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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 */
 25 struct 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 
 33 bool 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 }
 83 EXPORT_SYMBOL_GPL(pie_drop_early);
 84 
 85 static 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 
121 out:
122         q->stats.dropped++;
123         q->vars.accu_prob = 0;
124         return qdisc_drop(skb, sch, to_free);
125 }
126 
127 static 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 
138 static 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 
208 void 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 
290 burst_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 }
298 EXPORT_SYMBOL_GPL(pie_process_dequeue);
299 
300 void 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 atleast 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 }
421 EXPORT_SYMBOL_GPL(pie_calculate_probability);
422 
423 static 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 
438 static 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 
461 static 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 
487 nla_put_failure:
488         nla_nest_cancel(skb, opts);
489         return -1;
490 }
491 
492 static 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 
517 static 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 
529 static 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 
537 static 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 
545 static 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 
560 static int __init pie_module_init(void)
561 {
562         return register_qdisc(&pie_qdisc_ops);
563 }
564 
565 static void __exit pie_module_exit(void)
566 {
567         unregister_qdisc(&pie_qdisc_ops);
568 }
569 
570 module_init(pie_module_init);
571 module_exit(pie_module_exit);
572 
573 MODULE_DESCRIPTION("Proportional Integral controller Enhanced (PIE) scheduler");
574 MODULE_AUTHOR("Vijay Subramanian");
575 MODULE_AUTHOR("Mythili Prabhu");
576 MODULE_LICENSE("GPL");
577 

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