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TOMOYO Linux Cross Reference
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 
 23 #define QUEUE_THRESHOLD 16384
 24 #define DQCOUNT_INVALID -1
 25 #define MAX_PROB 0xffffffffffffffff
 26 #define PIE_SCALE 8
 27 
 28 /* parameters used */
 29 struct pie_params {
 30         psched_time_t target;   /* user specified target delay in pschedtime */
 31         u32 tupdate;            /* timer frequency (in jiffies) */
 32         u32 limit;              /* number of packets that can be enqueued */
 33         u32 alpha;              /* alpha and beta are between 0 and 32 */
 34         u32 beta;               /* and are used for shift relative to 1 */
 35         bool ecn;               /* true if ecn is enabled */
 36         bool bytemode;          /* to scale drop early prob based on pkt size */
 37 };
 38 
 39 /* variables used */
 40 struct pie_vars {
 41         u64 prob;               /* probability but scaled by u64 limit. */
 42         psched_time_t burst_time;
 43         psched_time_t qdelay;
 44         psched_time_t qdelay_old;
 45         u64 dq_count;           /* measured in bytes */
 46         psched_time_t dq_tstamp;        /* drain rate */
 47         u64 accu_prob;          /* accumulated drop probability */
 48         u32 avg_dq_rate;        /* bytes per pschedtime tick,scaled */
 49         u32 qlen_old;           /* in bytes */
 50         u8 accu_prob_overflows; /* overflows of accu_prob */
 51 };
 52 
 53 /* statistics gathering */
 54 struct pie_stats {
 55         u32 packets_in;         /* total number of packets enqueued */
 56         u32 dropped;            /* packets dropped due to pie_action */
 57         u32 overlimit;          /* dropped due to lack of space in queue */
 58         u32 maxq;               /* maximum queue size */
 59         u32 ecn_mark;           /* packets marked with ECN */
 60 };
 61 
 62 /* private data for the Qdisc */
 63 struct pie_sched_data {
 64         struct pie_params params;
 65         struct pie_vars vars;
 66         struct pie_stats stats;
 67         struct timer_list adapt_timer;
 68         struct Qdisc *sch;
 69 };
 70 
 71 static void pie_params_init(struct pie_params *params)
 72 {
 73         params->alpha = 2;
 74         params->beta = 20;
 75         params->tupdate = usecs_to_jiffies(15 * USEC_PER_MSEC); /* 15 ms */
 76         params->limit = 1000;   /* default of 1000 packets */
 77         params->target = PSCHED_NS2TICKS(15 * NSEC_PER_MSEC);   /* 15 ms */
 78         params->ecn = false;
 79         params->bytemode = false;
 80 }
 81 
 82 static void pie_vars_init(struct pie_vars *vars)
 83 {
 84         vars->dq_count = DQCOUNT_INVALID;
 85         vars->accu_prob = 0;
 86         vars->avg_dq_rate = 0;
 87         /* default of 150 ms in pschedtime */
 88         vars->burst_time = PSCHED_NS2TICKS(150 * NSEC_PER_MSEC);
 89         vars->accu_prob_overflows = 0;
 90 }
 91 
 92 static bool drop_early(struct Qdisc *sch, u32 packet_size)
 93 {
 94         struct pie_sched_data *q = qdisc_priv(sch);
 95         u64 rnd;
 96         u64 local_prob = q->vars.prob;
 97         u32 mtu = psched_mtu(qdisc_dev(sch));
 98 
 99         /* If there is still burst allowance left skip random early drop */
100         if (q->vars.burst_time > 0)
101                 return false;
102 
103         /* If current delay is less than half of target, and
104          * if drop prob is low already, disable early_drop
105          */
106         if ((q->vars.qdelay < q->params.target / 2) &&
107             (q->vars.prob < MAX_PROB / 5))
108                 return false;
109 
110         /* If we have fewer than 2 mtu-sized packets, disable drop_early,
111          * similar to min_th in RED
112          */
113         if (sch->qstats.backlog < 2 * mtu)
114                 return false;
115 
116         /* If bytemode is turned on, use packet size to compute new
117          * probablity. Smaller packets will have lower drop prob in this case
118          */
119         if (q->params.bytemode && packet_size <= mtu)
120                 local_prob = (u64)packet_size * div_u64(local_prob, mtu);
121         else
122                 local_prob = q->vars.prob;
123 
124         if (local_prob == 0) {
125                 q->vars.accu_prob = 0;
126                 q->vars.accu_prob_overflows = 0;
127         }
128 
129         if (local_prob > MAX_PROB - q->vars.accu_prob)
130                 q->vars.accu_prob_overflows++;
131 
132         q->vars.accu_prob += local_prob;
133 
134         if (q->vars.accu_prob_overflows == 0 &&
135             q->vars.accu_prob < (MAX_PROB / 100) * 85)
136                 return false;
137         if (q->vars.accu_prob_overflows == 8 &&
138             q->vars.accu_prob >= MAX_PROB / 2)
139                 return true;
140 
141         prandom_bytes(&rnd, 8);
142         if (rnd < local_prob) {
143                 q->vars.accu_prob = 0;
144                 q->vars.accu_prob_overflows = 0;
145                 return true;
146         }
147 
148         return false;
149 }
150 
151 static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
152                              struct sk_buff **to_free)
153 {
154         struct pie_sched_data *q = qdisc_priv(sch);
155         bool enqueue = false;
156 
157         if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
158                 q->stats.overlimit++;
159                 goto out;
160         }
161 
162         if (!drop_early(sch, skb->len)) {
163                 enqueue = true;
164         } else if (q->params.ecn && (q->vars.prob <= MAX_PROB / 10) &&
165                    INET_ECN_set_ce(skb)) {
166                 /* If packet is ecn capable, mark it if drop probability
167                  * is lower than 10%, else drop it.
168                  */
169                 q->stats.ecn_mark++;
170                 enqueue = true;
171         }
172 
173         /* we can enqueue the packet */
174         if (enqueue) {
175                 q->stats.packets_in++;
176                 if (qdisc_qlen(sch) > q->stats.maxq)
177                         q->stats.maxq = qdisc_qlen(sch);
178 
179                 return qdisc_enqueue_tail(skb, sch);
180         }
181 
182 out:
183         q->stats.dropped++;
184         q->vars.accu_prob = 0;
185         q->vars.accu_prob_overflows = 0;
186         return qdisc_drop(skb, sch, to_free);
187 }
188 
189 static const struct nla_policy pie_policy[TCA_PIE_MAX + 1] = {
190         [TCA_PIE_TARGET] = {.type = NLA_U32},
191         [TCA_PIE_LIMIT] = {.type = NLA_U32},
192         [TCA_PIE_TUPDATE] = {.type = NLA_U32},
193         [TCA_PIE_ALPHA] = {.type = NLA_U32},
194         [TCA_PIE_BETA] = {.type = NLA_U32},
195         [TCA_PIE_ECN] = {.type = NLA_U32},
196         [TCA_PIE_BYTEMODE] = {.type = NLA_U32},
197 };
198 
199 static int pie_change(struct Qdisc *sch, struct nlattr *opt,
200                       struct netlink_ext_ack *extack)
201 {
202         struct pie_sched_data *q = qdisc_priv(sch);
203         struct nlattr *tb[TCA_PIE_MAX + 1];
204         unsigned int qlen, dropped = 0;
205         int err;
206 
207         if (!opt)
208                 return -EINVAL;
209 
210         err = nla_parse_nested_deprecated(tb, TCA_PIE_MAX, opt, pie_policy,
211                                           NULL);
212         if (err < 0)
213                 return err;
214 
215         sch_tree_lock(sch);
216 
217         /* convert from microseconds to pschedtime */
218         if (tb[TCA_PIE_TARGET]) {
219                 /* target is in us */
220                 u32 target = nla_get_u32(tb[TCA_PIE_TARGET]);
221 
222                 /* convert to pschedtime */
223                 q->params.target = PSCHED_NS2TICKS((u64)target * NSEC_PER_USEC);
224         }
225 
226         /* tupdate is in jiffies */
227         if (tb[TCA_PIE_TUPDATE])
228                 q->params.tupdate =
229                         usecs_to_jiffies(nla_get_u32(tb[TCA_PIE_TUPDATE]));
230 
231         if (tb[TCA_PIE_LIMIT]) {
232                 u32 limit = nla_get_u32(tb[TCA_PIE_LIMIT]);
233 
234                 q->params.limit = limit;
235                 sch->limit = limit;
236         }
237 
238         if (tb[TCA_PIE_ALPHA])
239                 q->params.alpha = nla_get_u32(tb[TCA_PIE_ALPHA]);
240 
241         if (tb[TCA_PIE_BETA])
242                 q->params.beta = nla_get_u32(tb[TCA_PIE_BETA]);
243 
244         if (tb[TCA_PIE_ECN])
245                 q->params.ecn = nla_get_u32(tb[TCA_PIE_ECN]);
246 
247         if (tb[TCA_PIE_BYTEMODE])
248                 q->params.bytemode = nla_get_u32(tb[TCA_PIE_BYTEMODE]);
249 
250         /* Drop excess packets if new limit is lower */
251         qlen = sch->q.qlen;
252         while (sch->q.qlen > sch->limit) {
253                 struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);
254 
255                 dropped += qdisc_pkt_len(skb);
256                 qdisc_qstats_backlog_dec(sch, skb);
257                 rtnl_qdisc_drop(skb, sch);
258         }
259         qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, dropped);
260 
261         sch_tree_unlock(sch);
262         return 0;
263 }
264 
265 static void pie_process_dequeue(struct Qdisc *sch, struct sk_buff *skb)
266 {
267         struct pie_sched_data *q = qdisc_priv(sch);
268         int qlen = sch->qstats.backlog; /* current queue size in bytes */
269 
270         /* If current queue is about 10 packets or more and dq_count is unset
271          * we have enough packets to calculate the drain rate. Save
272          * current time as dq_tstamp and start measurement cycle.
273          */
274         if (qlen >= QUEUE_THRESHOLD && q->vars.dq_count == DQCOUNT_INVALID) {
275                 q->vars.dq_tstamp = psched_get_time();
276                 q->vars.dq_count = 0;
277         }
278 
279         /* Calculate the average drain rate from this value.  If queue length
280          * has receded to a small value viz., <= QUEUE_THRESHOLD bytes,reset
281          * the dq_count to -1 as we don't have enough packets to calculate the
282          * drain rate anymore The following if block is entered only when we
283          * have a substantial queue built up (QUEUE_THRESHOLD bytes or more)
284          * and we calculate the drain rate for the threshold here.  dq_count is
285          * in bytes, time difference in psched_time, hence rate is in
286          * bytes/psched_time.
287          */
288         if (q->vars.dq_count != DQCOUNT_INVALID) {
289                 q->vars.dq_count += skb->len;
290 
291                 if (q->vars.dq_count >= QUEUE_THRESHOLD) {
292                         psched_time_t now = psched_get_time();
293                         u32 dtime = now - q->vars.dq_tstamp;
294                         u32 count = q->vars.dq_count << PIE_SCALE;
295 
296                         if (dtime == 0)
297                                 return;
298 
299                         count = count / dtime;
300 
301                         if (q->vars.avg_dq_rate == 0)
302                                 q->vars.avg_dq_rate = count;
303                         else
304                                 q->vars.avg_dq_rate =
305                                     (q->vars.avg_dq_rate -
306                                      (q->vars.avg_dq_rate >> 3)) + (count >> 3);
307 
308                         /* If the queue has receded below the threshold, we hold
309                          * on to the last drain rate calculated, else we reset
310                          * dq_count to 0 to re-enter the if block when the next
311                          * packet is dequeued
312                          */
313                         if (qlen < QUEUE_THRESHOLD) {
314                                 q->vars.dq_count = DQCOUNT_INVALID;
315                         } else {
316                                 q->vars.dq_count = 0;
317                                 q->vars.dq_tstamp = psched_get_time();
318                         }
319 
320                         if (q->vars.burst_time > 0) {
321                                 if (q->vars.burst_time > dtime)
322                                         q->vars.burst_time -= dtime;
323                                 else
324                                         q->vars.burst_time = 0;
325                         }
326                 }
327         }
328 }
329 
330 static void calculate_probability(struct Qdisc *sch)
331 {
332         struct pie_sched_data *q = qdisc_priv(sch);
333         u32 qlen = sch->qstats.backlog; /* queue size in bytes */
334         psched_time_t qdelay = 0;       /* in pschedtime */
335         psched_time_t qdelay_old = q->vars.qdelay;      /* in pschedtime */
336         s64 delta = 0;          /* determines the change in probability */
337         u64 oldprob;
338         u64 alpha, beta;
339         u32 power;
340         bool update_prob = true;
341 
342         q->vars.qdelay_old = q->vars.qdelay;
343 
344         if (q->vars.avg_dq_rate > 0)
345                 qdelay = (qlen << PIE_SCALE) / q->vars.avg_dq_rate;
346         else
347                 qdelay = 0;
348 
349         /* If qdelay is zero and qlen is not, it means qlen is very small, less
350          * than dequeue_rate, so we do not update probabilty in this round
351          */
352         if (qdelay == 0 && qlen != 0)
353                 update_prob = false;
354 
355         /* In the algorithm, alpha and beta are between 0 and 2 with typical
356          * value for alpha as 0.125. In this implementation, we use values 0-32
357          * passed from user space to represent this. Also, alpha and beta have
358          * unit of HZ and need to be scaled before they can used to update
359          * probability. alpha/beta are updated locally below by scaling down
360          * by 16 to come to 0-2 range.
361          */
362         alpha = ((u64)q->params.alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
363         beta = ((u64)q->params.beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
364 
365         /* We scale alpha and beta differently depending on how heavy the
366          * congestion is. Please see RFC 8033 for details.
367          */
368         if (q->vars.prob < MAX_PROB / 10) {
369                 alpha >>= 1;
370                 beta >>= 1;
371 
372                 power = 100;
373                 while (q->vars.prob < div_u64(MAX_PROB, power) &&
374                        power <= 1000000) {
375                         alpha >>= 2;
376                         beta >>= 2;
377                         power *= 10;
378                 }
379         }
380 
381         /* alpha and beta should be between 0 and 32, in multiples of 1/16 */
382         delta += alpha * (u64)(qdelay - q->params.target);
383         delta += beta * (u64)(qdelay - qdelay_old);
384 
385         oldprob = q->vars.prob;
386 
387         /* to ensure we increase probability in steps of no more than 2% */
388         if (delta > (s64)(MAX_PROB / (100 / 2)) &&
389             q->vars.prob >= MAX_PROB / 10)
390                 delta = (MAX_PROB / 100) * 2;
391 
392         /* Non-linear drop:
393          * Tune drop probability to increase quickly for high delays(>= 250ms)
394          * 250ms is derived through experiments and provides error protection
395          */
396 
397         if (qdelay > (PSCHED_NS2TICKS(250 * NSEC_PER_MSEC)))
398                 delta += MAX_PROB / (100 / 2);
399 
400         q->vars.prob += delta;
401 
402         if (delta > 0) {
403                 /* prevent overflow */
404                 if (q->vars.prob < oldprob) {
405                         q->vars.prob = MAX_PROB;
406                         /* Prevent normalization error. If probability is at
407                          * maximum value already, we normalize it here, and
408                          * skip the check to do a non-linear drop in the next
409                          * section.
410                          */
411                         update_prob = false;
412                 }
413         } else {
414                 /* prevent underflow */
415                 if (q->vars.prob > oldprob)
416                         q->vars.prob = 0;
417         }
418 
419         /* Non-linear drop in probability: Reduce drop probability quickly if
420          * delay is 0 for 2 consecutive Tupdate periods.
421          */
422 
423         if (qdelay == 0 && qdelay_old == 0 && update_prob)
424                 /* Reduce drop probability to 98.4% */
425                 q->vars.prob -= q->vars.prob / 64u;
426 
427         q->vars.qdelay = qdelay;
428         q->vars.qlen_old = qlen;
429 
430         /* We restart the measurement cycle if the following conditions are met
431          * 1. If the delay has been low for 2 consecutive Tupdate periods
432          * 2. Calculated drop probability is zero
433          * 3. We have atleast one estimate for the avg_dq_rate ie.,
434          *    is a non-zero value
435          */
436         if ((q->vars.qdelay < q->params.target / 2) &&
437             (q->vars.qdelay_old < q->params.target / 2) &&
438             q->vars.prob == 0 &&
439             q->vars.avg_dq_rate > 0)
440                 pie_vars_init(&q->vars);
441 }
442 
443 static void pie_timer(struct timer_list *t)
444 {
445         struct pie_sched_data *q = from_timer(q, t, adapt_timer);
446         struct Qdisc *sch = q->sch;
447         spinlock_t *root_lock = qdisc_lock(qdisc_root_sleeping(sch));
448 
449         spin_lock(root_lock);
450         calculate_probability(sch);
451 
452         /* reset the timer to fire after 'tupdate'. tupdate is in jiffies. */
453         if (q->params.tupdate)
454                 mod_timer(&q->adapt_timer, jiffies + q->params.tupdate);
455         spin_unlock(root_lock);
456 }
457 
458 static int pie_init(struct Qdisc *sch, struct nlattr *opt,
459                     struct netlink_ext_ack *extack)
460 {
461         struct pie_sched_data *q = qdisc_priv(sch);
462 
463         pie_params_init(&q->params);
464         pie_vars_init(&q->vars);
465         sch->limit = q->params.limit;
466 
467         q->sch = sch;
468         timer_setup(&q->adapt_timer, pie_timer, 0);
469 
470         if (opt) {
471                 int err = pie_change(sch, opt, extack);
472 
473                 if (err)
474                         return err;
475         }
476 
477         mod_timer(&q->adapt_timer, jiffies + HZ / 2);
478         return 0;
479 }
480 
481 static int pie_dump(struct Qdisc *sch, struct sk_buff *skb)
482 {
483         struct pie_sched_data *q = qdisc_priv(sch);
484         struct nlattr *opts;
485 
486         opts = nla_nest_start_noflag(skb, TCA_OPTIONS);
487         if (!opts)
488                 goto nla_put_failure;
489 
490         /* convert target from pschedtime to us */
491         if (nla_put_u32(skb, TCA_PIE_TARGET,
492                         ((u32)PSCHED_TICKS2NS(q->params.target)) /
493                         NSEC_PER_USEC) ||
494             nla_put_u32(skb, TCA_PIE_LIMIT, sch->limit) ||
495             nla_put_u32(skb, TCA_PIE_TUPDATE,
496                         jiffies_to_usecs(q->params.tupdate)) ||
497             nla_put_u32(skb, TCA_PIE_ALPHA, q->params.alpha) ||
498             nla_put_u32(skb, TCA_PIE_BETA, q->params.beta) ||
499             nla_put_u32(skb, TCA_PIE_ECN, q->params.ecn) ||
500             nla_put_u32(skb, TCA_PIE_BYTEMODE, q->params.bytemode))
501                 goto nla_put_failure;
502 
503         return nla_nest_end(skb, opts);
504 
505 nla_put_failure:
506         nla_nest_cancel(skb, opts);
507         return -1;
508 }
509 
510 static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
511 {
512         struct pie_sched_data *q = qdisc_priv(sch);
513         struct tc_pie_xstats st = {
514                 .prob           = q->vars.prob,
515                 .delay          = ((u32)PSCHED_TICKS2NS(q->vars.qdelay)) /
516                                    NSEC_PER_USEC,
517                 /* unscale and return dq_rate in bytes per sec */
518                 .avg_dq_rate    = q->vars.avg_dq_rate *
519                                   (PSCHED_TICKS_PER_SEC) >> PIE_SCALE,
520                 .packets_in     = q->stats.packets_in,
521                 .overlimit      = q->stats.overlimit,
522                 .maxq           = q->stats.maxq,
523                 .dropped        = q->stats.dropped,
524                 .ecn_mark       = q->stats.ecn_mark,
525         };
526 
527         return gnet_stats_copy_app(d, &st, sizeof(st));
528 }
529 
530 static struct sk_buff *pie_qdisc_dequeue(struct Qdisc *sch)
531 {
532         struct sk_buff *skb = qdisc_dequeue_head(sch);
533 
534         if (!skb)
535                 return NULL;
536 
537         pie_process_dequeue(sch, skb);
538         return skb;
539 }
540 
541 static void pie_reset(struct Qdisc *sch)
542 {
543         struct pie_sched_data *q = qdisc_priv(sch);
544 
545         qdisc_reset_queue(sch);
546         pie_vars_init(&q->vars);
547 }
548 
549 static void pie_destroy(struct Qdisc *sch)
550 {
551         struct pie_sched_data *q = qdisc_priv(sch);
552 
553         q->params.tupdate = 0;
554         del_timer_sync(&q->adapt_timer);
555 }
556 
557 static struct Qdisc_ops pie_qdisc_ops __read_mostly = {
558         .id = "pie",
559         .priv_size      = sizeof(struct pie_sched_data),
560         .enqueue        = pie_qdisc_enqueue,
561         .dequeue        = pie_qdisc_dequeue,
562         .peek           = qdisc_peek_dequeued,
563         .init           = pie_init,
564         .destroy        = pie_destroy,
565         .reset          = pie_reset,
566         .change         = pie_change,
567         .dump           = pie_dump,
568         .dump_stats     = pie_dump_stats,
569         .owner          = THIS_MODULE,
570 };
571 
572 static int __init pie_module_init(void)
573 {
574         return register_qdisc(&pie_qdisc_ops);
575 }
576 
577 static void __exit pie_module_exit(void)
578 {
579         unregister_qdisc(&pie_qdisc_ops);
580 }
581 
582 module_init(pie_module_init);
583 module_exit(pie_module_exit);
584 
585 MODULE_DESCRIPTION("Proportional Integral controller Enhanced (PIE) scheduler");
586 MODULE_AUTHOR("Vijay Subramanian");
587 MODULE_AUTHOR("Mythili Prabhu");
588 MODULE_LICENSE("GPL");
589 

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