (vars->avg_dq_rate == 0)
 266                                vars->avg_dq_rate = count;
 267                        else
 268                                vars->avg_dq_rate =
 269                                    (vars->avg_dq_rate -
 270                                     (vars->avg_dq_rate >> 3)) + (count >> 3);
 271
 272                        /* If the queue has receded below the threshold, we hold
 273                         * on to the last drain rate calculated, else we reset
 274                         * dq_count to 0 to re-enter the if block when the next
 275                         * packet is dequeued
 276                         */
 277                        if (backlog < QUEUE_THRESHOLD) {
 278                                vars->dq_count = DQCOUNT_INVALID;
 279                        } else {
 280                                vars->dq_count = 0;
 281                                vars->dq_tstamp = psched_get_time();
 282                        }
 283
 284                        goto burst_allowance_reduction;
 285                }
 286        }
 287
 288        return;
 289
 290burst_allowance_reduction:
 291        if (vars->burst_time > 0) {
 292                if (vars->burst_time > dtime)
 293                        vars->burst_time -= dtime;
 294                else
 295                        vars->burst_time = 0;
 296        }
 297}
 298EXPORT_SYMBOL_GPL(pie_process_dequeue);
 299
 300void pie_calculate_probability(struct pie_params *params, struct pie_vars *vars,
 301                               u32 backlog)
 302{
 303        psched_time_t qdelay = 0;       /* in pschedtime */
 304        psched_time_t qdelay_old = 0;   /* in pschedtime */
 305        s64 delta = 0;          /* determines the change in probability */
 306        u64 oldprob;
 307        u64 alpha, beta;
 308        u32 power;
 309        bool update_prob = true;
 310
 311        if (params->dq_rate_estimator) {
 312                qdelay_old = vars->qdelay;
 313                vars->qdelay_old = vars->qdelay;
 314
 315                if (vars->avg_dq_rate > 0)
 316                        qdelay = (backlog << PIE_SCALE) / vars->avg_dq_rate;
 317                else
 318                        qdelay = 0;
 319        } else {
 320                qdelay = vars->qdelay;
 321                qdelay_old = vars->qdelay_old;
 322        }
 323
 324        /* If qdelay is zero and backlog is not, it means backlog is very small,
 325         * so we do not update probabilty in this round.
 326         */
 327        if (qdelay == 0 && backlog != 0)
 328                update_prob = false;
 329
 330        /* In the algorithm, alpha and beta are between 0 and 2 with typical
 331         * value for alpha as 0.125. In this implementation, we use values 0-32
 332         * passed from user space to represent this. Also, alpha and beta have
 333         * unit of HZ and need to be scaled before they can used to update
 334         * probability. alpha/beta are updated locally below by scaling down
 335         * by 16 to come to 0-2 range.
 336         */
 337        alpha = ((u64)params->alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
 338        beta = ((u64)params->beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
 339
 340        /* We scale alpha and beta differently depending on how heavy the
 341         * congestion is. Please see RFC 8033 for details.
 342         */
 343        if (vars->prob < MAX_PROB / 10) {
 344                alpha >>= 1;
 345                beta >>= 1;
 346
 347                power = 100;
 348                while (vars->prob < div_u64(MAX_PROB, power) &&
 349                       power <= 1000000) {
 350                        alpha >>= 2;
 351                        beta >>= 2;
 352                        power *= 10;
 353                }
 354        }
 355
 356        /* alpha and beta should be between 0 and 32, in multiples of 1/16 */
 357        delta += alpha * (qdelay - params->target);
 358        delta += beta * (qdelay - qdelay_old);
 359
 360        oldprob = vars->prob;
 361
 362        /* to ensure we increase probability in steps of no more than 2% */
 363        if (delta > (s64)(MAX_PROB / (100 / 2)) &&
 364            vars->prob >= MAX_PROB / 10)
 365                delta = (MAX_PROB / 100) * 2;
 366
 367        /* Non-linear drop:
 368         * Tune drop probability to increase quickly for high delays(>= 250ms)
 369         * 250ms is derived through experiments and provides error protection
 370         */
 371
 372        if (qdelay > (PSCHED_NS2TICKS(250 * NSEC_PER_MSEC)))
 373                delta += MAX_PROB / (100 / 2);
 374
 375        vars->prob += delta;
 376
 377        if (delta > 0) {
 378                /* prevent overflow */
 379                if (vars->prob < oldprob) {
 380                        vars->prob = MAX_PROB;
 381                        /* Prevent normalization error. If probability is at
 382                         * maximum value already, we normalize it here, and
 383                         * skip the check to do a non-linear drop in the next
 384                         * section.
 385                         */
 386                        update_prob = false;
 387                }
 388        } else {
 389                /* prevent underflow */
 390                if (vars->prob > oldprob)
 391                        vars->prob = 0;
 392        }
 393
 394        /* Non-linear drop in probability: Reduce drop probability quickly if
 395         * delay is 0 for 2 consecutive Tupdate periods.
 396         */
 397
 398        if (qdelay == 0 && qdelay_old == 0 && update_prob)
 399                /* Reduce drop probability to 98.4% */
 400                vars->prob -= vars->prob / 64;
 401
 402        vars->qdelay = qdelay;
 403        vars->backlog_old = backlog;
 404
 405        /* We restart the measurement cycle if the following conditions are met
 406         * 1. If the delay has been low for 2 consecutive Tupdate periods
 407         * 2. Calculated drop probability is zero
 408         * 3. If average dq_rate_estimator is enabled, we have at least one
 409         *    estimate for the avg_dq_rate ie., is a non-zero value
 410         */
 411        if ((vars->qdelay < params->target / 2) &&
 412            (vars->qdelay_old < params->target / 2) &&
 413            vars->prob == 0 &&
 414            (!params->dq_rate_estimator || vars->avg_dq_rate > 0)) {
 415                pie_vars_init(vars);
 416        }
 417
 418        if (!params->dq_rate_estimator)
 419                vars->qdelay_old = qdelay;
 420}
 421EXPORT_SYMBOL_GPL(pie_calculate_probability);
 422
 423static void pie_timer(struct timer_list *t)
 424{
 425        struct pie_sched_data *q = from_timer(q, t, adapt_timer);
 426        struct Qdisc *sch = q->sch;
 427        spinlock_t *root_lock = qdisc_lock(qdisc_root_sleeping(sch));
 428
 429        spin_lock(root_lock);
 430        pie_calculate_probability(&q->params, &q->vars, sch->qstats.backlog);
 431
 432        /* reset the timer to fire after 'tupdate'. tupdate is in jiffies. */
 433        if (q->params.tupdate)
 434                mod_timer(&q->adapt_timer, jiffies + q->params.tupdate);
 435        spin_unlock(root_lock);
 436}
 437
 438static int pie_init(struct Qdisc *sch, struct nlattr *opt,
 439                    struct netlink_ext_ack *extack)
 440{
 441        struct pie_sched_data *q = qdisc_priv(sch);
 442
 443        pie_params_init(&q->params);
 444        pie_vars_init(&q->vars);
 445        sch->limit = q->params.limit;
 446
 447        q->sch = sch;
 448        timer_setup(&q->adapt_timer, pie_timer, 0);
 449
 450        if (opt) {
 451                int err = pie_change(sch, opt, extack);
 452
 453                if (err)
 454                        return err;
 455        }
 456
 457        mod_timer(&q->adapt_timer, jiffies + HZ / 2);
 458        return 0;
 459}
 460
 461static int pie_dump(struct Qdisc *sch, struct sk_buff *skb)
 462{
 463        struct pie_sched_data *q = qdisc_priv(sch);
 464        struct nlattr *opts;
 465
 466        opts = nla_nest_start_noflag(skb, TCA_OPTIONS);
 467        if (!opts)
 468                goto nla_put_failure;
 469
 470        /* convert target from pschedtime to us */
 471        if (nla_put_u32(skb, TCA_PIE_TARGET,
 472                        ((u32)PSCHED_TICKS2NS(q->params.target)) /
 473                        NSEC_PER_USEC) ||
 474            nla_put_u32(skb, TCA_PIE_LIMIT, sch->limit) ||
 475            nla_put_u32(skb, TCA_PIE_TUPDATE,
 476                        jiffies_to_usecs(q->params.tupdate)) ||
 477            nla_put_u32(skb, TCA_PIE_ALPHA, q->params.alpha) ||
 478            nla_put_u32(skb, TCA_PIE_BETA, q->params.beta) ||
 479            nla_put_u32(skb, TCA_PIE_ECN, q->params.ecn) ||
 480            nla_put_u32(skb, TCA_PIE_BYTEMODE, q->params.bytemode) ||
 481            nla_put_u32(skb, TCA_PIE_DQ_RATE_ESTIMATOR,
 482                        q->params.dq_rate_estimator))
 483                goto nla_put_failure;
 484
 485        return nla_nest_end(skb, opts);
 486
 487nla_put_failure:
 488        nla_nest_cancel(skb, opts);
 489        return -1;
 490}
 491
 492static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
 493{
 494        struct pie_sched_data *q = qdisc_priv(sch);
 495        struct tc_pie_xstats st = {
 496                .prob           = q->vars.prob << BITS_PER_BYTE,
 497                .delay          = ((u32)PSCHED_TICKS2NS(q->vars.qdelay)) /
 498                                   NSEC_PER_USEC,
 499                .packets_in     = q->stats.packets_in,
 500                .overlimit      = q->stats.overlimit,
 501                .maxq           = q->stats.maxq,
 502                .dropped        = q->stats.dropped,
 503                .ecn_mark       = q->stats.ecn_mark,
 504        };
 505
 506        /* avg_dq_rate is only valid if dq_rate_estimator is enabled */
 507        st.dq_rate_estimating = q->params.dq_rate_estimator;
 508
 509        /* unscale and return dq_rate in bytes per sec */
 510        if (q->params.dq_rate_estimator)
 511                st.avg_dq_rate = q->vars.avg_dq_rate *
 512                                 (PSCHED_TICKS_PER_SEC) >> PIE_SCALE;
 513
 514        return gnet_stats_copy_app(d, &st, sizeof(st));
 515}
 516
 517static struct sk_buff *pie_qdisc_dequeue(struct Qdisc *sch)
 518{
 519        struct pie_sched_data *q = qdisc_priv(sch);
 520        struct sk_buff *skb = qdisc_dequeue_head(sch);
 521
 522        if (!skb)
 523                return NULL;
 524
 525        pie_process_dequeue(skb, &q->params, &q->vars, sch->qstats.backlog);
 526        return skb;
 527}
 528
 529static void pie_reset(struct Qdisc *sch)
 530{
 531        struct pie_sched_data *q = qdisc_priv(sch);
 532
 533        qdisc_reset_queue(sch);
 534        pie_vars_init(&q->vars);
 535}
 536
 537static void pie_destroy(struct Qdisc *sch)
 538{
 539        struct pie_sched_data *q = qdisc_priv(sch);
 540
 541        q->params.tupdate = 0;
 542        del_timer_sync(&q->adapt_timer);
 543}
 544
 545static struct Qdisc_ops pie_qdisc_ops __read_mostly = {
 546        .id             = "pie",
 547        .priv_size      = sizeof(struct pie_sched_data),
 548        .enqueue        = pie_qdisc_enqueue,
 549        .dequeue        = pie_qdisc_dequeue,
 550        .peek           = qdisc_peek_dequeued,
 551        .init           = pie_init,
 552        .destroy        = pie_destroy,
 553        .reset          = pie_reset,
 554        .change         = pie_change,
 555        .dump           = pie_dump,
 556        .dump_stats     = pie_dump_stats,
 557        .owner          = THIS_MODULE,
 558};
 559
 560static int __init pie_module_init(void)
 561{
 562        return register_qdisc(&pie_qdisc_ops);
 563}
 564
 565static void __exit pie_module_exit(void)
 566{
 567        unregister_qdisc(&pie_qdisc_ops);
 568}
 569
 570module_init(pie_module_init);
 571module_exit(pie_module_exit);
 572
 573MODULE_DESCRIPTION("Proportional Integral controller Enhanced (PIE) scheduler");
 574MODULE_AUTHOR("Vijay Subramanian");
 575MODULE_AUTHOR("Mythili Prabhu");
 576MODULE_LICENSE("GPL");
 577