xref: /linux/net/sched/sch_pie.c (revision 4b132aacb0768ac1e652cf517097ea6f237214b9) 
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 	get_random_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 	err = nla_parse_nested_deprecated(tb, TCA_PIE_MAX, opt, pie_policy,
147 					  NULL);
148 	if (err < 0)
149 		return err;
150 
151 	sch_tree_lock(sch);
152 
153 	/* convert from microseconds to pschedtime */
154 	if (tb[TCA_PIE_TARGET]) {
155 		/* target is in us */
156 		u32 target = nla_get_u32(tb[TCA_PIE_TARGET]);
157 
158 		/* convert to pschedtime */
159 		WRITE_ONCE(q->params.target,
160 			   PSCHED_NS2TICKS((u64)target * NSEC_PER_USEC));
161 	}
162 
163 	/* tupdate is in jiffies */
164 	if (tb[TCA_PIE_TUPDATE])
165 		WRITE_ONCE(q->params.tupdate,
166 			   usecs_to_jiffies(nla_get_u32(tb[TCA_PIE_TUPDATE])));
167 
168 	if (tb[TCA_PIE_LIMIT]) {
169 		u32 limit = nla_get_u32(tb[TCA_PIE_LIMIT]);
170 
171 		WRITE_ONCE(q->params.limit, limit);
172 		WRITE_ONCE(sch->limit, limit);
173 	}
174 
175 	if (tb[TCA_PIE_ALPHA])
176 		WRITE_ONCE(q->params.alpha, nla_get_u32(tb[TCA_PIE_ALPHA]));
177 
178 	if (tb[TCA_PIE_BETA])
179 		WRITE_ONCE(q->params.beta, nla_get_u32(tb[TCA_PIE_BETA]));
180 
181 	if (tb[TCA_PIE_ECN])
182 		WRITE_ONCE(q->params.ecn, nla_get_u32(tb[TCA_PIE_ECN]));
183 
184 	if (tb[TCA_PIE_BYTEMODE])
185 		WRITE_ONCE(q->params.bytemode,
186 			   nla_get_u32(tb[TCA_PIE_BYTEMODE]));
187 
188 	if (tb[TCA_PIE_DQ_RATE_ESTIMATOR])
189 		WRITE_ONCE(q->params.dq_rate_estimator,
190 			   nla_get_u32(tb[TCA_PIE_DQ_RATE_ESTIMATOR]));
191 
192 	/* Drop excess packets if new limit is lower */
193 	qlen = sch->q.qlen;
194 	while (sch->q.qlen > sch->limit) {
195 		struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);
196 
197 		dropped += qdisc_pkt_len(skb);
198 		qdisc_qstats_backlog_dec(sch, skb);
199 		rtnl_qdisc_drop(skb, sch);
200 	}
201 	qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, dropped);
202 
203 	sch_tree_unlock(sch);
204 	return 0;
205 }
206 
207 void pie_process_dequeue(struct sk_buff *skb, struct pie_params *params,
208 			 struct pie_vars *vars, u32 backlog)
209 {
210 	psched_time_t now = psched_get_time();
211 	u32 dtime = 0;
212 
213 	/* If dq_rate_estimator is disabled, calculate qdelay using the
214 	 * packet timestamp.
215 	 */
216 	if (!params->dq_rate_estimator) {
217 		vars->qdelay = now - pie_get_enqueue_time(skb);
218 
219 		if (vars->dq_tstamp != DTIME_INVALID)
220 			dtime = now - vars->dq_tstamp;
221 
222 		vars->dq_tstamp = now;
223 
224 		if (backlog == 0)
225 			vars->qdelay = 0;
226 
227 		if (dtime == 0)
228 			return;
229 
230 		goto burst_allowance_reduction;
231 	}
232 
233 	/* If current queue is about 10 packets or more and dq_count is unset
234 	 * we have enough packets to calculate the drain rate. Save
235 	 * current time as dq_tstamp and start measurement cycle.
236 	 */
237 	if (backlog >= QUEUE_THRESHOLD && vars->dq_count == DQCOUNT_INVALID) {
238 		vars->dq_tstamp = psched_get_time();
239 		vars->dq_count = 0;
240 	}
241 
242 	/* Calculate the average drain rate from this value. If queue length
243 	 * has receded to a small value viz., <= QUEUE_THRESHOLD bytes, reset
244 	 * the dq_count to -1 as we don't have enough packets to calculate the
245 	 * drain rate anymore. The following if block is entered only when we
246 	 * have a substantial queue built up (QUEUE_THRESHOLD bytes or more)
247 	 * and we calculate the drain rate for the threshold here.  dq_count is
248 	 * in bytes, time difference in psched_time, hence rate is in
249 	 * bytes/psched_time.
250 	 */
251 	if (vars->dq_count != DQCOUNT_INVALID) {
252 		vars->dq_count += skb->len;
253 
254 		if (vars->dq_count >= QUEUE_THRESHOLD) {
255 			u32 count = vars->dq_count << PIE_SCALE;
256 
257 			dtime = now - vars->dq_tstamp;
258 
259 			if (dtime == 0)
260 				return;
261 
262 			count = count / dtime;
263 
264 			if (vars->avg_dq_rate == 0)
265 				vars->avg_dq_rate = count;
266 			else
267 				vars->avg_dq_rate =
268 				    (vars->avg_dq_rate -
269 				     (vars->avg_dq_rate >> 3)) + (count >> 3);
270 
271 			/* If the queue has receded below the threshold, we hold
272 			 * on to the last drain rate calculated, else we reset
273 			 * dq_count to 0 to re-enter the if block when the next
274 			 * packet is dequeued
275 			 */
276 			if (backlog < QUEUE_THRESHOLD) {
277 				vars->dq_count = DQCOUNT_INVALID;
278 			} else {
279 				vars->dq_count = 0;
280 				vars->dq_tstamp = psched_get_time();
281 			}
282 
283 			goto burst_allowance_reduction;
284 		}
285 	}
286 
287 	return;
288 
289 burst_allowance_reduction:
290 	if (vars->burst_time > 0) {
291 		if (vars->burst_time > dtime)
292 			vars->burst_time -= dtime;
293 		else
294 			vars->burst_time = 0;
295 	}
296 }
297 EXPORT_SYMBOL_GPL(pie_process_dequeue);
298 
299 void pie_calculate_probability(struct pie_params *params, struct pie_vars *vars,
300 			       u32 backlog)
301 {
302 	psched_time_t qdelay = 0;	/* in pschedtime */
303 	psched_time_t qdelay_old = 0;	/* in pschedtime */
304 	s64 delta = 0;		/* determines the change in probability */
305 	u64 oldprob;
306 	u64 alpha, beta;
307 	u32 power;
308 	bool update_prob = true;
309 
310 	if (params->dq_rate_estimator) {
311 		qdelay_old = vars->qdelay;
312 		vars->qdelay_old = vars->qdelay;
313 
314 		if (vars->avg_dq_rate > 0)
315 			qdelay = (backlog << PIE_SCALE) / vars->avg_dq_rate;
316 		else
317 			qdelay = 0;
318 	} else {
319 		qdelay = vars->qdelay;
320 		qdelay_old = vars->qdelay_old;
321 	}
322 
323 	/* If qdelay is zero and backlog is not, it means backlog is very small,
324 	 * so we do not update probability in this round.
325 	 */
326 	if (qdelay == 0 && backlog != 0)
327 		update_prob = false;
328 
329 	/* In the algorithm, alpha and beta are between 0 and 2 with typical
330 	 * value for alpha as 0.125. In this implementation, we use values 0-32
331 	 * passed from user space to represent this. Also, alpha and beta have
332 	 * unit of HZ and need to be scaled before they can used to update
333 	 * probability. alpha/beta are updated locally below by scaling down
334 	 * by 16 to come to 0-2 range.
335 	 */
336 	alpha = ((u64)params->alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
337 	beta = ((u64)params->beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
338 
339 	/* We scale alpha and beta differently depending on how heavy the
340 	 * congestion is. Please see RFC 8033 for details.
341 	 */
342 	if (vars->prob < MAX_PROB / 10) {
343 		alpha >>= 1;
344 		beta >>= 1;
345 
346 		power = 100;
347 		while (vars->prob < div_u64(MAX_PROB, power) &&
348 		       power <= 1000000) {
349 			alpha >>= 2;
350 			beta >>= 2;
351 			power *= 10;
352 		}
353 	}
354 
355 	/* alpha and beta should be between 0 and 32, in multiples of 1/16 */
356 	delta += alpha * (qdelay - params->target);
357 	delta += beta * (qdelay - qdelay_old);
358 
359 	oldprob = vars->prob;
360 
361 	/* to ensure we increase probability in steps of no more than 2% */
362 	if (delta > (s64)(MAX_PROB / (100 / 2)) &&
363 	    vars->prob >= MAX_PROB / 10)
364 		delta = (MAX_PROB / 100) * 2;
365 
366 	/* Non-linear drop:
367 	 * Tune drop probability to increase quickly for high delays(>= 250ms)
368 	 * 250ms is derived through experiments and provides error protection
369 	 */
370 
371 	if (qdelay > (PSCHED_NS2TICKS(250 * NSEC_PER_MSEC)))
372 		delta += MAX_PROB / (100 / 2);
373 
374 	vars->prob += delta;
375 
376 	if (delta > 0) {
377 		/* prevent overflow */
378 		if (vars->prob < oldprob) {
379 			vars->prob = MAX_PROB;
380 			/* Prevent normalization error. If probability is at
381 			 * maximum value already, we normalize it here, and
382 			 * skip the check to do a non-linear drop in the next
383 			 * section.
384 			 */
385 			update_prob = false;
386 		}
387 	} else {
388 		/* prevent underflow */
389 		if (vars->prob > oldprob)
390 			vars->prob = 0;
391 	}
392 
393 	/* Non-linear drop in probability: Reduce drop probability quickly if
394 	 * delay is 0 for 2 consecutive Tupdate periods.
395 	 */
396 
397 	if (qdelay == 0 && qdelay_old == 0 && update_prob)
398 		/* Reduce drop probability to 98.4% */
399 		vars->prob -= vars->prob / 64;
400 
401 	vars->qdelay = qdelay;
402 	vars->backlog_old = backlog;
403 
404 	/* We restart the measurement cycle if the following conditions are met
405 	 * 1. If the delay has been low for 2 consecutive Tupdate periods
406 	 * 2. Calculated drop probability is zero
407 	 * 3. If average dq_rate_estimator is enabled, we have at least one
408 	 *    estimate for the avg_dq_rate ie., is a non-zero value
409 	 */
410 	if ((vars->qdelay < params->target / 2) &&
411 	    (vars->qdelay_old < params->target / 2) &&
412 	    vars->prob == 0 &&
413 	    (!params->dq_rate_estimator || vars->avg_dq_rate > 0)) {
414 		pie_vars_init(vars);
415 	}
416 
417 	if (!params->dq_rate_estimator)
418 		vars->qdelay_old = qdelay;
419 }
420 EXPORT_SYMBOL_GPL(pie_calculate_probability);
421 
422 static void pie_timer(struct timer_list *t)
423 {
424 	struct pie_sched_data *q = from_timer(q, t, adapt_timer);
425 	struct Qdisc *sch = q->sch;
426 	spinlock_t *root_lock;
427 
428 	rcu_read_lock();
429 	root_lock = qdisc_lock(qdisc_root_sleeping(sch));
430 	spin_lock(root_lock);
431 	pie_calculate_probability(&q->params, &q->vars, sch->qstats.backlog);
432 
433 	/* reset the timer to fire after 'tupdate'. tupdate is in jiffies. */
434 	if (q->params.tupdate)
435 		mod_timer(&q->adapt_timer, jiffies + q->params.tupdate);
436 	spin_unlock(root_lock);
437 	rcu_read_unlock();
438 }
439 
440 static int pie_init(struct Qdisc *sch, struct nlattr *opt,
441 		    struct netlink_ext_ack *extack)
442 {
443 	struct pie_sched_data *q = qdisc_priv(sch);
444 
445 	pie_params_init(&q->params);
446 	pie_vars_init(&q->vars);
447 	sch->limit = q->params.limit;
448 
449 	q->sch = sch;
450 	timer_setup(&q->adapt_timer, pie_timer, 0);
451 
452 	if (opt) {
453 		int err = pie_change(sch, opt, extack);
454 
455 		if (err)
456 			return err;
457 	}
458 
459 	mod_timer(&q->adapt_timer, jiffies + HZ / 2);
460 	return 0;
461 }
462 
463 static int pie_dump(struct Qdisc *sch, struct sk_buff *skb)
464 {
465 	struct pie_sched_data *q = qdisc_priv(sch);
466 	struct nlattr *opts;
467 
468 	opts = nla_nest_start_noflag(skb, TCA_OPTIONS);
469 	if (!opts)
470 		goto nla_put_failure;
471 
472 	/* convert target from pschedtime to us */
473 	if (nla_put_u32(skb, TCA_PIE_TARGET,
474 			((u32)PSCHED_TICKS2NS(READ_ONCE(q->params.target))) /
475 			NSEC_PER_USEC) ||
476 	    nla_put_u32(skb, TCA_PIE_LIMIT, READ_ONCE(sch->limit)) ||
477 	    nla_put_u32(skb, TCA_PIE_TUPDATE,
478 			jiffies_to_usecs(READ_ONCE(q->params.tupdate))) ||
479 	    nla_put_u32(skb, TCA_PIE_ALPHA, READ_ONCE(q->params.alpha)) ||
480 	    nla_put_u32(skb, TCA_PIE_BETA, READ_ONCE(q->params.beta)) ||
481 	    nla_put_u32(skb, TCA_PIE_ECN, q->params.ecn) ||
482 	    nla_put_u32(skb, TCA_PIE_BYTEMODE,
483 			READ_ONCE(q->params.bytemode)) ||
484 	    nla_put_u32(skb, TCA_PIE_DQ_RATE_ESTIMATOR,
485 			READ_ONCE(q->params.dq_rate_estimator)))
486 		goto nla_put_failure;
487 
488 	return nla_nest_end(skb, opts);
489 
490 nla_put_failure:
491 	nla_nest_cancel(skb, opts);
492 	return -1;
493 }
494 
495 static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
496 {
497 	struct pie_sched_data *q = qdisc_priv(sch);
498 	struct tc_pie_xstats st = {
499 		.prob		= q->vars.prob << BITS_PER_BYTE,
500 		.delay		= ((u32)PSCHED_TICKS2NS(q->vars.qdelay)) /
501 				   NSEC_PER_USEC,
502 		.packets_in	= q->stats.packets_in,
503 		.overlimit	= q->stats.overlimit,
504 		.maxq		= q->stats.maxq,
505 		.dropped	= q->stats.dropped,
506 		.ecn_mark	= q->stats.ecn_mark,
507 	};
508 
509 	/* avg_dq_rate is only valid if dq_rate_estimator is enabled */
510 	st.dq_rate_estimating = q->params.dq_rate_estimator;
511 
512 	/* unscale and return dq_rate in bytes per sec */
513 	if (q->params.dq_rate_estimator)
514 		st.avg_dq_rate = q->vars.avg_dq_rate *
515 				 (PSCHED_TICKS_PER_SEC) >> PIE_SCALE;
516 
517 	return gnet_stats_copy_app(d, &st, sizeof(st));
518 }
519 
520 static struct sk_buff *pie_qdisc_dequeue(struct Qdisc *sch)
521 {
522 	struct pie_sched_data *q = qdisc_priv(sch);
523 	struct sk_buff *skb = qdisc_dequeue_head(sch);
524 
525 	if (!skb)
526 		return NULL;
527 
528 	pie_process_dequeue(skb, &q->params, &q->vars, sch->qstats.backlog);
529 	return skb;
530 }
531 
532 static void pie_reset(struct Qdisc *sch)
533 {
534 	struct pie_sched_data *q = qdisc_priv(sch);
535 
536 	qdisc_reset_queue(sch);
537 	pie_vars_init(&q->vars);
538 }
539 
540 static void pie_destroy(struct Qdisc *sch)
541 {
542 	struct pie_sched_data *q = qdisc_priv(sch);
543 
544 	q->params.tupdate = 0;
545 	del_timer_sync(&q->adapt_timer);
546 }
547 
548 static struct Qdisc_ops pie_qdisc_ops __read_mostly = {
549 	.id		= "pie",
550 	.priv_size	= sizeof(struct pie_sched_data),
551 	.enqueue	= pie_qdisc_enqueue,
552 	.dequeue	= pie_qdisc_dequeue,
553 	.peek		= qdisc_peek_dequeued,
554 	.init		= pie_init,
555 	.destroy	= pie_destroy,
556 	.reset		= pie_reset,
557 	.change		= pie_change,
558 	.dump		= pie_dump,
559 	.dump_stats	= pie_dump_stats,
560 	.owner		= THIS_MODULE,
561 };
562 MODULE_ALIAS_NET_SCH("pie");
563 
564 static int __init pie_module_init(void)
565 {
566 	return register_qdisc(&pie_qdisc_ops);
567 }
568 
569 static void __exit pie_module_exit(void)
570 {
571 	unregister_qdisc(&pie_qdisc_ops);
572 }
573 
574 module_init(pie_module_init);
575 module_exit(pie_module_exit);
576 
577 MODULE_DESCRIPTION("Proportional Integral controller Enhanced (PIE) scheduler");
578 MODULE_AUTHOR("Vijay Subramanian");
579 MODULE_AUTHOR("Mythili Prabhu");
580 MODULE_LICENSE("GPL");
581