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TOMOYO Linux Cross Reference
Linux/block/blk-iolatency.c

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  1 // SPDX-License-Identifier: GPL-2.0
  2 /*
  3  * Block rq-qos base io controller
  4  *
  5  * This works similar to wbt with a few exceptions
  6  *
  7  * - It's bio based, so the latency covers the whole block layer in addition to
  8  *   the actual io.
  9  * - We will throttle all IO that comes in here if we need to.
 10  * - We use the mean latency over the 100ms window.  This is because writes can
 11  *   be particularly fast, which could give us a false sense of the impact of
 12  *   other workloads on our protected workload.
 13  * - By default there's no throttling, we set the queue_depth to UINT_MAX so
 14  *   that we can have as many outstanding bio's as we're allowed to.  Only at
 15  *   throttle time do we pay attention to the actual queue depth.
 16  *
 17  * The hierarchy works like the cpu controller does, we track the latency at
 18  * every configured node, and each configured node has it's own independent
 19  * queue depth.  This means that we only care about our latency targets at the
 20  * peer level.  Some group at the bottom of the hierarchy isn't going to affect
 21  * a group at the end of some other path if we're only configred at leaf level.
 22  *
 23  * Consider the following
 24  *
 25  *                   root blkg
 26  *             /                     \
 27  *        fast (target=5ms)     slow (target=10ms)
 28  *         /     \                  /        \
 29  *       a        b          normal(15ms)   unloved
 30  *
 31  * "a" and "b" have no target, but their combined io under "fast" cannot exceed
 32  * an average latency of 5ms.  If it does then we will throttle the "slow"
 33  * group.  In the case of "normal", if it exceeds its 15ms target, we will
 34  * throttle "unloved", but nobody else.
 35  *
 36  * In this example "fast", "slow", and "normal" will be the only groups actually
 37  * accounting their io latencies.  We have to walk up the heirarchy to the root
 38  * on every submit and complete so we can do the appropriate stat recording and
 39  * adjust the queue depth of ourselves if needed.
 40  *
 41  * There are 2 ways we throttle IO.
 42  *
 43  * 1) Queue depth throttling.  As we throttle down we will adjust the maximum
 44  * number of IO's we're allowed to have in flight.  This starts at (u64)-1 down
 45  * to 1.  If the group is only ever submitting IO for itself then this is the
 46  * only way we throttle.
 47  *
 48  * 2) Induced delay throttling.  This is for the case that a group is generating
 49  * IO that has to be issued by the root cg to avoid priority inversion. So think
 50  * REQ_META or REQ_SWAP.  If we are already at qd == 1 and we're getting a lot
 51  * of work done for us on behalf of the root cg and are being asked to scale
 52  * down more then we induce a latency at userspace return.  We accumulate the
 53  * total amount of time we need to be punished by doing
 54  *
 55  * total_time += min_lat_nsec - actual_io_completion
 56  *
 57  * and then at throttle time will do
 58  *
 59  * throttle_time = min(total_time, NSEC_PER_SEC)
 60  *
 61  * This induced delay will throttle back the activity that is generating the
 62  * root cg issued io's, wethere that's some metadata intensive operation or the
 63  * group is using so much memory that it is pushing us into swap.
 64  *
 65  * Copyright (C) 2018 Josef Bacik
 66  */
 67 #include <linux/kernel.h>
 68 #include <linux/blk_types.h>
 69 #include <linux/backing-dev.h>
 70 #include <linux/module.h>
 71 #include <linux/timer.h>
 72 #include <linux/memcontrol.h>
 73 #include <linux/sched/loadavg.h>
 74 #include <linux/sched/signal.h>
 75 #include <trace/events/block.h>
 76 #include <linux/blk-mq.h>
 77 #include "blk-rq-qos.h"
 78 #include "blk-stat.h"
 79 #include "blk-cgroup.h"
 80 #include "blk.h"
 81 
 82 #define DEFAULT_SCALE_COOKIE 1000000U
 83 
 84 static struct blkcg_policy blkcg_policy_iolatency;
 85 struct iolatency_grp;
 86 
 87 struct blk_iolatency {
 88         struct rq_qos rqos;
 89         struct timer_list timer;
 90 
 91         /*
 92          * ->enabled is the master enable switch gating the throttling logic and
 93          * inflight tracking. The number of cgroups which have iolat enabled is
 94          * tracked in ->enable_cnt, and ->enable is flipped on/off accordingly
 95          * from ->enable_work with the request_queue frozen. For details, See
 96          * blkiolatency_enable_work_fn().
 97          */
 98         bool enabled;
 99         atomic_t enable_cnt;
100         struct work_struct enable_work;
101 };
102 
103 static inline struct blk_iolatency *BLKIOLATENCY(struct rq_qos *rqos)
104 {
105         return container_of(rqos, struct blk_iolatency, rqos);
106 }
107 
108 struct child_latency_info {
109         spinlock_t lock;
110 
111         /* Last time we adjusted the scale of everybody. */
112         u64 last_scale_event;
113 
114         /* The latency that we missed. */
115         u64 scale_lat;
116 
117         /* Total io's from all of our children for the last summation. */
118         u64 nr_samples;
119 
120         /* The guy who actually changed the latency numbers. */
121         struct iolatency_grp *scale_grp;
122 
123         /* Cookie to tell if we need to scale up or down. */
124         atomic_t scale_cookie;
125 };
126 
127 struct percentile_stats {
128         u64 total;
129         u64 missed;
130 };
131 
132 struct latency_stat {
133         union {
134                 struct percentile_stats ps;
135                 struct blk_rq_stat rqs;
136         };
137 };
138 
139 struct iolatency_grp {
140         struct blkg_policy_data pd;
141         struct latency_stat __percpu *stats;
142         struct latency_stat cur_stat;
143         struct blk_iolatency *blkiolat;
144         struct rq_depth rq_depth;
145         struct rq_wait rq_wait;
146         atomic64_t window_start;
147         atomic_t scale_cookie;
148         u64 min_lat_nsec;
149         u64 cur_win_nsec;
150 
151         /* total running average of our io latency. */
152         u64 lat_avg;
153 
154         /* Our current number of IO's for the last summation. */
155         u64 nr_samples;
156 
157         bool ssd;
158         struct child_latency_info child_lat;
159 };
160 
161 #define BLKIOLATENCY_MIN_WIN_SIZE (100 * NSEC_PER_MSEC)
162 #define BLKIOLATENCY_MAX_WIN_SIZE NSEC_PER_SEC
163 /*
164  * These are the constants used to fake the fixed-point moving average
165  * calculation just like load average.  The call to calc_load() folds
166  * (FIXED_1 (2048) - exp_factor) * new_sample into lat_avg.  The sampling
167  * window size is bucketed to try to approximately calculate average
168  * latency such that 1/exp (decay rate) is [1 min, 2.5 min) when windows
169  * elapse immediately.  Note, windows only elapse with IO activity.  Idle
170  * periods extend the most recent window.
171  */
172 #define BLKIOLATENCY_NR_EXP_FACTORS 5
173 #define BLKIOLATENCY_EXP_BUCKET_SIZE (BLKIOLATENCY_MAX_WIN_SIZE / \
174                                       (BLKIOLATENCY_NR_EXP_FACTORS - 1))
175 static const u64 iolatency_exp_factors[BLKIOLATENCY_NR_EXP_FACTORS] = {
176         2045, // exp(1/600) - 600 samples
177         2039, // exp(1/240) - 240 samples
178         2031, // exp(1/120) - 120 samples
179         2023, // exp(1/80)  - 80 samples
180         2014, // exp(1/60)  - 60 samples
181 };
182 
183 static inline struct iolatency_grp *pd_to_lat(struct blkg_policy_data *pd)
184 {
185         return pd ? container_of(pd, struct iolatency_grp, pd) : NULL;
186 }
187 
188 static inline struct iolatency_grp *blkg_to_lat(struct blkcg_gq *blkg)
189 {
190         return pd_to_lat(blkg_to_pd(blkg, &blkcg_policy_iolatency));
191 }
192 
193 static inline struct blkcg_gq *lat_to_blkg(struct iolatency_grp *iolat)
194 {
195         return pd_to_blkg(&iolat->pd);
196 }
197 
198 static inline void latency_stat_init(struct iolatency_grp *iolat,
199                                      struct latency_stat *stat)
200 {
201         if (iolat->ssd) {
202                 stat->ps.total = 0;
203                 stat->ps.missed = 0;
204         } else
205                 blk_rq_stat_init(&stat->rqs);
206 }
207 
208 static inline void latency_stat_sum(struct iolatency_grp *iolat,
209                                     struct latency_stat *sum,
210                                     struct latency_stat *stat)
211 {
212         if (iolat->ssd) {
213                 sum->ps.total += stat->ps.total;
214                 sum->ps.missed += stat->ps.missed;
215         } else
216                 blk_rq_stat_sum(&sum->rqs, &stat->rqs);
217 }
218 
219 static inline void latency_stat_record_time(struct iolatency_grp *iolat,
220                                             u64 req_time)
221 {
222         struct latency_stat *stat = get_cpu_ptr(iolat->stats);
223         if (iolat->ssd) {
224                 if (req_time >= iolat->min_lat_nsec)
225                         stat->ps.missed++;
226                 stat->ps.total++;
227         } else
228                 blk_rq_stat_add(&stat->rqs, req_time);
229         put_cpu_ptr(stat);
230 }
231 
232 static inline bool latency_sum_ok(struct iolatency_grp *iolat,
233                                   struct latency_stat *stat)
234 {
235         if (iolat->ssd) {
236                 u64 thresh = div64_u64(stat->ps.total, 10);
237                 thresh = max(thresh, 1ULL);
238                 return stat->ps.missed < thresh;
239         }
240         return stat->rqs.mean <= iolat->min_lat_nsec;
241 }
242 
243 static inline u64 latency_stat_samples(struct iolatency_grp *iolat,
244                                        struct latency_stat *stat)
245 {
246         if (iolat->ssd)
247                 return stat->ps.total;
248         return stat->rqs.nr_samples;
249 }
250 
251 static inline void iolat_update_total_lat_avg(struct iolatency_grp *iolat,
252                                               struct latency_stat *stat)
253 {
254         int exp_idx;
255 
256         if (iolat->ssd)
257                 return;
258 
259         /*
260          * calc_load() takes in a number stored in fixed point representation.
261          * Because we are using this for IO time in ns, the values stored
262          * are significantly larger than the FIXED_1 denominator (2048).
263          * Therefore, rounding errors in the calculation are negligible and
264          * can be ignored.
265          */
266         exp_idx = min_t(int, BLKIOLATENCY_NR_EXP_FACTORS - 1,
267                         div64_u64(iolat->cur_win_nsec,
268                                   BLKIOLATENCY_EXP_BUCKET_SIZE));
269         iolat->lat_avg = calc_load(iolat->lat_avg,
270                                    iolatency_exp_factors[exp_idx],
271                                    stat->rqs.mean);
272 }
273 
274 static void iolat_cleanup_cb(struct rq_wait *rqw, void *private_data)
275 {
276         atomic_dec(&rqw->inflight);
277         wake_up(&rqw->wait);
278 }
279 
280 static bool iolat_acquire_inflight(struct rq_wait *rqw, void *private_data)
281 {
282         struct iolatency_grp *iolat = private_data;
283         return rq_wait_inc_below(rqw, iolat->rq_depth.max_depth);
284 }
285 
286 static void __blkcg_iolatency_throttle(struct rq_qos *rqos,
287                                        struct iolatency_grp *iolat,
288                                        bool issue_as_root,
289                                        bool use_memdelay)
290 {
291         struct rq_wait *rqw = &iolat->rq_wait;
292         unsigned use_delay = atomic_read(&lat_to_blkg(iolat)->use_delay);
293 
294         if (use_delay)
295                 blkcg_schedule_throttle(rqos->q, use_memdelay);
296 
297         /*
298          * To avoid priority inversions we want to just take a slot if we are
299          * issuing as root.  If we're being killed off there's no point in
300          * delaying things, we may have been killed by OOM so throttling may
301          * make recovery take even longer, so just let the IO's through so the
302          * task can go away.
303          */
304         if (issue_as_root || fatal_signal_pending(current)) {
305                 atomic_inc(&rqw->inflight);
306                 return;
307         }
308 
309         rq_qos_wait(rqw, iolat, iolat_acquire_inflight, iolat_cleanup_cb);
310 }
311 
312 #define SCALE_DOWN_FACTOR 2
313 #define SCALE_UP_FACTOR 4
314 
315 static inline unsigned long scale_amount(unsigned long qd, bool up)
316 {
317         return max(up ? qd >> SCALE_UP_FACTOR : qd >> SCALE_DOWN_FACTOR, 1UL);
318 }
319 
320 /*
321  * We scale the qd down faster than we scale up, so we need to use this helper
322  * to adjust the scale_cookie accordingly so we don't prematurely get
323  * scale_cookie at DEFAULT_SCALE_COOKIE and unthrottle too much.
324  *
325  * Each group has their own local copy of the last scale cookie they saw, so if
326  * the global scale cookie goes up or down they know which way they need to go
327  * based on their last knowledge of it.
328  */
329 static void scale_cookie_change(struct blk_iolatency *blkiolat,
330                                 struct child_latency_info *lat_info,
331                                 bool up)
332 {
333         unsigned long qd = blkiolat->rqos.q->nr_requests;
334         unsigned long scale = scale_amount(qd, up);
335         unsigned long old = atomic_read(&lat_info->scale_cookie);
336         unsigned long max_scale = qd << 1;
337         unsigned long diff = 0;
338 
339         if (old < DEFAULT_SCALE_COOKIE)
340                 diff = DEFAULT_SCALE_COOKIE - old;
341 
342         if (up) {
343                 if (scale + old > DEFAULT_SCALE_COOKIE)
344                         atomic_set(&lat_info->scale_cookie,
345                                    DEFAULT_SCALE_COOKIE);
346                 else if (diff > qd)
347                         atomic_inc(&lat_info->scale_cookie);
348                 else
349                         atomic_add(scale, &lat_info->scale_cookie);
350         } else {
351                 /*
352                  * We don't want to dig a hole so deep that it takes us hours to
353                  * dig out of it.  Just enough that we don't throttle/unthrottle
354                  * with jagged workloads but can still unthrottle once pressure
355                  * has sufficiently dissipated.
356                  */
357                 if (diff > qd) {
358                         if (diff < max_scale)
359                                 atomic_dec(&lat_info->scale_cookie);
360                 } else {
361                         atomic_sub(scale, &lat_info->scale_cookie);
362                 }
363         }
364 }
365 
366 /*
367  * Change the queue depth of the iolatency_grp.  We add/subtract 1/16th of the
368  * queue depth at a time so we don't get wild swings and hopefully dial in to
369  * fairer distribution of the overall queue depth.
370  */
371 static void scale_change(struct iolatency_grp *iolat, bool up)
372 {
373         unsigned long qd = iolat->blkiolat->rqos.q->nr_requests;
374         unsigned long scale = scale_amount(qd, up);
375         unsigned long old = iolat->rq_depth.max_depth;
376 
377         if (old > qd)
378                 old = qd;
379 
380         if (up) {
381                 if (old == 1 && blkcg_unuse_delay(lat_to_blkg(iolat)))
382                         return;
383 
384                 if (old < qd) {
385                         old += scale;
386                         old = min(old, qd);
387                         iolat->rq_depth.max_depth = old;
388                         wake_up_all(&iolat->rq_wait.wait);
389                 }
390         } else {
391                 old >>= 1;
392                 iolat->rq_depth.max_depth = max(old, 1UL);
393         }
394 }
395 
396 /* Check our parent and see if the scale cookie has changed. */
397 static void check_scale_change(struct iolatency_grp *iolat)
398 {
399         struct iolatency_grp *parent;
400         struct child_latency_info *lat_info;
401         unsigned int cur_cookie;
402         unsigned int our_cookie = atomic_read(&iolat->scale_cookie);
403         u64 scale_lat;
404         int direction = 0;
405 
406         if (lat_to_blkg(iolat)->parent == NULL)
407                 return;
408 
409         parent = blkg_to_lat(lat_to_blkg(iolat)->parent);
410         if (!parent)
411                 return;
412 
413         lat_info = &parent->child_lat;
414         cur_cookie = atomic_read(&lat_info->scale_cookie);
415         scale_lat = READ_ONCE(lat_info->scale_lat);
416 
417         if (cur_cookie < our_cookie)
418                 direction = -1;
419         else if (cur_cookie > our_cookie)
420                 direction = 1;
421         else
422                 return;
423 
424         if (!atomic_try_cmpxchg(&iolat->scale_cookie, &our_cookie, cur_cookie)) {
425                 /* Somebody beat us to the punch, just bail. */
426                 return;
427         }
428 
429         if (direction < 0 && iolat->min_lat_nsec) {
430                 u64 samples_thresh;
431 
432                 if (!scale_lat || iolat->min_lat_nsec <= scale_lat)
433                         return;
434 
435                 /*
436                  * Sometimes high priority groups are their own worst enemy, so
437                  * instead of taking it out on some poor other group that did 5%
438                  * or less of the IO's for the last summation just skip this
439                  * scale down event.
440                  */
441                 samples_thresh = lat_info->nr_samples * 5;
442                 samples_thresh = max(1ULL, div64_u64(samples_thresh, 100));
443                 if (iolat->nr_samples <= samples_thresh)
444                         return;
445         }
446 
447         /* We're as low as we can go. */
448         if (iolat->rq_depth.max_depth == 1 && direction < 0) {
449                 blkcg_use_delay(lat_to_blkg(iolat));
450                 return;
451         }
452 
453         /* We're back to the default cookie, unthrottle all the things. */
454         if (cur_cookie == DEFAULT_SCALE_COOKIE) {
455                 blkcg_clear_delay(lat_to_blkg(iolat));
456                 iolat->rq_depth.max_depth = UINT_MAX;
457                 wake_up_all(&iolat->rq_wait.wait);
458                 return;
459         }
460 
461         scale_change(iolat, direction > 0);
462 }
463 
464 static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio)
465 {
466         struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
467         struct blkcg_gq *blkg = bio->bi_blkg;
468         bool issue_as_root = bio_issue_as_root_blkg(bio);
469 
470         if (!blkiolat->enabled)
471                 return;
472 
473         while (blkg && blkg->parent) {
474                 struct iolatency_grp *iolat = blkg_to_lat(blkg);
475                 if (!iolat) {
476                         blkg = blkg->parent;
477                         continue;
478                 }
479 
480                 check_scale_change(iolat);
481                 __blkcg_iolatency_throttle(rqos, iolat, issue_as_root,
482                                      (bio->bi_opf & REQ_SWAP) == REQ_SWAP);
483                 blkg = blkg->parent;
484         }
485         if (!timer_pending(&blkiolat->timer))
486                 mod_timer(&blkiolat->timer, jiffies + HZ);
487 }
488 
489 static void iolatency_record_time(struct iolatency_grp *iolat,
490                                   struct bio_issue *issue, u64 now,
491                                   bool issue_as_root)
492 {
493         u64 start = bio_issue_time(issue);
494         u64 req_time;
495 
496         /*
497          * Have to do this so we are truncated to the correct time that our
498          * issue is truncated to.
499          */
500         now = __bio_issue_time(now);
501 
502         if (now <= start)
503                 return;
504 
505         req_time = now - start;
506 
507         /*
508          * We don't want to count issue_as_root bio's in the cgroups latency
509          * statistics as it could skew the numbers downwards.
510          */
511         if (unlikely(issue_as_root && iolat->rq_depth.max_depth != UINT_MAX)) {
512                 u64 sub = iolat->min_lat_nsec;
513                 if (req_time < sub)
514                         blkcg_add_delay(lat_to_blkg(iolat), now, sub - req_time);
515                 return;
516         }
517 
518         latency_stat_record_time(iolat, req_time);
519 }
520 
521 #define BLKIOLATENCY_MIN_ADJUST_TIME (500 * NSEC_PER_MSEC)
522 #define BLKIOLATENCY_MIN_GOOD_SAMPLES 5
523 
524 static void iolatency_check_latencies(struct iolatency_grp *iolat, u64 now)
525 {
526         struct blkcg_gq *blkg = lat_to_blkg(iolat);
527         struct iolatency_grp *parent;
528         struct child_latency_info *lat_info;
529         struct latency_stat stat;
530         unsigned long flags;
531         int cpu;
532 
533         latency_stat_init(iolat, &stat);
534         preempt_disable();
535         for_each_online_cpu(cpu) {
536                 struct latency_stat *s;
537                 s = per_cpu_ptr(iolat->stats, cpu);
538                 latency_stat_sum(iolat, &stat, s);
539                 latency_stat_init(iolat, s);
540         }
541         preempt_enable();
542 
543         parent = blkg_to_lat(blkg->parent);
544         if (!parent)
545                 return;
546 
547         lat_info = &parent->child_lat;
548 
549         iolat_update_total_lat_avg(iolat, &stat);
550 
551         /* Everything is ok and we don't need to adjust the scale. */
552         if (latency_sum_ok(iolat, &stat) &&
553             atomic_read(&lat_info->scale_cookie) == DEFAULT_SCALE_COOKIE)
554                 return;
555 
556         /* Somebody beat us to the punch, just bail. */
557         spin_lock_irqsave(&lat_info->lock, flags);
558 
559         latency_stat_sum(iolat, &iolat->cur_stat, &stat);
560         lat_info->nr_samples -= iolat->nr_samples;
561         lat_info->nr_samples += latency_stat_samples(iolat, &iolat->cur_stat);
562         iolat->nr_samples = latency_stat_samples(iolat, &iolat->cur_stat);
563 
564         if ((lat_info->last_scale_event >= now ||
565             now - lat_info->last_scale_event < BLKIOLATENCY_MIN_ADJUST_TIME))
566                 goto out;
567 
568         if (latency_sum_ok(iolat, &iolat->cur_stat) &&
569             latency_sum_ok(iolat, &stat)) {
570                 if (latency_stat_samples(iolat, &iolat->cur_stat) <
571                     BLKIOLATENCY_MIN_GOOD_SAMPLES)
572                         goto out;
573                 if (lat_info->scale_grp == iolat) {
574                         lat_info->last_scale_event = now;
575                         scale_cookie_change(iolat->blkiolat, lat_info, true);
576                 }
577         } else if (lat_info->scale_lat == 0 ||
578                    lat_info->scale_lat >= iolat->min_lat_nsec) {
579                 lat_info->last_scale_event = now;
580                 if (!lat_info->scale_grp ||
581                     lat_info->scale_lat > iolat->min_lat_nsec) {
582                         WRITE_ONCE(lat_info->scale_lat, iolat->min_lat_nsec);
583                         lat_info->scale_grp = iolat;
584                 }
585                 scale_cookie_change(iolat->blkiolat, lat_info, false);
586         }
587         latency_stat_init(iolat, &iolat->cur_stat);
588 out:
589         spin_unlock_irqrestore(&lat_info->lock, flags);
590 }
591 
592 static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
593 {
594         struct blkcg_gq *blkg;
595         struct rq_wait *rqw;
596         struct iolatency_grp *iolat;
597         u64 window_start;
598         u64 now;
599         bool issue_as_root = bio_issue_as_root_blkg(bio);
600         int inflight = 0;
601 
602         blkg = bio->bi_blkg;
603         if (!blkg || !bio_flagged(bio, BIO_QOS_THROTTLED))
604                 return;
605 
606         iolat = blkg_to_lat(bio->bi_blkg);
607         if (!iolat)
608                 return;
609 
610         if (!iolat->blkiolat->enabled)
611                 return;
612 
613         now = ktime_to_ns(ktime_get());
614         while (blkg && blkg->parent) {
615                 iolat = blkg_to_lat(blkg);
616                 if (!iolat) {
617                         blkg = blkg->parent;
618                         continue;
619                 }
620                 rqw = &iolat->rq_wait;
621 
622                 inflight = atomic_dec_return(&rqw->inflight);
623                 WARN_ON_ONCE(inflight < 0);
624                 /*
625                  * If bi_status is BLK_STS_AGAIN, the bio wasn't actually
626                  * submitted, so do not account for it.
627                  */
628                 if (iolat->min_lat_nsec && bio->bi_status != BLK_STS_AGAIN) {
629                         iolatency_record_time(iolat, &bio->bi_issue, now,
630                                               issue_as_root);
631                         window_start = atomic64_read(&iolat->window_start);
632                         if (now > window_start &&
633                             (now - window_start) >= iolat->cur_win_nsec) {
634                                 if (atomic64_try_cmpxchg(&iolat->window_start,
635                                                          &window_start, now))
636                                         iolatency_check_latencies(iolat, now);
637                         }
638                 }
639                 wake_up(&rqw->wait);
640                 blkg = blkg->parent;
641         }
642 }
643 
644 static void blkcg_iolatency_exit(struct rq_qos *rqos)
645 {
646         struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
647 
648         del_timer_sync(&blkiolat->timer);
649         flush_work(&blkiolat->enable_work);
650         blkcg_deactivate_policy(rqos->q, &blkcg_policy_iolatency);
651         kfree(blkiolat);
652 }
653 
654 static struct rq_qos_ops blkcg_iolatency_ops = {
655         .throttle = blkcg_iolatency_throttle,
656         .done_bio = blkcg_iolatency_done_bio,
657         .exit = blkcg_iolatency_exit,
658 };
659 
660 static void blkiolatency_timer_fn(struct timer_list *t)
661 {
662         struct blk_iolatency *blkiolat = from_timer(blkiolat, t, timer);
663         struct blkcg_gq *blkg;
664         struct cgroup_subsys_state *pos_css;
665         u64 now = ktime_to_ns(ktime_get());
666 
667         rcu_read_lock();
668         blkg_for_each_descendant_pre(blkg, pos_css,
669                                      blkiolat->rqos.q->root_blkg) {
670                 struct iolatency_grp *iolat;
671                 struct child_latency_info *lat_info;
672                 unsigned long flags;
673                 u64 cookie;
674 
675                 /*
676                  * We could be exiting, don't access the pd unless we have a
677                  * ref on the blkg.
678                  */
679                 if (!blkg_tryget(blkg))
680                         continue;
681 
682                 iolat = blkg_to_lat(blkg);
683                 if (!iolat)
684                         goto next;
685 
686                 lat_info = &iolat->child_lat;
687                 cookie = atomic_read(&lat_info->scale_cookie);
688 
689                 if (cookie >= DEFAULT_SCALE_COOKIE)
690                         goto next;
691 
692                 spin_lock_irqsave(&lat_info->lock, flags);
693                 if (lat_info->last_scale_event >= now)
694                         goto next_lock;
695 
696                 /*
697                  * We scaled down but don't have a scale_grp, scale up and carry
698                  * on.
699                  */
700                 if (lat_info->scale_grp == NULL) {
701                         scale_cookie_change(iolat->blkiolat, lat_info, true);
702                         goto next_lock;
703                 }
704 
705                 /*
706                  * It's been 5 seconds since our last scale event, clear the
707                  * scale grp in case the group that needed the scale down isn't
708                  * doing any IO currently.
709                  */
710                 if (now - lat_info->last_scale_event >=
711                     ((u64)NSEC_PER_SEC * 5))
712                         lat_info->scale_grp = NULL;
713 next_lock:
714                 spin_unlock_irqrestore(&lat_info->lock, flags);
715 next:
716                 blkg_put(blkg);
717         }
718         rcu_read_unlock();
719 }
720 
721 /**
722  * blkiolatency_enable_work_fn - Enable or disable iolatency on the device
723  * @work: enable_work of the blk_iolatency of interest
724  *
725  * iolatency needs to keep track of the number of in-flight IOs per cgroup. This
726  * is relatively expensive as it involves walking up the hierarchy twice for
727  * every IO. Thus, if iolatency is not enabled in any cgroup for the device, we
728  * want to disable the in-flight tracking.
729  *
730  * We have to make sure that the counting is balanced - we don't want to leak
731  * the in-flight counts by disabling accounting in the completion path while IOs
732  * are in flight. This is achieved by ensuring that no IO is in flight by
733  * freezing the queue while flipping ->enabled. As this requires a sleepable
734  * context, ->enabled flipping is punted to this work function.
735  */
736 static void blkiolatency_enable_work_fn(struct work_struct *work)
737 {
738         struct blk_iolatency *blkiolat = container_of(work, struct blk_iolatency,
739                                                       enable_work);
740         bool enabled;
741 
742         /*
743          * There can only be one instance of this function running for @blkiolat
744          * and it's guaranteed to be executed at least once after the latest
745          * ->enabled_cnt modification. Acting on the latest ->enable_cnt is
746          * sufficient.
747          *
748          * Also, we know @blkiolat is safe to access as ->enable_work is flushed
749          * in blkcg_iolatency_exit().
750          */
751         enabled = atomic_read(&blkiolat->enable_cnt);
752         if (enabled != blkiolat->enabled) {
753                 blk_mq_freeze_queue(blkiolat->rqos.q);
754                 blkiolat->enabled = enabled;
755                 blk_mq_unfreeze_queue(blkiolat->rqos.q);
756         }
757 }
758 
759 int blk_iolatency_init(struct request_queue *q)
760 {
761         struct blk_iolatency *blkiolat;
762         struct rq_qos *rqos;
763         int ret;
764 
765         blkiolat = kzalloc(sizeof(*blkiolat), GFP_KERNEL);
766         if (!blkiolat)
767                 return -ENOMEM;
768 
769         rqos = &blkiolat->rqos;
770         rqos->id = RQ_QOS_LATENCY;
771         rqos->ops = &blkcg_iolatency_ops;
772         rqos->q = q;
773 
774         ret = rq_qos_add(q, rqos);
775         if (ret)
776                 goto err_free;
777         ret = blkcg_activate_policy(q, &blkcg_policy_iolatency);
778         if (ret)
779                 goto err_qos_del;
780 
781         timer_setup(&blkiolat->timer, blkiolatency_timer_fn, 0);
782         INIT_WORK(&blkiolat->enable_work, blkiolatency_enable_work_fn);
783 
784         return 0;
785 
786 err_qos_del:
787         rq_qos_del(q, rqos);
788 err_free:
789         kfree(blkiolat);
790         return ret;
791 }
792 
793 static void iolatency_set_min_lat_nsec(struct blkcg_gq *blkg, u64 val)
794 {
795         struct iolatency_grp *iolat = blkg_to_lat(blkg);
796         struct blk_iolatency *blkiolat = iolat->blkiolat;
797         u64 oldval = iolat->min_lat_nsec;
798 
799         iolat->min_lat_nsec = val;
800         iolat->cur_win_nsec = max_t(u64, val << 4, BLKIOLATENCY_MIN_WIN_SIZE);
801         iolat->cur_win_nsec = min_t(u64, iolat->cur_win_nsec,
802                                     BLKIOLATENCY_MAX_WIN_SIZE);
803 
804         if (!oldval && val) {
805                 if (atomic_inc_return(&blkiolat->enable_cnt) == 1)
806                         schedule_work(&blkiolat->enable_work);
807         }
808         if (oldval && !val) {
809                 blkcg_clear_delay(blkg);
810                 if (atomic_dec_return(&blkiolat->enable_cnt) == 0)
811                         schedule_work(&blkiolat->enable_work);
812         }
813 }
814 
815 static void iolatency_clear_scaling(struct blkcg_gq *blkg)
816 {
817         if (blkg->parent) {
818                 struct iolatency_grp *iolat = blkg_to_lat(blkg->parent);
819                 struct child_latency_info *lat_info;
820                 if (!iolat)
821                         return;
822 
823                 lat_info = &iolat->child_lat;
824                 spin_lock(&lat_info->lock);
825                 atomic_set(&lat_info->scale_cookie, DEFAULT_SCALE_COOKIE);
826                 lat_info->last_scale_event = 0;
827                 lat_info->scale_grp = NULL;
828                 lat_info->scale_lat = 0;
829                 spin_unlock(&lat_info->lock);
830         }
831 }
832 
833 static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
834                              size_t nbytes, loff_t off)
835 {
836         struct blkcg *blkcg = css_to_blkcg(of_css(of));
837         struct blkcg_gq *blkg;
838         struct blkg_conf_ctx ctx;
839         struct iolatency_grp *iolat;
840         char *p, *tok;
841         u64 lat_val = 0;
842         u64 oldval;
843         int ret;
844 
845         ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, buf, &ctx);
846         if (ret)
847                 return ret;
848 
849         iolat = blkg_to_lat(ctx.blkg);
850         p = ctx.body;
851 
852         ret = -EINVAL;
853         while ((tok = strsep(&p, " "))) {
854                 char key[16];
855                 char val[21];   /* 18446744073709551616 */
856 
857                 if (sscanf(tok, "%15[^=]=%20s", key, val) != 2)
858                         goto out;
859 
860                 if (!strcmp(key, "target")) {
861                         u64 v;
862 
863                         if (!strcmp(val, "max"))
864                                 lat_val = 0;
865                         else if (sscanf(val, "%llu", &v) == 1)
866                                 lat_val = v * NSEC_PER_USEC;
867                         else
868                                 goto out;
869                 } else {
870                         goto out;
871                 }
872         }
873 
874         /* Walk up the tree to see if our new val is lower than it should be. */
875         blkg = ctx.blkg;
876         oldval = iolat->min_lat_nsec;
877 
878         iolatency_set_min_lat_nsec(blkg, lat_val);
879         if (oldval != iolat->min_lat_nsec)
880                 iolatency_clear_scaling(blkg);
881         ret = 0;
882 out:
883         blkg_conf_finish(&ctx);
884         return ret ?: nbytes;
885 }
886 
887 static u64 iolatency_prfill_limit(struct seq_file *sf,
888                                   struct blkg_policy_data *pd, int off)
889 {
890         struct iolatency_grp *iolat = pd_to_lat(pd);
891         const char *dname = blkg_dev_name(pd->blkg);
892 
893         if (!dname || !iolat->min_lat_nsec)
894                 return 0;
895         seq_printf(sf, "%s target=%llu\n",
896                    dname, div_u64(iolat->min_lat_nsec, NSEC_PER_USEC));
897         return 0;
898 }
899 
900 static int iolatency_print_limit(struct seq_file *sf, void *v)
901 {
902         blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
903                           iolatency_prfill_limit,
904                           &blkcg_policy_iolatency, seq_cft(sf)->private, false);
905         return 0;
906 }
907 
908 static void iolatency_ssd_stat(struct iolatency_grp *iolat, struct seq_file *s)
909 {
910         struct latency_stat stat;
911         int cpu;
912 
913         latency_stat_init(iolat, &stat);
914         preempt_disable();
915         for_each_online_cpu(cpu) {
916                 struct latency_stat *s;
917                 s = per_cpu_ptr(iolat->stats, cpu);
918                 latency_stat_sum(iolat, &stat, s);
919         }
920         preempt_enable();
921 
922         if (iolat->rq_depth.max_depth == UINT_MAX)
923                 seq_printf(s, " missed=%llu total=%llu depth=max",
924                         (unsigned long long)stat.ps.missed,
925                         (unsigned long long)stat.ps.total);
926         else
927                 seq_printf(s, " missed=%llu total=%llu depth=%u",
928                         (unsigned long long)stat.ps.missed,
929                         (unsigned long long)stat.ps.total,
930                         iolat->rq_depth.max_depth);
931 }
932 
933 static void iolatency_pd_stat(struct blkg_policy_data *pd, struct seq_file *s)
934 {
935         struct iolatency_grp *iolat = pd_to_lat(pd);
936         unsigned long long avg_lat;
937         unsigned long long cur_win;
938 
939         if (!blkcg_debug_stats)
940                 return;
941 
942         if (iolat->ssd)
943                 return iolatency_ssd_stat(iolat, s);
944 
945         avg_lat = div64_u64(iolat->lat_avg, NSEC_PER_USEC);
946         cur_win = div64_u64(iolat->cur_win_nsec, NSEC_PER_MSEC);
947         if (iolat->rq_depth.max_depth == UINT_MAX)
948                 seq_printf(s, " depth=max avg_lat=%llu win=%llu",
949                         avg_lat, cur_win);
950         else
951                 seq_printf(s, " depth=%u avg_lat=%llu win=%llu",
952                         iolat->rq_depth.max_depth, avg_lat, cur_win);
953 }
954 
955 static struct blkg_policy_data *iolatency_pd_alloc(gfp_t gfp,
956                                                    struct request_queue *q,
957                                                    struct blkcg *blkcg)
958 {
959         struct iolatency_grp *iolat;
960 
961         iolat = kzalloc_node(sizeof(*iolat), gfp, q->node);
962         if (!iolat)
963                 return NULL;
964         iolat->stats = __alloc_percpu_gfp(sizeof(struct latency_stat),
965                                        __alignof__(struct latency_stat), gfp);
966         if (!iolat->stats) {
967                 kfree(iolat);
968                 return NULL;
969         }
970         return &iolat->pd;
971 }
972 
973 static void iolatency_pd_init(struct blkg_policy_data *pd)
974 {
975         struct iolatency_grp *iolat = pd_to_lat(pd);
976         struct blkcg_gq *blkg = lat_to_blkg(iolat);
977         struct rq_qos *rqos = blkcg_rq_qos(blkg->q);
978         struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
979         u64 now = ktime_to_ns(ktime_get());
980         int cpu;
981 
982         if (blk_queue_nonrot(blkg->q))
983                 iolat->ssd = true;
984         else
985                 iolat->ssd = false;
986 
987         for_each_possible_cpu(cpu) {
988                 struct latency_stat *stat;
989                 stat = per_cpu_ptr(iolat->stats, cpu);
990                 latency_stat_init(iolat, stat);
991         }
992 
993         latency_stat_init(iolat, &iolat->cur_stat);
994         rq_wait_init(&iolat->rq_wait);
995         spin_lock_init(&iolat->child_lat.lock);
996         iolat->rq_depth.queue_depth = blkg->q->nr_requests;
997         iolat->rq_depth.max_depth = UINT_MAX;
998         iolat->rq_depth.default_depth = iolat->rq_depth.queue_depth;
999         iolat->blkiolat = blkiolat;
1000         iolat->cur_win_nsec = 100 * NSEC_PER_MSEC;
1001         atomic64_set(&iolat->window_start, now);
1002 
1003         /*
1004          * We init things in list order, so the pd for the parent may not be
1005          * init'ed yet for whatever reason.
1006          */
1007         if (blkg->parent && blkg_to_pd(blkg->parent, &blkcg_policy_iolatency)) {
1008                 struct iolatency_grp *parent = blkg_to_lat(blkg->parent);
1009                 atomic_set(&iolat->scale_cookie,
1010                            atomic_read(&parent->child_lat.scale_cookie));
1011         } else {
1012                 atomic_set(&iolat->scale_cookie, DEFAULT_SCALE_COOKIE);
1013         }
1014 
1015         atomic_set(&iolat->child_lat.scale_cookie, DEFAULT_SCALE_COOKIE);
1016 }
1017 
1018 static void iolatency_pd_offline(struct blkg_policy_data *pd)
1019 {
1020         struct iolatency_grp *iolat = pd_to_lat(pd);
1021         struct blkcg_gq *blkg = lat_to_blkg(iolat);
1022 
1023         iolatency_set_min_lat_nsec(blkg, 0);
1024         iolatency_clear_scaling(blkg);
1025 }
1026 
1027 static void iolatency_pd_free(struct blkg_policy_data *pd)
1028 {
1029         struct iolatency_grp *iolat = pd_to_lat(pd);
1030         free_percpu(iolat->stats);
1031         kfree(iolat);
1032 }
1033 
1034 static struct cftype iolatency_files[] = {
1035         {
1036                 .name = "latency",
1037                 .flags = CFTYPE_NOT_ON_ROOT,
1038                 .seq_show = iolatency_print_limit,
1039                 .write = iolatency_set_limit,
1040         },
1041         {}
1042 };
1043 
1044 static struct blkcg_policy blkcg_policy_iolatency = {
1045         .dfl_cftypes    = iolatency_files,
1046         .pd_alloc_fn    = iolatency_pd_alloc,
1047         .pd_init_fn     = iolatency_pd_init,
1048         .pd_offline_fn  = iolatency_pd_offline,
1049         .pd_free_fn     = iolatency_pd_free,
1050         .pd_stat_fn     = iolatency_pd_stat,
1051 };
1052 
1053 static int __init iolatency_init(void)
1054 {
1055         return blkcg_policy_register(&blkcg_policy_iolatency);
1056 }
1057 
1058 static void __exit iolatency_exit(void)
1059 {
1060         blkcg_policy_unregister(&blkcg_policy_iolatency);
1061 }
1062 
1063 module_init(iolatency_init);
1064 module_exit(iolatency_exit);
1065 

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