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Linux/block/blk-wbt.c

  1 /*
  2  * buffered writeback throttling. loosely based on CoDel. We can't drop
  3  * packets for IO scheduling, so the logic is something like this:
  4  *
  5  * - Monitor latencies in a defined window of time.
  6  * - If the minimum latency in the above window exceeds some target, increment
  7  *   scaling step and scale down queue depth by a factor of 2x. The monitoring
  8  *   window is then shrunk to 100 / sqrt(scaling step + 1).
  9  * - For any window where we don't have solid data on what the latencies
 10  *   look like, retain status quo.
 11  * - If latencies look good, decrement scaling step.
 12  * - If we're only doing writes, allow the scaling step to go negative. This
 13  *   will temporarily boost write performance, snapping back to a stable
 14  *   scaling step of 0 if reads show up or the heavy writers finish. Unlike
 15  *   positive scaling steps where we shrink the monitoring window, a negative
 16  *   scaling step retains the default step==0 window size.
 17  *
 18  * Copyright (C) 2016 Jens Axboe
 19  *
 20  */
 21 #include <linux/kernel.h>
 22 #include <linux/blk_types.h>
 23 #include <linux/slab.h>
 24 #include <linux/backing-dev.h>
 25 #include <linux/swap.h>
 26 
 27 #include "blk-wbt.h"
 28 
 29 #define CREATE_TRACE_POINTS
 30 #include <trace/events/wbt.h>
 31 
 32 enum {
 33         /*
 34          * Default setting, we'll scale up (to 75% of QD max) or down (min 1)
 35          * from here depending on device stats
 36          */
 37         RWB_DEF_DEPTH   = 16,
 38 
 39         /*
 40          * 100msec window
 41          */
 42         RWB_WINDOW_NSEC         = 100 * 1000 * 1000ULL,
 43 
 44         /*
 45          * Disregard stats, if we don't meet this minimum
 46          */
 47         RWB_MIN_WRITE_SAMPLES   = 3,
 48 
 49         /*
 50          * If we have this number of consecutive windows with not enough
 51          * information to scale up or down, scale up.
 52          */
 53         RWB_UNKNOWN_BUMP        = 5,
 54 };
 55 
 56 static inline bool rwb_enabled(struct rq_wb *rwb)
 57 {
 58         return rwb && rwb->wb_normal != 0;
 59 }
 60 
 61 /*
 62  * Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
 63  * false if 'v' + 1 would be bigger than 'below'.
 64  */
 65 static bool atomic_inc_below(atomic_t *v, int below)
 66 {
 67         int cur = atomic_read(v);
 68 
 69         for (;;) {
 70                 int old;
 71 
 72                 if (cur >= below)
 73                         return false;
 74                 old = atomic_cmpxchg(v, cur, cur + 1);
 75                 if (old == cur)
 76                         break;
 77                 cur = old;
 78         }
 79 
 80         return true;
 81 }
 82 
 83 static void wb_timestamp(struct rq_wb *rwb, unsigned long *var)
 84 {
 85         if (rwb_enabled(rwb)) {
 86                 const unsigned long cur = jiffies;
 87 
 88                 if (cur != *var)
 89                         *var = cur;
 90         }
 91 }
 92 
 93 /*
 94  * If a task was rate throttled in balance_dirty_pages() within the last
 95  * second or so, use that to indicate a higher cleaning rate.
 96  */
 97 static bool wb_recent_wait(struct rq_wb *rwb)
 98 {
 99         struct bdi_writeback *wb = &rwb->queue->backing_dev_info.wb;
100 
101         return time_before(jiffies, wb->dirty_sleep + HZ);
102 }
103 
104 static inline struct rq_wait *get_rq_wait(struct rq_wb *rwb, bool is_kswapd)
105 {
106         return &rwb->rq_wait[is_kswapd];
107 }
108 
109 static void rwb_wake_all(struct rq_wb *rwb)
110 {
111         int i;
112 
113         for (i = 0; i < WBT_NUM_RWQ; i++) {
114                 struct rq_wait *rqw = &rwb->rq_wait[i];
115 
116                 if (waitqueue_active(&rqw->wait))
117                         wake_up_all(&rqw->wait);
118         }
119 }
120 
121 void __wbt_done(struct rq_wb *rwb, enum wbt_flags wb_acct)
122 {
123         struct rq_wait *rqw;
124         int inflight, limit;
125 
126         if (!(wb_acct & WBT_TRACKED))
127                 return;
128 
129         rqw = get_rq_wait(rwb, wb_acct & WBT_KSWAPD);
130         inflight = atomic_dec_return(&rqw->inflight);
131 
132         /*
133          * wbt got disabled with IO in flight. Wake up any potential
134          * waiters, we don't have to do more than that.
135          */
136         if (unlikely(!rwb_enabled(rwb))) {
137                 rwb_wake_all(rwb);
138                 return;
139         }
140 
141         /*
142          * If the device does write back caching, drop further down
143          * before we wake people up.
144          */
145         if (rwb->wc && !wb_recent_wait(rwb))
146                 limit = 0;
147         else
148                 limit = rwb->wb_normal;
149 
150         /*
151          * Don't wake anyone up if we are above the normal limit.
152          */
153         if (inflight && inflight >= limit)
154                 return;
155 
156         if (waitqueue_active(&rqw->wait)) {
157                 int diff = limit - inflight;
158 
159                 if (!inflight || diff >= rwb->wb_background / 2)
160                         wake_up_all(&rqw->wait);
161         }
162 }
163 
164 /*
165  * Called on completion of a request. Note that it's also called when
166  * a request is merged, when the request gets freed.
167  */
168 void wbt_done(struct rq_wb *rwb, struct blk_issue_stat *stat)
169 {
170         if (!rwb)
171                 return;
172 
173         if (!wbt_is_tracked(stat)) {
174                 if (rwb->sync_cookie == stat) {
175                         rwb->sync_issue = 0;
176                         rwb->sync_cookie = NULL;
177                 }
178 
179                 if (wbt_is_read(stat))
180                         wb_timestamp(rwb, &rwb->last_comp);
181                 wbt_clear_state(stat);
182         } else {
183                 WARN_ON_ONCE(stat == rwb->sync_cookie);
184                 __wbt_done(rwb, wbt_stat_to_mask(stat));
185                 wbt_clear_state(stat);
186         }
187 }
188 
189 /*
190  * Return true, if we can't increase the depth further by scaling
191  */
192 static bool calc_wb_limits(struct rq_wb *rwb)
193 {
194         unsigned int depth;
195         bool ret = false;
196 
197         if (!rwb->min_lat_nsec) {
198                 rwb->wb_max = rwb->wb_normal = rwb->wb_background = 0;
199                 return false;
200         }
201 
202         /*
203          * For QD=1 devices, this is a special case. It's important for those
204          * to have one request ready when one completes, so force a depth of
205          * 2 for those devices. On the backend, it'll be a depth of 1 anyway,
206          * since the device can't have more than that in flight. If we're
207          * scaling down, then keep a setting of 1/1/1.
208          */
209         if (rwb->queue_depth == 1) {
210                 if (rwb->scale_step > 0)
211                         rwb->wb_max = rwb->wb_normal = 1;
212                 else {
213                         rwb->wb_max = rwb->wb_normal = 2;
214                         ret = true;
215                 }
216                 rwb->wb_background = 1;
217         } else {
218                 /*
219                  * scale_step == 0 is our default state. If we have suffered
220                  * latency spikes, step will be > 0, and we shrink the
221                  * allowed write depths. If step is < 0, we're only doing
222                  * writes, and we allow a temporarily higher depth to
223                  * increase performance.
224                  */
225                 depth = min_t(unsigned int, RWB_DEF_DEPTH, rwb->queue_depth);
226                 if (rwb->scale_step > 0)
227                         depth = 1 + ((depth - 1) >> min(31, rwb->scale_step));
228                 else if (rwb->scale_step < 0) {
229                         unsigned int maxd = 3 * rwb->queue_depth / 4;
230 
231                         depth = 1 + ((depth - 1) << -rwb->scale_step);
232                         if (depth > maxd) {
233                                 depth = maxd;
234                                 ret = true;
235                         }
236                 }
237 
238                 /*
239                  * Set our max/normal/bg queue depths based on how far
240                  * we have scaled down (->scale_step).
241                  */
242                 rwb->wb_max = depth;
243                 rwb->wb_normal = (rwb->wb_max + 1) / 2;
244                 rwb->wb_background = (rwb->wb_max + 3) / 4;
245         }
246 
247         return ret;
248 }
249 
250 static inline bool stat_sample_valid(struct blk_rq_stat *stat)
251 {
252         /*
253          * We need at least one read sample, and a minimum of
254          * RWB_MIN_WRITE_SAMPLES. We require some write samples to know
255          * that it's writes impacting us, and not just some sole read on
256          * a device that is in a lower power state.
257          */
258         return stat[BLK_STAT_READ].nr_samples >= 1 &&
259                 stat[BLK_STAT_WRITE].nr_samples >= RWB_MIN_WRITE_SAMPLES;
260 }
261 
262 static u64 rwb_sync_issue_lat(struct rq_wb *rwb)
263 {
264         u64 now, issue = ACCESS_ONCE(rwb->sync_issue);
265 
266         if (!issue || !rwb->sync_cookie)
267                 return 0;
268 
269         now = ktime_to_ns(ktime_get());
270         return now - issue;
271 }
272 
273 enum {
274         LAT_OK = 1,
275         LAT_UNKNOWN,
276         LAT_UNKNOWN_WRITES,
277         LAT_EXCEEDED,
278 };
279 
280 static int __latency_exceeded(struct rq_wb *rwb, struct blk_rq_stat *stat)
281 {
282         struct backing_dev_info *bdi = &rwb->queue->backing_dev_info;
283         u64 thislat;
284 
285         /*
286          * If our stored sync issue exceeds the window size, or it
287          * exceeds our min target AND we haven't logged any entries,
288          * flag the latency as exceeded. wbt works off completion latencies,
289          * but for a flooded device, a single sync IO can take a long time
290          * to complete after being issued. If this time exceeds our
291          * monitoring window AND we didn't see any other completions in that
292          * window, then count that sync IO as a violation of the latency.
293          */
294         thislat = rwb_sync_issue_lat(rwb);
295         if (thislat > rwb->cur_win_nsec ||
296             (thislat > rwb->min_lat_nsec && !stat[BLK_STAT_READ].nr_samples)) {
297                 trace_wbt_lat(bdi, thislat);
298                 return LAT_EXCEEDED;
299         }
300 
301         /*
302          * No read/write mix, if stat isn't valid
303          */
304         if (!stat_sample_valid(stat)) {
305                 /*
306                  * If we had writes in this stat window and the window is
307                  * current, we're only doing writes. If a task recently
308                  * waited or still has writes in flights, consider us doing
309                  * just writes as well.
310                  */
311                 if ((stat[BLK_STAT_WRITE].nr_samples && blk_stat_is_current(stat)) ||
312                     wb_recent_wait(rwb) || wbt_inflight(rwb))
313                         return LAT_UNKNOWN_WRITES;
314                 return LAT_UNKNOWN;
315         }
316 
317         /*
318          * If the 'min' latency exceeds our target, step down.
319          */
320         if (stat[BLK_STAT_READ].min > rwb->min_lat_nsec) {
321                 trace_wbt_lat(bdi, stat[BLK_STAT_READ].min);
322                 trace_wbt_stat(bdi, stat);
323                 return LAT_EXCEEDED;
324         }
325 
326         if (rwb->scale_step)
327                 trace_wbt_stat(bdi, stat);
328 
329         return LAT_OK;
330 }
331 
332 static int latency_exceeded(struct rq_wb *rwb)
333 {
334         struct blk_rq_stat stat[2];
335 
336         blk_queue_stat_get(rwb->queue, stat);
337         return __latency_exceeded(rwb, stat);
338 }
339 
340 static void rwb_trace_step(struct rq_wb *rwb, const char *msg)
341 {
342         struct backing_dev_info *bdi = &rwb->queue->backing_dev_info;
343 
344         trace_wbt_step(bdi, msg, rwb->scale_step, rwb->cur_win_nsec,
345                         rwb->wb_background, rwb->wb_normal, rwb->wb_max);
346 }
347 
348 static void scale_up(struct rq_wb *rwb)
349 {
350         /*
351          * Hit max in previous round, stop here
352          */
353         if (rwb->scaled_max)
354                 return;
355 
356         rwb->scale_step--;
357         rwb->unknown_cnt = 0;
358         blk_stat_clear(rwb->queue);
359 
360         rwb->scaled_max = calc_wb_limits(rwb);
361 
362         rwb_wake_all(rwb);
363 
364         rwb_trace_step(rwb, "step up");
365 }
366 
367 /*
368  * Scale rwb down. If 'hard_throttle' is set, do it quicker, since we
369  * had a latency violation.
370  */
371 static void scale_down(struct rq_wb *rwb, bool hard_throttle)
372 {
373         /*
374          * Stop scaling down when we've hit the limit. This also prevents
375          * ->scale_step from going to crazy values, if the device can't
376          * keep up.
377          */
378         if (rwb->wb_max == 1)
379                 return;
380 
381         if (rwb->scale_step < 0 && hard_throttle)
382                 rwb->scale_step = 0;
383         else
384                 rwb->scale_step++;
385 
386         rwb->scaled_max = false;
387         rwb->unknown_cnt = 0;
388         blk_stat_clear(rwb->queue);
389         calc_wb_limits(rwb);
390         rwb_trace_step(rwb, "step down");
391 }
392 
393 static void rwb_arm_timer(struct rq_wb *rwb)
394 {
395         unsigned long expires;
396 
397         if (rwb->scale_step > 0) {
398                 /*
399                  * We should speed this up, using some variant of a fast
400                  * integer inverse square root calculation. Since we only do
401                  * this for every window expiration, it's not a huge deal,
402                  * though.
403                  */
404                 rwb->cur_win_nsec = div_u64(rwb->win_nsec << 4,
405                                         int_sqrt((rwb->scale_step + 1) << 8));
406         } else {
407                 /*
408                  * For step < 0, we don't want to increase/decrease the
409                  * window size.
410                  */
411                 rwb->cur_win_nsec = rwb->win_nsec;
412         }
413 
414         expires = jiffies + nsecs_to_jiffies(rwb->cur_win_nsec);
415         mod_timer(&rwb->window_timer, expires);
416 }
417 
418 static void wb_timer_fn(unsigned long data)
419 {
420         struct rq_wb *rwb = (struct rq_wb *) data;
421         unsigned int inflight = wbt_inflight(rwb);
422         int status;
423 
424         status = latency_exceeded(rwb);
425 
426         trace_wbt_timer(&rwb->queue->backing_dev_info, status, rwb->scale_step,
427                         inflight);
428 
429         /*
430          * If we exceeded the latency target, step down. If we did not,
431          * step one level up. If we don't know enough to say either exceeded
432          * or ok, then don't do anything.
433          */
434         switch (status) {
435         case LAT_EXCEEDED:
436                 scale_down(rwb, true);
437                 break;
438         case LAT_OK:
439                 scale_up(rwb);
440                 break;
441         case LAT_UNKNOWN_WRITES:
442                 /*
443                  * We started a the center step, but don't have a valid
444                  * read/write sample, but we do have writes going on.
445                  * Allow step to go negative, to increase write perf.
446                  */
447                 scale_up(rwb);
448                 break;
449         case LAT_UNKNOWN:
450                 if (++rwb->unknown_cnt < RWB_UNKNOWN_BUMP)
451                         break;
452                 /*
453                  * We get here when previously scaled reduced depth, and we
454                  * currently don't have a valid read/write sample. For that
455                  * case, slowly return to center state (step == 0).
456                  */
457                 if (rwb->scale_step > 0)
458                         scale_up(rwb);
459                 else if (rwb->scale_step < 0)
460                         scale_down(rwb, false);
461                 break;
462         default:
463                 break;
464         }
465 
466         /*
467          * Re-arm timer, if we have IO in flight
468          */
469         if (rwb->scale_step || inflight)
470                 rwb_arm_timer(rwb);
471 }
472 
473 void wbt_update_limits(struct rq_wb *rwb)
474 {
475         rwb->scale_step = 0;
476         rwb->scaled_max = false;
477         calc_wb_limits(rwb);
478 
479         rwb_wake_all(rwb);
480 }
481 
482 static bool close_io(struct rq_wb *rwb)
483 {
484         const unsigned long now = jiffies;
485 
486         return time_before(now, rwb->last_issue + HZ / 10) ||
487                 time_before(now, rwb->last_comp + HZ / 10);
488 }
489 
490 #define REQ_HIPRIO      (REQ_SYNC | REQ_META | REQ_PRIO)
491 
492 static inline unsigned int get_limit(struct rq_wb *rwb, unsigned long rw)
493 {
494         unsigned int limit;
495 
496         /*
497          * At this point we know it's a buffered write. If this is
498          * kswapd trying to free memory, or REQ_SYNC is set, set, then
499          * it's WB_SYNC_ALL writeback, and we'll use the max limit for
500          * that. If the write is marked as a background write, then use
501          * the idle limit, or go to normal if we haven't had competing
502          * IO for a bit.
503          */
504         if ((rw & REQ_HIPRIO) || wb_recent_wait(rwb) || current_is_kswapd())
505                 limit = rwb->wb_max;
506         else if ((rw & REQ_BACKGROUND) || close_io(rwb)) {
507                 /*
508                  * If less than 100ms since we completed unrelated IO,
509                  * limit us to half the depth for background writeback.
510                  */
511                 limit = rwb->wb_background;
512         } else
513                 limit = rwb->wb_normal;
514 
515         return limit;
516 }
517 
518 static inline bool may_queue(struct rq_wb *rwb, struct rq_wait *rqw,
519                              wait_queue_t *wait, unsigned long rw)
520 {
521         /*
522          * inc it here even if disabled, since we'll dec it at completion.
523          * this only happens if the task was sleeping in __wbt_wait(),
524          * and someone turned it off at the same time.
525          */
526         if (!rwb_enabled(rwb)) {
527                 atomic_inc(&rqw->inflight);
528                 return true;
529         }
530 
531         /*
532          * If the waitqueue is already active and we are not the next
533          * in line to be woken up, wait for our turn.
534          */
535         if (waitqueue_active(&rqw->wait) &&
536             rqw->wait.task_list.next != &wait->task_list)
537                 return false;
538 
539         return atomic_inc_below(&rqw->inflight, get_limit(rwb, rw));
540 }
541 
542 /*
543  * Block if we will exceed our limit, or if we are currently waiting for
544  * the timer to kick off queuing again.
545  */
546 static void __wbt_wait(struct rq_wb *rwb, unsigned long rw, spinlock_t *lock)
547         __releases(lock)
548         __acquires(lock)
549 {
550         struct rq_wait *rqw = get_rq_wait(rwb, current_is_kswapd());
551         DEFINE_WAIT(wait);
552 
553         if (may_queue(rwb, rqw, &wait, rw))
554                 return;
555 
556         do {
557                 prepare_to_wait_exclusive(&rqw->wait, &wait,
558                                                 TASK_UNINTERRUPTIBLE);
559 
560                 if (may_queue(rwb, rqw, &wait, rw))
561                         break;
562 
563                 if (lock) {
564                         spin_unlock_irq(lock);
565                         io_schedule();
566                         spin_lock_irq(lock);
567                 } else
568                         io_schedule();
569         } while (1);
570 
571         finish_wait(&rqw->wait, &wait);
572 }
573 
574 static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio *bio)
575 {
576         const int op = bio_op(bio);
577 
578         /*
579          * If not a WRITE, do nothing
580          */
581         if (op != REQ_OP_WRITE)
582                 return false;
583 
584         /*
585          * Don't throttle WRITE_ODIRECT
586          */
587         if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) == (REQ_SYNC | REQ_IDLE))
588                 return false;
589 
590         return true;
591 }
592 
593 /*
594  * Returns true if the IO request should be accounted, false if not.
595  * May sleep, if we have exceeded the writeback limits. Caller can pass
596  * in an irq held spinlock, if it holds one when calling this function.
597  * If we do sleep, we'll release and re-grab it.
598  */
599 enum wbt_flags wbt_wait(struct rq_wb *rwb, struct bio *bio, spinlock_t *lock)
600 {
601         unsigned int ret = 0;
602 
603         if (!rwb_enabled(rwb))
604                 return 0;
605 
606         if (bio_op(bio) == REQ_OP_READ)
607                 ret = WBT_READ;
608 
609         if (!wbt_should_throttle(rwb, bio)) {
610                 if (ret & WBT_READ)
611                         wb_timestamp(rwb, &rwb->last_issue);
612                 return ret;
613         }
614 
615         __wbt_wait(rwb, bio->bi_opf, lock);
616 
617         if (!timer_pending(&rwb->window_timer))
618                 rwb_arm_timer(rwb);
619 
620         if (current_is_kswapd())
621                 ret |= WBT_KSWAPD;
622 
623         return ret | WBT_TRACKED;
624 }
625 
626 void wbt_issue(struct rq_wb *rwb, struct blk_issue_stat *stat)
627 {
628         if (!rwb_enabled(rwb))
629                 return;
630 
631         /*
632          * Track sync issue, in case it takes a long time to complete. Allows
633          * us to react quicker, if a sync IO takes a long time to complete.
634          * Note that this is just a hint. 'stat' can go away when the
635          * request completes, so it's important we never dereference it. We
636          * only use the address to compare with, which is why we store the
637          * sync_issue time locally.
638          */
639         if (wbt_is_read(stat) && !rwb->sync_issue) {
640                 rwb->sync_cookie = stat;
641                 rwb->sync_issue = blk_stat_time(stat);
642         }
643 }
644 
645 void wbt_requeue(struct rq_wb *rwb, struct blk_issue_stat *stat)
646 {
647         if (!rwb_enabled(rwb))
648                 return;
649         if (stat == rwb->sync_cookie) {
650                 rwb->sync_issue = 0;
651                 rwb->sync_cookie = NULL;
652         }
653 }
654 
655 void wbt_set_queue_depth(struct rq_wb *rwb, unsigned int depth)
656 {
657         if (rwb) {
658                 rwb->queue_depth = depth;
659                 wbt_update_limits(rwb);
660         }
661 }
662 
663 void wbt_set_write_cache(struct rq_wb *rwb, bool write_cache_on)
664 {
665         if (rwb)
666                 rwb->wc = write_cache_on;
667 }
668 
669  /*
670  * Disable wbt, if enabled by default. Only called from CFQ, if we have
671  * cgroups enabled
672  */
673 void wbt_disable_default(struct request_queue *q)
674 {
675         struct rq_wb *rwb = q->rq_wb;
676 
677         if (rwb && rwb->enable_state == WBT_STATE_ON_DEFAULT) {
678                 del_timer_sync(&rwb->window_timer);
679                 rwb->win_nsec = rwb->min_lat_nsec = 0;
680                 wbt_update_limits(rwb);
681         }
682 }
683 EXPORT_SYMBOL_GPL(wbt_disable_default);
684 
685 u64 wbt_default_latency_nsec(struct request_queue *q)
686 {
687         /*
688          * We default to 2msec for non-rotational storage, and 75msec
689          * for rotational storage.
690          */
691         if (blk_queue_nonrot(q))
692                 return 2000000ULL;
693         else
694                 return 75000000ULL;
695 }
696 
697 int wbt_init(struct request_queue *q)
698 {
699         struct rq_wb *rwb;
700         int i;
701 
702         /*
703          * For now, we depend on the stats window being larger than
704          * our monitoring window. Ensure that this isn't inadvertently
705          * violated.
706          */
707         BUILD_BUG_ON(RWB_WINDOW_NSEC > BLK_STAT_NSEC);
708         BUILD_BUG_ON(WBT_NR_BITS > BLK_STAT_RES_BITS);
709 
710         rwb = kzalloc(sizeof(*rwb), GFP_KERNEL);
711         if (!rwb)
712                 return -ENOMEM;
713 
714         for (i = 0; i < WBT_NUM_RWQ; i++) {
715                 atomic_set(&rwb->rq_wait[i].inflight, 0);
716                 init_waitqueue_head(&rwb->rq_wait[i].wait);
717         }
718 
719         setup_timer(&rwb->window_timer, wb_timer_fn, (unsigned long) rwb);
720         rwb->wc = 1;
721         rwb->queue_depth = RWB_DEF_DEPTH;
722         rwb->last_comp = rwb->last_issue = jiffies;
723         rwb->queue = q;
724         rwb->win_nsec = RWB_WINDOW_NSEC;
725         rwb->enable_state = WBT_STATE_ON_DEFAULT;
726         wbt_update_limits(rwb);
727 
728         /*
729          * Assign rwb, and turn on stats tracking for this queue
730          */
731         q->rq_wb = rwb;
732         blk_stat_enable(q);
733 
734         rwb->min_lat_nsec = wbt_default_latency_nsec(q);
735 
736         wbt_set_queue_depth(rwb, blk_queue_depth(q));
737         wbt_set_write_cache(rwb, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
738 
739         return 0;
740 }
741 
742 void wbt_exit(struct request_queue *q)
743 {
744         struct rq_wb *rwb = q->rq_wb;
745 
746         if (rwb) {
747                 del_timer_sync(&rwb->window_timer);
748                 q->rq_wb = NULL;
749                 kfree(rwb);
750         }
751 }
752 

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