xref: /linux/block/blk-mq-sched.c (revision 4f3c8320c78cdd11c8fdd23c33787407f719322e)
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * blk-mq scheduling framework
4  *
5  * Copyright (C) 2016 Jens Axboe
6  */
7 #include <linux/kernel.h>
8 #include <linux/module.h>
9 #include <linux/blk-mq.h>
10 #include <linux/list_sort.h>
11 
12 #include <trace/events/block.h>
13 
14 #include "blk.h"
15 #include "blk-mq.h"
16 #include "blk-mq-debugfs.h"
17 #include "blk-mq-sched.h"
18 #include "blk-mq-tag.h"
19 #include "blk-wbt.h"
20 
21 void blk_mq_sched_assign_ioc(struct request *rq)
22 {
23 	struct request_queue *q = rq->q;
24 	struct io_context *ioc;
25 	struct io_cq *icq;
26 
27 	/*
28 	 * May not have an IO context if it's a passthrough request
29 	 */
30 	ioc = current->io_context;
31 	if (!ioc)
32 		return;
33 
34 	spin_lock_irq(&q->queue_lock);
35 	icq = ioc_lookup_icq(ioc, q);
36 	spin_unlock_irq(&q->queue_lock);
37 
38 	if (!icq) {
39 		icq = ioc_create_icq(ioc, q, GFP_ATOMIC);
40 		if (!icq)
41 			return;
42 	}
43 	get_io_context(icq->ioc);
44 	rq->elv.icq = icq;
45 }
46 
47 /*
48  * Mark a hardware queue as needing a restart. For shared queues, maintain
49  * a count of how many hardware queues are marked for restart.
50  */
51 void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
52 {
53 	if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
54 		return;
55 
56 	set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
57 }
58 EXPORT_SYMBOL_GPL(blk_mq_sched_mark_restart_hctx);
59 
60 void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
61 {
62 	if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
63 		return;
64 	clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
65 
66 	/*
67 	 * Order clearing SCHED_RESTART and list_empty_careful(&hctx->dispatch)
68 	 * in blk_mq_run_hw_queue(). Its pair is the barrier in
69 	 * blk_mq_dispatch_rq_list(). So dispatch code won't see SCHED_RESTART,
70 	 * meantime new request added to hctx->dispatch is missed to check in
71 	 * blk_mq_run_hw_queue().
72 	 */
73 	smp_mb();
74 
75 	blk_mq_run_hw_queue(hctx, true);
76 }
77 
78 static int sched_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
79 {
80 	struct request *rqa = container_of(a, struct request, queuelist);
81 	struct request *rqb = container_of(b, struct request, queuelist);
82 
83 	return rqa->mq_hctx > rqb->mq_hctx;
84 }
85 
86 static bool blk_mq_dispatch_hctx_list(struct list_head *rq_list)
87 {
88 	struct blk_mq_hw_ctx *hctx =
89 		list_first_entry(rq_list, struct request, queuelist)->mq_hctx;
90 	struct request *rq;
91 	LIST_HEAD(hctx_list);
92 	unsigned int count = 0;
93 
94 	list_for_each_entry(rq, rq_list, queuelist) {
95 		if (rq->mq_hctx != hctx) {
96 			list_cut_before(&hctx_list, rq_list, &rq->queuelist);
97 			goto dispatch;
98 		}
99 		count++;
100 	}
101 	list_splice_tail_init(rq_list, &hctx_list);
102 
103 dispatch:
104 	return blk_mq_dispatch_rq_list(hctx, &hctx_list, count);
105 }
106 
107 #define BLK_MQ_BUDGET_DELAY	3		/* ms units */
108 
109 /*
110  * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
111  * its queue by itself in its completion handler, so we don't need to
112  * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
113  *
114  * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
115  * be run again.  This is necessary to avoid starving flushes.
116  */
117 static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
118 {
119 	struct request_queue *q = hctx->queue;
120 	struct elevator_queue *e = q->elevator;
121 	bool multi_hctxs = false, run_queue = false;
122 	bool dispatched = false, busy = false;
123 	unsigned int max_dispatch;
124 	LIST_HEAD(rq_list);
125 	int count = 0;
126 
127 	if (hctx->dispatch_busy)
128 		max_dispatch = 1;
129 	else
130 		max_dispatch = hctx->queue->nr_requests;
131 
132 	do {
133 		struct request *rq;
134 
135 		if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
136 			break;
137 
138 		if (!list_empty_careful(&hctx->dispatch)) {
139 			busy = true;
140 			break;
141 		}
142 
143 		if (!blk_mq_get_dispatch_budget(q))
144 			break;
145 
146 		rq = e->type->ops.dispatch_request(hctx);
147 		if (!rq) {
148 			blk_mq_put_dispatch_budget(q);
149 			/*
150 			 * We're releasing without dispatching. Holding the
151 			 * budget could have blocked any "hctx"s with the
152 			 * same queue and if we didn't dispatch then there's
153 			 * no guarantee anyone will kick the queue.  Kick it
154 			 * ourselves.
155 			 */
156 			run_queue = true;
157 			break;
158 		}
159 
160 		/*
161 		 * Now this rq owns the budget which has to be released
162 		 * if this rq won't be queued to driver via .queue_rq()
163 		 * in blk_mq_dispatch_rq_list().
164 		 */
165 		list_add_tail(&rq->queuelist, &rq_list);
166 		if (rq->mq_hctx != hctx)
167 			multi_hctxs = true;
168 	} while (++count < max_dispatch);
169 
170 	if (!count) {
171 		if (run_queue)
172 			blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
173 	} else if (multi_hctxs) {
174 		/*
175 		 * Requests from different hctx may be dequeued from some
176 		 * schedulers, such as bfq and deadline.
177 		 *
178 		 * Sort the requests in the list according to their hctx,
179 		 * dispatch batching requests from same hctx at a time.
180 		 */
181 		list_sort(NULL, &rq_list, sched_rq_cmp);
182 		do {
183 			dispatched |= blk_mq_dispatch_hctx_list(&rq_list);
184 		} while (!list_empty(&rq_list));
185 	} else {
186 		dispatched = blk_mq_dispatch_rq_list(hctx, &rq_list, count);
187 	}
188 
189 	if (busy)
190 		return -EAGAIN;
191 	return !!dispatched;
192 }
193 
194 static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
195 {
196 	int ret;
197 
198 	do {
199 		ret = __blk_mq_do_dispatch_sched(hctx);
200 	} while (ret == 1);
201 
202 	return ret;
203 }
204 
205 static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
206 					  struct blk_mq_ctx *ctx)
207 {
208 	unsigned short idx = ctx->index_hw[hctx->type];
209 
210 	if (++idx == hctx->nr_ctx)
211 		idx = 0;
212 
213 	return hctx->ctxs[idx];
214 }
215 
216 /*
217  * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
218  * its queue by itself in its completion handler, so we don't need to
219  * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
220  *
221  * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
222  * be run again.  This is necessary to avoid starving flushes.
223  */
224 static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
225 {
226 	struct request_queue *q = hctx->queue;
227 	LIST_HEAD(rq_list);
228 	struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
229 	int ret = 0;
230 	struct request *rq;
231 
232 	do {
233 		if (!list_empty_careful(&hctx->dispatch)) {
234 			ret = -EAGAIN;
235 			break;
236 		}
237 
238 		if (!sbitmap_any_bit_set(&hctx->ctx_map))
239 			break;
240 
241 		if (!blk_mq_get_dispatch_budget(q))
242 			break;
243 
244 		rq = blk_mq_dequeue_from_ctx(hctx, ctx);
245 		if (!rq) {
246 			blk_mq_put_dispatch_budget(q);
247 			/*
248 			 * We're releasing without dispatching. Holding the
249 			 * budget could have blocked any "hctx"s with the
250 			 * same queue and if we didn't dispatch then there's
251 			 * no guarantee anyone will kick the queue.  Kick it
252 			 * ourselves.
253 			 */
254 			blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
255 			break;
256 		}
257 
258 		/*
259 		 * Now this rq owns the budget which has to be released
260 		 * if this rq won't be queued to driver via .queue_rq()
261 		 * in blk_mq_dispatch_rq_list().
262 		 */
263 		list_add(&rq->queuelist, &rq_list);
264 
265 		/* round robin for fair dispatch */
266 		ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
267 
268 	} while (blk_mq_dispatch_rq_list(rq->mq_hctx, &rq_list, 1));
269 
270 	WRITE_ONCE(hctx->dispatch_from, ctx);
271 	return ret;
272 }
273 
274 static int __blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
275 {
276 	struct request_queue *q = hctx->queue;
277 	struct elevator_queue *e = q->elevator;
278 	const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
279 	int ret = 0;
280 	LIST_HEAD(rq_list);
281 
282 	/*
283 	 * If we have previous entries on our dispatch list, grab them first for
284 	 * more fair dispatch.
285 	 */
286 	if (!list_empty_careful(&hctx->dispatch)) {
287 		spin_lock(&hctx->lock);
288 		if (!list_empty(&hctx->dispatch))
289 			list_splice_init(&hctx->dispatch, &rq_list);
290 		spin_unlock(&hctx->lock);
291 	}
292 
293 	/*
294 	 * Only ask the scheduler for requests, if we didn't have residual
295 	 * requests from the dispatch list. This is to avoid the case where
296 	 * we only ever dispatch a fraction of the requests available because
297 	 * of low device queue depth. Once we pull requests out of the IO
298 	 * scheduler, we can no longer merge or sort them. So it's best to
299 	 * leave them there for as long as we can. Mark the hw queue as
300 	 * needing a restart in that case.
301 	 *
302 	 * We want to dispatch from the scheduler if there was nothing
303 	 * on the dispatch list or we were able to dispatch from the
304 	 * dispatch list.
305 	 */
306 	if (!list_empty(&rq_list)) {
307 		blk_mq_sched_mark_restart_hctx(hctx);
308 		if (blk_mq_dispatch_rq_list(hctx, &rq_list, 0)) {
309 			if (has_sched_dispatch)
310 				ret = blk_mq_do_dispatch_sched(hctx);
311 			else
312 				ret = blk_mq_do_dispatch_ctx(hctx);
313 		}
314 	} else if (has_sched_dispatch) {
315 		ret = blk_mq_do_dispatch_sched(hctx);
316 	} else if (hctx->dispatch_busy) {
317 		/* dequeue request one by one from sw queue if queue is busy */
318 		ret = blk_mq_do_dispatch_ctx(hctx);
319 	} else {
320 		blk_mq_flush_busy_ctxs(hctx, &rq_list);
321 		blk_mq_dispatch_rq_list(hctx, &rq_list, 0);
322 	}
323 
324 	return ret;
325 }
326 
327 void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
328 {
329 	struct request_queue *q = hctx->queue;
330 
331 	/* RCU or SRCU read lock is needed before checking quiesced flag */
332 	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
333 		return;
334 
335 	hctx->run++;
336 
337 	/*
338 	 * A return of -EAGAIN is an indication that hctx->dispatch is not
339 	 * empty and we must run again in order to avoid starving flushes.
340 	 */
341 	if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) {
342 		if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN)
343 			blk_mq_run_hw_queue(hctx, true);
344 	}
345 }
346 
347 bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
348 		unsigned int nr_segs)
349 {
350 	struct elevator_queue *e = q->elevator;
351 	struct blk_mq_ctx *ctx = blk_mq_get_ctx(q);
352 	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, bio->bi_opf, ctx);
353 	bool ret = false;
354 	enum hctx_type type;
355 
356 	if (e && e->type->ops.bio_merge)
357 		return e->type->ops.bio_merge(hctx, bio, nr_segs);
358 
359 	type = hctx->type;
360 	if (!(hctx->flags & BLK_MQ_F_SHOULD_MERGE) ||
361 	    list_empty_careful(&ctx->rq_lists[type]))
362 		return false;
363 
364 	/* default per sw-queue merge */
365 	spin_lock(&ctx->lock);
366 	/*
367 	 * Reverse check our software queue for entries that we could
368 	 * potentially merge with. Currently includes a hand-wavy stop
369 	 * count of 8, to not spend too much time checking for merges.
370 	 */
371 	if (blk_bio_list_merge(q, &ctx->rq_lists[type], bio, nr_segs)) {
372 		ctx->rq_merged++;
373 		ret = true;
374 	}
375 
376 	spin_unlock(&ctx->lock);
377 
378 	return ret;
379 }
380 
381 bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq)
382 {
383 	return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq);
384 }
385 EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);
386 
387 void blk_mq_sched_request_inserted(struct request *rq)
388 {
389 	trace_block_rq_insert(rq);
390 }
391 EXPORT_SYMBOL_GPL(blk_mq_sched_request_inserted);
392 
393 static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
394 				       bool has_sched,
395 				       struct request *rq)
396 {
397 	/*
398 	 * dispatch flush and passthrough rq directly
399 	 *
400 	 * passthrough request has to be added to hctx->dispatch directly.
401 	 * For some reason, device may be in one situation which can't
402 	 * handle FS request, so STS_RESOURCE is always returned and the
403 	 * FS request will be added to hctx->dispatch. However passthrough
404 	 * request may be required at that time for fixing the problem. If
405 	 * passthrough request is added to scheduler queue, there isn't any
406 	 * chance to dispatch it given we prioritize requests in hctx->dispatch.
407 	 */
408 	if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq))
409 		return true;
410 
411 	if (has_sched)
412 		rq->rq_flags |= RQF_SORTED;
413 
414 	return false;
415 }
416 
417 void blk_mq_sched_insert_request(struct request *rq, bool at_head,
418 				 bool run_queue, bool async)
419 {
420 	struct request_queue *q = rq->q;
421 	struct elevator_queue *e = q->elevator;
422 	struct blk_mq_ctx *ctx = rq->mq_ctx;
423 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
424 
425 	WARN_ON(e && (rq->tag != BLK_MQ_NO_TAG));
426 
427 	if (blk_mq_sched_bypass_insert(hctx, !!e, rq)) {
428 		/*
429 		 * Firstly normal IO request is inserted to scheduler queue or
430 		 * sw queue, meantime we add flush request to dispatch queue(
431 		 * hctx->dispatch) directly and there is at most one in-flight
432 		 * flush request for each hw queue, so it doesn't matter to add
433 		 * flush request to tail or front of the dispatch queue.
434 		 *
435 		 * Secondly in case of NCQ, flush request belongs to non-NCQ
436 		 * command, and queueing it will fail when there is any
437 		 * in-flight normal IO request(NCQ command). When adding flush
438 		 * rq to the front of hctx->dispatch, it is easier to introduce
439 		 * extra time to flush rq's latency because of S_SCHED_RESTART
440 		 * compared with adding to the tail of dispatch queue, then
441 		 * chance of flush merge is increased, and less flush requests
442 		 * will be issued to controller. It is observed that ~10% time
443 		 * is saved in blktests block/004 on disk attached to AHCI/NCQ
444 		 * drive when adding flush rq to the front of hctx->dispatch.
445 		 *
446 		 * Simply queue flush rq to the front of hctx->dispatch so that
447 		 * intensive flush workloads can benefit in case of NCQ HW.
448 		 */
449 		at_head = (rq->rq_flags & RQF_FLUSH_SEQ) ? true : at_head;
450 		blk_mq_request_bypass_insert(rq, at_head, false);
451 		goto run;
452 	}
453 
454 	if (e && e->type->ops.insert_requests) {
455 		LIST_HEAD(list);
456 
457 		list_add(&rq->queuelist, &list);
458 		e->type->ops.insert_requests(hctx, &list, at_head);
459 	} else {
460 		spin_lock(&ctx->lock);
461 		__blk_mq_insert_request(hctx, rq, at_head);
462 		spin_unlock(&ctx->lock);
463 	}
464 
465 run:
466 	if (run_queue)
467 		blk_mq_run_hw_queue(hctx, async);
468 }
469 
470 void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx,
471 				  struct blk_mq_ctx *ctx,
472 				  struct list_head *list, bool run_queue_async)
473 {
474 	struct elevator_queue *e;
475 	struct request_queue *q = hctx->queue;
476 
477 	/*
478 	 * blk_mq_sched_insert_requests() is called from flush plug
479 	 * context only, and hold one usage counter to prevent queue
480 	 * from being released.
481 	 */
482 	percpu_ref_get(&q->q_usage_counter);
483 
484 	e = hctx->queue->elevator;
485 	if (e && e->type->ops.insert_requests)
486 		e->type->ops.insert_requests(hctx, list, false);
487 	else {
488 		/*
489 		 * try to issue requests directly if the hw queue isn't
490 		 * busy in case of 'none' scheduler, and this way may save
491 		 * us one extra enqueue & dequeue to sw queue.
492 		 */
493 		if (!hctx->dispatch_busy && !e && !run_queue_async) {
494 			blk_mq_try_issue_list_directly(hctx, list);
495 			if (list_empty(list))
496 				goto out;
497 		}
498 		blk_mq_insert_requests(hctx, ctx, list);
499 	}
500 
501 	blk_mq_run_hw_queue(hctx, run_queue_async);
502  out:
503 	percpu_ref_put(&q->q_usage_counter);
504 }
505 
506 static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
507 				   struct blk_mq_hw_ctx *hctx,
508 				   unsigned int hctx_idx)
509 {
510 	unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
511 
512 	if (hctx->sched_tags) {
513 		blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
514 		blk_mq_free_rq_map(hctx->sched_tags, flags);
515 		hctx->sched_tags = NULL;
516 	}
517 }
518 
519 static int blk_mq_sched_alloc_tags(struct request_queue *q,
520 				   struct blk_mq_hw_ctx *hctx,
521 				   unsigned int hctx_idx)
522 {
523 	struct blk_mq_tag_set *set = q->tag_set;
524 	/* Clear HCTX_SHARED so tags are init'ed */
525 	unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
526 	int ret;
527 
528 	hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
529 					       set->reserved_tags, flags);
530 	if (!hctx->sched_tags)
531 		return -ENOMEM;
532 
533 	ret = blk_mq_alloc_rqs(set, hctx->sched_tags, hctx_idx, q->nr_requests);
534 	if (ret)
535 		blk_mq_sched_free_tags(set, hctx, hctx_idx);
536 
537 	return ret;
538 }
539 
540 /* called in queue's release handler, tagset has gone away */
541 static void blk_mq_sched_tags_teardown(struct request_queue *q)
542 {
543 	struct blk_mq_hw_ctx *hctx;
544 	int i;
545 
546 	queue_for_each_hw_ctx(q, hctx, i) {
547 		/* Clear HCTX_SHARED so tags are freed */
548 		unsigned int flags = hctx->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
549 
550 		if (hctx->sched_tags) {
551 			blk_mq_free_rq_map(hctx->sched_tags, flags);
552 			hctx->sched_tags = NULL;
553 		}
554 	}
555 }
556 
557 int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
558 {
559 	struct blk_mq_hw_ctx *hctx;
560 	struct elevator_queue *eq;
561 	unsigned int i;
562 	int ret;
563 
564 	if (!e) {
565 		q->elevator = NULL;
566 		q->nr_requests = q->tag_set->queue_depth;
567 		return 0;
568 	}
569 
570 	/*
571 	 * Default to double of smaller one between hw queue_depth and 128,
572 	 * since we don't split into sync/async like the old code did.
573 	 * Additionally, this is a per-hw queue depth.
574 	 */
575 	q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
576 				   BLKDEV_MAX_RQ);
577 
578 	queue_for_each_hw_ctx(q, hctx, i) {
579 		ret = blk_mq_sched_alloc_tags(q, hctx, i);
580 		if (ret)
581 			goto err;
582 	}
583 
584 	ret = e->ops.init_sched(q, e);
585 	if (ret)
586 		goto err;
587 
588 	blk_mq_debugfs_register_sched(q);
589 
590 	queue_for_each_hw_ctx(q, hctx, i) {
591 		if (e->ops.init_hctx) {
592 			ret = e->ops.init_hctx(hctx, i);
593 			if (ret) {
594 				eq = q->elevator;
595 				blk_mq_sched_free_requests(q);
596 				blk_mq_exit_sched(q, eq);
597 				kobject_put(&eq->kobj);
598 				return ret;
599 			}
600 		}
601 		blk_mq_debugfs_register_sched_hctx(q, hctx);
602 	}
603 
604 	return 0;
605 
606 err:
607 	blk_mq_sched_free_requests(q);
608 	blk_mq_sched_tags_teardown(q);
609 	q->elevator = NULL;
610 	return ret;
611 }
612 
613 /*
614  * called in either blk_queue_cleanup or elevator_switch, tagset
615  * is required for freeing requests
616  */
617 void blk_mq_sched_free_requests(struct request_queue *q)
618 {
619 	struct blk_mq_hw_ctx *hctx;
620 	int i;
621 
622 	queue_for_each_hw_ctx(q, hctx, i) {
623 		if (hctx->sched_tags)
624 			blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);
625 	}
626 }
627 
628 void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
629 {
630 	struct blk_mq_hw_ctx *hctx;
631 	unsigned int i;
632 
633 	queue_for_each_hw_ctx(q, hctx, i) {
634 		blk_mq_debugfs_unregister_sched_hctx(hctx);
635 		if (e->type->ops.exit_hctx && hctx->sched_data) {
636 			e->type->ops.exit_hctx(hctx, i);
637 			hctx->sched_data = NULL;
638 		}
639 	}
640 	blk_mq_debugfs_unregister_sched(q);
641 	if (e->type->ops.exit_sched)
642 		e->type->ops.exit_sched(e);
643 	blk_mq_sched_tags_teardown(q);
644 	q->elevator = NULL;
645 }
646