block/blk-mq-sched.c

   1 /*
   2  * blk-mq scheduling framework
   3  *
   4  * Copyright (C) 2016 Jens Axboe
   5  */
   6 #include <linux/kernel.h>
   7 #include <linux/module.h>
   8 #include <linux/blk-mq.h>
   9
  10 #include <trace/events/block.h>
  11
  12 #include "blk.h"
  13 #include "blk-mq.h"
  14 #include "blk-mq-debugfs.h"
  15 #include "blk-mq-sched.h"
  16 #include "blk-mq-tag.h"
  17 #include "blk-wbt.h"
  18
  19 void blk_mq_sched_free_hctx_data(struct request_queue *q,
  20                                  void (*exit)(struct blk_mq_hw_ctx *))
  21 {
  22         struct blk_mq_hw_ctx *hctx;
  23         int i;
  24
  25         queue_for_each_hw_ctx(q, hctx, i) {
  26                 if (exit && hctx->sched_data)
  27                         exit(hctx);
  28                 kfree(hctx->sched_data);
  29                 hctx->sched_data = NULL;
  30         }
  31 }
  32 EXPORT_SYMBOL_GPL(blk_mq_sched_free_hctx_data);
  33
  34 void blk_mq_sched_assign_ioc(struct request *rq, struct bio *bio)
  35 {
  36         struct request_queue *q = rq->q;
  37         struct io_context *ioc = rq_ioc(bio);
  38         struct io_cq *icq;
  39
  40         spin_lock_irq(q->queue_lock);
  41         icq = ioc_lookup_icq(ioc, q);
  42         spin_unlock_irq(q->queue_lock);
  43
  44         if (!icq) {
  45                 icq = ioc_create_icq(ioc, q, GFP_ATOMIC);
  46                 if (!icq)
  47                         return;
  48         }
  49         get_io_context(icq->ioc);
  50         rq->elv.icq = icq;
  51 }
  52
  53 /*
  54  * Mark a hardware queue as needing a restart. For shared queues, maintain
  55  * a count of how many hardware queues are marked for restart.
  56  */
  57 static void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
  58 {
  59         if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
  60                 return;
  61
  62         if (hctx->flags & BLK_MQ_F_TAG_SHARED) {
  63                 struct request_queue *q = hctx->queue;
  64
  65                 if (!test_and_set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
  66                         atomic_inc(&q->shared_hctx_restart);
  67         } else
  68                 set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
  69 }
  70
  71 static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
  72 {
  73         if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
  74                 return false;
  75
  76         if (hctx->flags & BLK_MQ_F_TAG_SHARED) {
  77                 struct request_queue *q = hctx->queue;
  78
  79                 if (test_and_clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
  80                         atomic_dec(&q->shared_hctx_restart);
  81         } else
  82                 clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
  83
  84         return blk_mq_run_hw_queue(hctx, true);
  85 }
  86
  87 /*
  88  * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
  89  * its queue by itself in its completion handler, so we don't need to
  90  * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
  91  */
  92 static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
  93 {
  94         struct request_queue *q = hctx->queue;
  95         struct elevator_queue *e = q->elevator;
  96         LIST_HEAD(rq_list);
  97
  98         do {
  99                 struct request *rq;
 100
 101                 if (e->type->ops.mq.has_work &&
 102                                 !e->type->ops.mq.has_work(hctx))
 103                         break;
 104
 105                 if (!blk_mq_get_dispatch_budget(hctx))
 106                         break;
 107
 108                 rq = e->type->ops.mq.dispatch_request(hctx);
 109                 if (!rq) {
 110                         blk_mq_put_dispatch_budget(hctx);
 111                         break;
 112                 }
 113
 114                 /*
 115                  * Now this rq owns the budget which has to be released
 116                  * if this rq won't be queued to driver via .queue_rq()
 117                  * in blk_mq_dispatch_rq_list().
 118                  */
 119                 list_add(&rq->queuelist, &rq_list);
 120         } while (blk_mq_dispatch_rq_list(q, &rq_list, true));
 121 }
 122
 123 static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
 124                                           struct blk_mq_ctx *ctx)
 125 {
 126         unsigned idx = ctx->index_hw;
 127
 128         if (++idx == hctx->nr_ctx)
 129                 idx = 0;
 130
 131         return hctx->ctxs[idx];
 132 }
 133
 134 /*
 135  * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
 136  * its queue by itself in its completion handler, so we don't need to
 137  * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
 138  */
 139 static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
 140 {
 141         struct request_queue *q = hctx->queue;
 142         LIST_HEAD(rq_list);
 143         struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
 144
 145         do {
 146                 struct request *rq;
 147
 148                 if (!sbitmap_any_bit_set(&hctx->ctx_map))
 149                         break;
 150
 151                 if (!blk_mq_get_dispatch_budget(hctx))
 152                         break;
 153
 154                 rq = blk_mq_dequeue_from_ctx(hctx, ctx);
 155                 if (!rq) {
 156                         blk_mq_put_dispatch_budget(hctx);
 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(&rq->queuelist, &rq_list);
 166
 167                 /* round robin for fair dispatch */
 168                 ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
 169
 170         } while (blk_mq_dispatch_rq_list(q, &rq_list, true));
 171
 172         WRITE_ONCE(hctx->dispatch_from, ctx);
 173 }
 174
 175 /* return true if hw queue need to be run again */
 176 void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 177 {
 178         struct request_queue *q = hctx->queue;
 179         struct elevator_queue *e = q->elevator;
 180         const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
 181         LIST_HEAD(rq_list);
 182
 183         /* RCU or SRCU read lock is needed before checking quiesced flag */
 184         if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
 185                 return;
 186
 187         hctx->run++;
 188
 189         /*
 190          * If we have previous entries on our dispatch list, grab them first for
 191          * more fair dispatch.
 192          */
 193         if (!list_empty_careful(&hctx->dispatch)) {
 194                 spin_lock(&hctx->lock);
 195                 if (!list_empty(&hctx->dispatch))
 196                         list_splice_init(&hctx->dispatch, &rq_list);
 197                 spin_unlock(&hctx->lock);
 198         }
 199
 200         /*
 201          * Only ask the scheduler for requests, if we didn't have residual
 202          * requests from the dispatch list. This is to avoid the case where
 203          * we only ever dispatch a fraction of the requests available because
 204          * of low device queue depth. Once we pull requests out of the IO
 205          * scheduler, we can no longer merge or sort them. So it's best to
 206          * leave them there for as long as we can. Mark the hw queue as
 207          * needing a restart in that case.
 208          *
 209          * We want to dispatch from the scheduler if there was nothing
 210          * on the dispatch list or we were able to dispatch from the
 211          * dispatch list.
 212          */
 213         if (!list_empty(&rq_list)) {
 214                 blk_mq_sched_mark_restart_hctx(hctx);
 215                 if (blk_mq_dispatch_rq_list(q, &rq_list, false)) {
 216                         if (has_sched_dispatch)
 217                                 blk_mq_do_dispatch_sched(hctx);
 218                         else
 219                                 blk_mq_do_dispatch_ctx(hctx);
 220                 }
 221         } else if (has_sched_dispatch) {
 222                 blk_mq_do_dispatch_sched(hctx);
 223         } else if (q->mq_ops->get_budget) {
 224                 /*
 225                  * If we need to get budget before queuing request, we
 226                  * dequeue request one by one from sw queue for avoiding
 227                  * to mess up I/O merge when dispatch runs out of resource.
 228                  *
 229                  * TODO: get more budgets, and dequeue more requests in
 230                  * one time.
 231                  */
 232                 blk_mq_do_dispatch_ctx(hctx);
 233         } else {
 234                 blk_mq_flush_busy_ctxs(hctx, &rq_list);
 235                 blk_mq_dispatch_rq_list(q, &rq_list, false);
 236         }
 237 }
 238
 239 bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
 240                             struct request **merged_request)
 241 {
 242         struct request *rq;
 243
 244         switch (elv_merge(q, &rq, bio)) {
 245         case ELEVATOR_BACK_MERGE:
 246                 if (!blk_mq_sched_allow_merge(q, rq, bio))
 247                         return false;
 248                 if (!bio_attempt_back_merge(q, rq, bio))
 249                         return false;
 250                 *merged_request = attempt_back_merge(q, rq);
 251                 if (!*merged_request)
 252                         elv_merged_request(q, rq, ELEVATOR_BACK_MERGE);
 253                 return true;
 254         case ELEVATOR_FRONT_MERGE:
 255                 if (!blk_mq_sched_allow_merge(q, rq, bio))
 256                         return false;
 257                 if (!bio_attempt_front_merge(q, rq, bio))
 258                         return false;
 259                 *merged_request = attempt_front_merge(q, rq);
 260                 if (!*merged_request)
 261                         elv_merged_request(q, rq, ELEVATOR_FRONT_MERGE);
 262                 return true;
 263         default:
 264                 return false;
 265         }
 266 }
 267 EXPORT_SYMBOL_GPL(blk_mq_sched_try_merge);
 268
 269 /*
 270  * Reverse check our software queue for entries that we could potentially
 271  * merge with. Currently includes a hand-wavy stop count of 8, to not spend
 272  * too much time checking for merges.
 273  */
 274 static bool blk_mq_attempt_merge(struct request_queue *q,
 275                                  struct blk_mq_ctx *ctx, struct bio *bio)
 276 {
 277         struct request *rq;
 278         int checked = 8;
 279
 280         lockdep_assert_held(&ctx->lock);
 281
 282         list_for_each_entry_reverse(rq, &ctx->rq_list, queuelist) {
 283                 bool merged = false;
 284
 285                 if (!checked--)
 286                         break;
 287
 288                 if (!blk_rq_merge_ok(rq, bio))
 289                         continue;
 290
 291                 switch (blk_try_merge(rq, bio)) {
 292                 case ELEVATOR_BACK_MERGE:
 293                         if (blk_mq_sched_allow_merge(q, rq, bio))
 294                                 merged = bio_attempt_back_merge(q, rq, bio);
 295                         break;
 296                 case ELEVATOR_FRONT_MERGE:
 297                         if (blk_mq_sched_allow_merge(q, rq, bio))
 298                                 merged = bio_attempt_front_merge(q, rq, bio);
 299                         break;
 300                 case ELEVATOR_DISCARD_MERGE:
 301                         merged = bio_attempt_discard_merge(q, rq, bio);
 302                         break;
 303                 default:
 304                         continue;
 305                 }
 306
 307                 if (merged)
 308                         ctx->rq_merged++;
 309                 return merged;
 310         }
 311
 312         return false;
 313 }
 314
 315 bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio)
 316 {
 317         struct elevator_queue *e = q->elevator;
 318         struct blk_mq_ctx *ctx = blk_mq_get_ctx(q);
 319         struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
 320         bool ret = false;
 321
 322         if (e && e->type->ops.mq.bio_merge) {
 323                 blk_mq_put_ctx(ctx);
 324                 return e->type->ops.mq.bio_merge(hctx, bio);
 325         }
 326
 327         if (hctx->flags & BLK_MQ_F_SHOULD_MERGE) {
 328                 /* default per sw-queue merge */
 329                 spin_lock(&ctx->lock);
 330                 ret = blk_mq_attempt_merge(q, ctx, bio);
 331                 spin_unlock(&ctx->lock);
 332         }
 333
 334         blk_mq_put_ctx(ctx);
 335         return ret;
 336 }
 337
 338 bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq)
 339 {
 340         return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq);
 341 }
 342 EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);
 343
 344 void blk_mq_sched_request_inserted(struct request *rq)
 345 {
 346         trace_block_rq_insert(rq->q, rq);
 347 }
 348 EXPORT_SYMBOL_GPL(blk_mq_sched_request_inserted);
 349
 350 static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
 351                                        bool has_sched,
 352                                        struct request *rq)
 353 {
 354         /* dispatch flush rq directly */
 355         if (rq->rq_flags & RQF_FLUSH_SEQ) {
 356                 spin_lock(&hctx->lock);
 357                 list_add(&rq->queuelist, &hctx->dispatch);
 358                 spin_unlock(&hctx->lock);
 359                 return true;
 360         }
 361
 362         if (has_sched)
 363                 rq->rq_flags |= RQF_SORTED;
 364
 365         return false;
 366 }
 367
 368 /**
 369  * list_for_each_entry_rcu_rr - iterate in a round-robin fashion over rcu list
 370  * @pos:    loop cursor.
 371  * @skip:   the list element that will not be examined. Iteration starts at
 372  *          @skip->next.
 373  * @head:   head of the list to examine. This list must have at least one
 374  *          element, namely @skip.
 375  * @member: name of the list_head structure within typeof(*pos).
 376  */
 377 #define list_for_each_entry_rcu_rr(pos, skip, head, member)             \
 378         for ((pos) = (skip);                                            \
 379              (pos = (pos)->member.next != (head) ? list_entry_rcu(      \
 380                         (pos)->member.next, typeof(*pos), member) :     \
 381               list_entry_rcu((pos)->member.next->next, typeof(*pos), member)), \
 382              (pos) != (skip); )
 383
 384 /*
 385  * Called after a driver tag has been freed to check whether a hctx needs to
 386  * be restarted. Restarts @hctx if its tag set is not shared. Restarts hardware
 387  * queues in a round-robin fashion if the tag set of @hctx is shared with other
 388  * hardware queues.
 389  */
 390 void blk_mq_sched_restart(struct blk_mq_hw_ctx *const hctx)
 391 {
 392         struct blk_mq_tags *const tags = hctx->tags;
 393         struct blk_mq_tag_set *const set = hctx->queue->tag_set;
 394         struct request_queue *const queue = hctx->queue, *q;
 395         struct blk_mq_hw_ctx *hctx2;
 396         unsigned int i, j;
 397
 398         if (set->flags & BLK_MQ_F_TAG_SHARED) {
 399                 /*
 400                  * If this is 0, then we know that no hardware queues
 401                  * have RESTART marked. We're done.
 402                  */
 403                 if (!atomic_read(&queue->shared_hctx_restart))
 404                         return;
 405
 406                 rcu_read_lock();
 407                 list_for_each_entry_rcu_rr(q, queue, &set->tag_list,
 408                                            tag_set_list) {
 409                         queue_for_each_hw_ctx(q, hctx2, i)
 410                                 if (hctx2->tags == tags &&
 411                                     blk_mq_sched_restart_hctx(hctx2))
 412                                         goto done;
 413                 }
 414                 j = hctx->queue_num + 1;
 415                 for (i = 0; i < queue->nr_hw_queues; i++, j++) {
 416                         if (j == queue->nr_hw_queues)
 417                                 j = 0;
 418                         hctx2 = queue->queue_hw_ctx[j];
 419                         if (hctx2->tags == tags &&
 420                             blk_mq_sched_restart_hctx(hctx2))
 421                                 break;
 422                 }
 423 done:
 424                 rcu_read_unlock();
 425         } else {
 426                 blk_mq_sched_restart_hctx(hctx);
 427         }
 428 }
 429
 430 void blk_mq_sched_insert_request(struct request *rq, bool at_head,
 431                                  bool run_queue, bool async, bool can_block)
 432 {
 433         struct request_queue *q = rq->q;
 434         struct elevator_queue *e = q->elevator;
 435         struct blk_mq_ctx *ctx = rq->mq_ctx;
 436         struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
 437
 438         /* flush rq in flush machinery need to be dispatched directly */
 439         if (!(rq->rq_flags & RQF_FLUSH_SEQ) && op_is_flush(rq->cmd_flags)) {
 440                 blk_insert_flush(rq);
 441                 goto run;
 442         }
 443
 444         WARN_ON(e && (rq->tag != -1));
 445
 446         if (blk_mq_sched_bypass_insert(hctx, !!e, rq))
 447                 goto run;
 448
 449         if (e && e->type->ops.mq.insert_requests) {
 450                 LIST_HEAD(list);
 451
 452                 list_add(&rq->queuelist, &list);
 453                 e->type->ops.mq.insert_requests(hctx, &list, at_head);
 454         } else {
 455                 spin_lock(&ctx->lock);
 456                 __blk_mq_insert_request(hctx, rq, at_head);
 457                 spin_unlock(&ctx->lock);
 458         }
 459
 460 run:
 461         if (run_queue)
 462                 blk_mq_run_hw_queue(hctx, async);
 463 }
 464
 465 void blk_mq_sched_insert_requests(struct request_queue *q,
 466                                   struct blk_mq_ctx *ctx,
 467                                   struct list_head *list, bool run_queue_async)
 468 {
 469         struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
 470         struct elevator_queue *e = hctx->queue->elevator;
 471
 472         if (e && e->type->ops.mq.insert_requests)
 473                 e->type->ops.mq.insert_requests(hctx, list, false);
 474         else
 475                 blk_mq_insert_requests(hctx, ctx, list);
 476
 477         blk_mq_run_hw_queue(hctx, run_queue_async);
 478 }
 479
 480 static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
 481                                    struct blk_mq_hw_ctx *hctx,
 482                                    unsigned int hctx_idx)
 483 {
 484         if (hctx->sched_tags) {
 485                 blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
 486                 blk_mq_free_rq_map(hctx->sched_tags);
 487                 hctx->sched_tags = NULL;
 488         }
 489 }
 490
 491 static int blk_mq_sched_alloc_tags(struct request_queue *q,
 492                                    struct blk_mq_hw_ctx *hctx,
 493                                    unsigned int hctx_idx)
 494 {
 495         struct blk_mq_tag_set *set = q->tag_set;
 496         int ret;
 497
 498         hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
 499                                                set->reserved_tags);
 500         if (!hctx->sched_tags)
 501                 return -ENOMEM;
 502
 503         ret = blk_mq_alloc_rqs(set, hctx->sched_tags, hctx_idx, q->nr_requests);
 504         if (ret)
 505                 blk_mq_sched_free_tags(set, hctx, hctx_idx);
 506
 507         return ret;
 508 }
 509
 510 static void blk_mq_sched_tags_teardown(struct request_queue *q)
 511 {
 512         struct blk_mq_tag_set *set = q->tag_set;
 513         struct blk_mq_hw_ctx *hctx;
 514         int i;
 515
 516         queue_for_each_hw_ctx(q, hctx, i)
 517                 blk_mq_sched_free_tags(set, hctx, i);
 518 }
 519
 520 int blk_mq_sched_init_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx,
 521                            unsigned int hctx_idx)
 522 {
 523         struct elevator_queue *e = q->elevator;
 524         int ret;
 525
 526         if (!e)
 527                 return 0;
 528
 529         ret = blk_mq_sched_alloc_tags(q, hctx, hctx_idx);
 530         if (ret)
 531                 return ret;
 532
 533         if (e->type->ops.mq.init_hctx) {
 534                 ret = e->type->ops.mq.init_hctx(hctx, hctx_idx);
 535                 if (ret) {
 536                         blk_mq_sched_free_tags(q->tag_set, hctx, hctx_idx);
 537                         return ret;
 538                 }
 539         }
 540
 541         blk_mq_debugfs_register_sched_hctx(q, hctx);
 542
 543         return 0;
 544 }
 545
 546 void blk_mq_sched_exit_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx,
 547                             unsigned int hctx_idx)
 548 {
 549         struct elevator_queue *e = q->elevator;
 550
 551         if (!e)
 552                 return;
 553
 554         blk_mq_debugfs_unregister_sched_hctx(hctx);
 555
 556         if (e->type->ops.mq.exit_hctx && hctx->sched_data) {
 557                 e->type->ops.mq.exit_hctx(hctx, hctx_idx);
 558                 hctx->sched_data = NULL;
 559         }
 560
 561         blk_mq_sched_free_tags(q->tag_set, hctx, hctx_idx);
 562 }
 563
 564 int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
 565 {
 566         struct blk_mq_hw_ctx *hctx;
 567         struct elevator_queue *eq;
 568         unsigned int i;
 569         int ret;
 570
 571         if (!e) {
 572                 q->elevator = NULL;
 573                 return 0;
 574         }
 575
 576         /*
 577          * Default to double of smaller one between hw queue_depth and 128,
 578          * since we don't split into sync/async like the old code did.
 579          * Additionally, this is a per-hw queue depth.
 580          */
 581         q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
 582                                    BLKDEV_MAX_RQ);
 583
 584         queue_for_each_hw_ctx(q, hctx, i) {
 585                 ret = blk_mq_sched_alloc_tags(q, hctx, i);
 586                 if (ret)
 587                         goto err;
 588         }
 589
 590         ret = e->ops.mq.init_sched(q, e);
 591         if (ret)
 592                 goto err;
 593
 594         blk_mq_debugfs_register_sched(q);
 595
 596         queue_for_each_hw_ctx(q, hctx, i) {
 597                 if (e->ops.mq.init_hctx) {
 598                         ret = e->ops.mq.init_hctx(hctx, i);
 599                         if (ret) {
 600                                 eq = q->elevator;
 601                                 blk_mq_exit_sched(q, eq);
 602                                 kobject_put(&eq->kobj);
 603                                 return ret;
 604                         }
 605                 }
 606                 blk_mq_debugfs_register_sched_hctx(q, hctx);
 607         }
 608
 609         return 0;
 610
 611 err:
 612         blk_mq_sched_tags_teardown(q);
 613         q->elevator = NULL;
 614         return ret;
 615 }
 616
 617 void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
 618 {
 619         struct blk_mq_hw_ctx *hctx;
 620         unsigned int i;
 621
 622         queue_for_each_hw_ctx(q, hctx, i) {
 623                 blk_mq_debugfs_unregister_sched_hctx(hctx);
 624                 if (e->type->ops.mq.exit_hctx && hctx->sched_data) {
 625                         e->type->ops.mq.exit_hctx(hctx, i);
 626                         hctx->sched_data = NULL;
 627                 }
 628         }
 629         blk_mq_debugfs_unregister_sched(q);
 630         if (e->type->ops.mq.exit_sched)
 631                 e->type->ops.mq.exit_sched(e);
 632         blk_mq_sched_tags_teardown(q);
 633         q->elevator = NULL;
 634 }
 635
 636 int blk_mq_sched_init(struct request_queue *q)
 637 {
 638         int ret;
 639
 640         mutex_lock(&q->sysfs_lock);
 641         ret = elevator_init(q, NULL);
 642         mutex_unlock(&q->sysfs_lock);
 643
 644         return ret;
 645 }