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 (hctx->dispatch_busy) {
 224                 /* dequeue request one by one from sw queue if queue is busy */
 225                 blk_mq_do_dispatch_ctx(hctx);
 226         } else {
 227                 blk_mq_flush_busy_ctxs(hctx, &rq_list);
 228                 blk_mq_dispatch_rq_list(q, &rq_list, false);
 229         }
 230 }
 231
 232 bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
 233                             struct request **merged_request)
 234 {
 235         struct request *rq;
 236
 237         switch (elv_merge(q, &rq, bio)) {
 238         case ELEVATOR_BACK_MERGE:
 239                 if (!blk_mq_sched_allow_merge(q, rq, bio))
 240                         return false;
 241                 if (!bio_attempt_back_merge(q, rq, bio))
 242                         return false;
 243                 *merged_request = attempt_back_merge(q, rq);
 244                 if (!*merged_request)
 245                         elv_merged_request(q, rq, ELEVATOR_BACK_MERGE);
 246                 return true;
 247         case ELEVATOR_FRONT_MERGE:
 248                 if (!blk_mq_sched_allow_merge(q, rq, bio))
 249                         return false;
 250                 if (!bio_attempt_front_merge(q, rq, bio))
 251                         return false;
 252                 *merged_request = attempt_front_merge(q, rq);
 253                 if (!*merged_request)
 254                         elv_merged_request(q, rq, ELEVATOR_FRONT_MERGE);
 255                 return true;
 256         default:
 257                 return false;
 258         }
 259 }
 260 EXPORT_SYMBOL_GPL(blk_mq_sched_try_merge);
 261
 262 /*
 263  * Reverse check our software queue for entries that we could potentially
 264  * merge with. Currently includes a hand-wavy stop count of 8, to not spend
 265  * too much time checking for merges.
 266  */
 267 static bool blk_mq_attempt_merge(struct request_queue *q,
 268                                  struct blk_mq_ctx *ctx, struct bio *bio)
 269 {
 270         struct request *rq;
 271         int checked = 8;
 272
 273         lockdep_assert_held(&ctx->lock);
 274
 275         list_for_each_entry_reverse(rq, &ctx->rq_list, queuelist) {
 276                 bool merged = false;
 277
 278                 if (!checked--)
 279                         break;
 280
 281                 if (!blk_rq_merge_ok(rq, bio))
 282                         continue;
 283
 284                 switch (blk_try_merge(rq, bio)) {
 285                 case ELEVATOR_BACK_MERGE:
 286                         if (blk_mq_sched_allow_merge(q, rq, bio))
 287                                 merged = bio_attempt_back_merge(q, rq, bio);
 288                         break;
 289                 case ELEVATOR_FRONT_MERGE:
 290                         if (blk_mq_sched_allow_merge(q, rq, bio))
 291                                 merged = bio_attempt_front_merge(q, rq, bio);
 292                         break;
 293                 case ELEVATOR_DISCARD_MERGE:
 294                         merged = bio_attempt_discard_merge(q, rq, bio);
 295                         break;
 296                 default:
 297                         continue;
 298                 }
 299
 300                 if (merged)
 301                         ctx->rq_merged++;
 302                 return merged;
 303         }
 304
 305         return false;
 306 }
 307
 308 bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio)
 309 {
 310         struct elevator_queue *e = q->elevator;
 311         struct blk_mq_ctx *ctx = blk_mq_get_ctx(q);
 312         struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
 313         bool ret = false;
 314
 315         if (e && e->type->ops.mq.bio_merge) {
 316                 blk_mq_put_ctx(ctx);
 317                 return e->type->ops.mq.bio_merge(hctx, bio);
 318         }
 319
 320         if ((hctx->flags & BLK_MQ_F_SHOULD_MERGE) &&
 321                         !list_empty_careful(&ctx->rq_list)) {
 322                 /* default per sw-queue merge */
 323                 spin_lock(&ctx->lock);
 324                 ret = blk_mq_attempt_merge(q, ctx, bio);
 325                 spin_unlock(&ctx->lock);
 326         }
 327
 328         blk_mq_put_ctx(ctx);
 329         return ret;
 330 }
 331
 332 bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq)
 333 {
 334         return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq);
 335 }
 336 EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);
 337
 338 void blk_mq_sched_request_inserted(struct request *rq)
 339 {
 340         trace_block_rq_insert(rq->q, rq);
 341 }
 342 EXPORT_SYMBOL_GPL(blk_mq_sched_request_inserted);
 343
 344 static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
 345                                        bool has_sched,
 346                                        struct request *rq)
 347 {
 348         /* dispatch flush rq directly */
 349         if (rq->rq_flags & RQF_FLUSH_SEQ) {
 350                 spin_lock(&hctx->lock);
 351                 list_add(&rq->queuelist, &hctx->dispatch);
 352                 spin_unlock(&hctx->lock);
 353                 return true;
 354         }
 355
 356         if (has_sched)
 357                 rq->rq_flags |= RQF_SORTED;
 358
 359         return false;
 360 }
 361
 362 /**
 363  * list_for_each_entry_rcu_rr - iterate in a round-robin fashion over rcu list
 364  * @pos:    loop cursor.
 365  * @skip:   the list element that will not be examined. Iteration starts at
 366  *          @skip->next.
 367  * @head:   head of the list to examine. This list must have at least one
 368  *          element, namely @skip.
 369  * @member: name of the list_head structure within typeof(*pos).
 370  */
 371 #define list_for_each_entry_rcu_rr(pos, skip, head, member)             \
 372         for ((pos) = (skip);                                            \
 373              (pos = (pos)->member.next != (head) ? list_entry_rcu(      \
 374                         (pos)->member.next, typeof(*pos), member) :     \
 375               list_entry_rcu((pos)->member.next->next, typeof(*pos), member)), \
 376              (pos) != (skip); )
 377
 378 /*
 379  * Called after a driver tag has been freed to check whether a hctx needs to
 380  * be restarted. Restarts @hctx if its tag set is not shared. Restarts hardware
 381  * queues in a round-robin fashion if the tag set of @hctx is shared with other
 382  * hardware queues.
 383  */
 384 void blk_mq_sched_restart(struct blk_mq_hw_ctx *const hctx)
 385 {
 386         struct blk_mq_tags *const tags = hctx->tags;
 387         struct blk_mq_tag_set *const set = hctx->queue->tag_set;
 388         struct request_queue *const queue = hctx->queue, *q;
 389         struct blk_mq_hw_ctx *hctx2;
 390         unsigned int i, j;
 391
 392         if (set->flags & BLK_MQ_F_TAG_SHARED) {
 393                 /*
 394                  * If this is 0, then we know that no hardware queues
 395                  * have RESTART marked. We're done.
 396                  */
 397                 if (!atomic_read(&queue->shared_hctx_restart))
 398                         return;
 399
 400                 rcu_read_lock();
 401                 list_for_each_entry_rcu_rr(q, queue, &set->tag_list,
 402                                            tag_set_list) {
 403                         queue_for_each_hw_ctx(q, hctx2, i)
 404                                 if (hctx2->tags == tags &&
 405                                     blk_mq_sched_restart_hctx(hctx2))
 406                                         goto done;
 407                 }
 408                 j = hctx->queue_num + 1;
 409                 for (i = 0; i < queue->nr_hw_queues; i++, j++) {
 410                         if (j == queue->nr_hw_queues)
 411                                 j = 0;
 412                         hctx2 = queue->queue_hw_ctx[j];
 413                         if (hctx2->tags == tags &&
 414                             blk_mq_sched_restart_hctx(hctx2))
 415                                 break;
 416                 }
 417 done:
 418                 rcu_read_unlock();
 419         } else {
 420                 blk_mq_sched_restart_hctx(hctx);
 421         }
 422 }
 423
 424 void blk_mq_sched_insert_request(struct request *rq, bool at_head,
 425                                  bool run_queue, bool async)
 426 {
 427         struct request_queue *q = rq->q;
 428         struct elevator_queue *e = q->elevator;
 429         struct blk_mq_ctx *ctx = rq->mq_ctx;
 430         struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
 431
 432         /* flush rq in flush machinery need to be dispatched directly */
 433         if (!(rq->rq_flags & RQF_FLUSH_SEQ) && op_is_flush(rq->cmd_flags)) {
 434                 blk_insert_flush(rq);
 435                 goto run;
 436         }
 437
 438         WARN_ON(e && (rq->tag != -1));
 439
 440         if (blk_mq_sched_bypass_insert(hctx, !!e, rq))
 441                 goto run;
 442
 443         if (e && e->type->ops.mq.insert_requests) {
 444                 LIST_HEAD(list);
 445
 446                 list_add(&rq->queuelist, &list);
 447                 e->type->ops.mq.insert_requests(hctx, &list, at_head);
 448         } else {
 449                 spin_lock(&ctx->lock);
 450                 __blk_mq_insert_request(hctx, rq, at_head);
 451                 spin_unlock(&ctx->lock);
 452         }
 453
 454 run:
 455         if (run_queue)
 456                 blk_mq_run_hw_queue(hctx, async);
 457 }
 458
 459 void blk_mq_sched_insert_requests(struct request_queue *q,
 460                                   struct blk_mq_ctx *ctx,
 461                                   struct list_head *list, bool run_queue_async)
 462 {
 463         struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
 464         struct elevator_queue *e = hctx->queue->elevator;
 465
 466         if (e && e->type->ops.mq.insert_requests)
 467                 e->type->ops.mq.insert_requests(hctx, list, false);
 468         else
 469                 blk_mq_insert_requests(hctx, ctx, list);
 470
 471         blk_mq_run_hw_queue(hctx, run_queue_async);
 472 }
 473
 474 static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
 475                                    struct blk_mq_hw_ctx *hctx,
 476                                    unsigned int hctx_idx)
 477 {
 478         if (hctx->sched_tags) {
 479                 blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
 480                 blk_mq_free_rq_map(hctx->sched_tags);
 481                 hctx->sched_tags = NULL;
 482         }
 483 }
 484
 485 static int blk_mq_sched_alloc_tags(struct request_queue *q,
 486                                    struct blk_mq_hw_ctx *hctx,
 487                                    unsigned int hctx_idx)
 488 {
 489         struct blk_mq_tag_set *set = q->tag_set;
 490         int ret;
 491
 492         hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
 493                                                set->reserved_tags);
 494         if (!hctx->sched_tags)
 495                 return -ENOMEM;
 496
 497         ret = blk_mq_alloc_rqs(set, hctx->sched_tags, hctx_idx, q->nr_requests);
 498         if (ret)
 499                 blk_mq_sched_free_tags(set, hctx, hctx_idx);
 500
 501         return ret;
 502 }
 503
 504 static void blk_mq_sched_tags_teardown(struct request_queue *q)
 505 {
 506         struct blk_mq_tag_set *set = q->tag_set;
 507         struct blk_mq_hw_ctx *hctx;
 508         int i;
 509
 510         queue_for_each_hw_ctx(q, hctx, i)
 511                 blk_mq_sched_free_tags(set, hctx, i);
 512 }
 513
 514 int blk_mq_sched_init_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx,
 515                            unsigned int hctx_idx)
 516 {
 517         struct elevator_queue *e = q->elevator;
 518         int ret;
 519
 520         if (!e)
 521                 return 0;
 522
 523         ret = blk_mq_sched_alloc_tags(q, hctx, hctx_idx);
 524         if (ret)
 525                 return ret;
 526
 527         if (e->type->ops.mq.init_hctx) {
 528                 ret = e->type->ops.mq.init_hctx(hctx, hctx_idx);
 529                 if (ret) {
 530                         blk_mq_sched_free_tags(q->tag_set, hctx, hctx_idx);
 531                         return ret;
 532                 }
 533         }
 534
 535         blk_mq_debugfs_register_sched_hctx(q, hctx);
 536
 537         return 0;
 538 }
 539
 540 void blk_mq_sched_exit_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx,
 541                             unsigned int hctx_idx)
 542 {
 543         struct elevator_queue *e = q->elevator;
 544
 545         if (!e)
 546                 return;
 547
 548         blk_mq_debugfs_unregister_sched_hctx(hctx);
 549
 550         if (e->type->ops.mq.exit_hctx && hctx->sched_data) {
 551                 e->type->ops.mq.exit_hctx(hctx, hctx_idx);
 552                 hctx->sched_data = NULL;
 553         }
 554
 555         blk_mq_sched_free_tags(q->tag_set, hctx, hctx_idx);
 556 }
 557
 558 int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
 559 {
 560         struct blk_mq_hw_ctx *hctx;
 561         struct elevator_queue *eq;
 562         unsigned int i;
 563         int ret;
 564
 565         if (!e) {
 566                 q->elevator = NULL;
 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.mq.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.mq.init_hctx) {
 592                         ret = e->ops.mq.init_hctx(hctx, i);
 593                         if (ret) {
 594                                 eq = q->elevator;
 595                                 blk_mq_exit_sched(q, eq);
 596                                 kobject_put(&eq->kobj);
 597                                 return ret;
 598                         }
 599                 }
 600                 blk_mq_debugfs_register_sched_hctx(q, hctx);
 601         }
 602
 603         return 0;
 604
 605 err:
 606         blk_mq_sched_tags_teardown(q);
 607         q->elevator = NULL;
 608         return ret;
 609 }
 610
 611 void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
 612 {
 613         struct blk_mq_hw_ctx *hctx;
 614         unsigned int i;
 615
 616         queue_for_each_hw_ctx(q, hctx, i) {
 617                 blk_mq_debugfs_unregister_sched_hctx(hctx);
 618                 if (e->type->ops.mq.exit_hctx && hctx->sched_data) {
 619                         e->type->ops.mq.exit_hctx(hctx, i);
 620                         hctx->sched_data = NULL;
 621                 }
 622         }
 623         blk_mq_debugfs_unregister_sched(q);
 624         if (e->type->ops.mq.exit_sched)
 625                 e->type->ops.mq.exit_sched(e);
 626         blk_mq_sched_tags_teardown(q);
 627         q->elevator = NULL;
 628 }
 629
 630 int blk_mq_sched_init(struct request_queue *q)
 631 {
 632         int ret;
 633
 634         mutex_lock(&q->sysfs_lock);
 635         ret = elevator_init(q, NULL);
 636         mutex_unlock(&q->sysfs_lock);
 637
 638         return ret;
 639 }