block/blk-mq-tag.c

   1 /*
   2  * Fast and scalable bitmap tagging variant. Uses sparser bitmaps spread
   3  * over multiple cachelines to avoid ping-pong between multiple submitters
   4  * or submitter and completer. Uses rolling wakeups to avoid falling of
   5  * the scaling cliff when we run out of tags and have to start putting
   6  * submitters to sleep.
   7  *
   8  * Uses active queue tracking to support fairer distribution of tags
   9  * between multiple submitters when a shared tag map is used.
  10  *
  11  * Copyright (C) 2013-2014 Jens Axboe
  12  */
  13 #include <linux/kernel.h>
  14 #include <linux/module.h>
  15 #include <linux/random.h>
  16
  17 #include <linux/blk-mq.h>
  18 #include "blk.h"
  19 #include "blk-mq.h"
  20 #include "blk-mq-tag.h"
  21
  22 static bool bt_has_free_tags(struct blk_mq_bitmap_tags *bt)
  23 {
  24         int i;
  25
  26         for (i = 0; i < bt->map_nr; i++) {
  27                 struct blk_align_bitmap *bm = &bt->map[i];
  28                 int ret;
  29
  30                 ret = find_first_zero_bit(&bm->word, bm->depth);
  31                 if (ret < bm->depth)
  32                         return true;
  33         }
  34
  35         return false;
  36 }
  37
  38 bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
  39 {
  40         if (!tags)
  41                 return true;
  42
  43         return bt_has_free_tags(&tags->bitmap_tags);
  44 }
  45
  46 static inline int bt_index_inc(int index)
  47 {
  48         return (index + 1) & (BT_WAIT_QUEUES - 1);
  49 }
  50
  51 static inline void bt_index_atomic_inc(atomic_t *index)
  52 {
  53         int old = atomic_read(index);
  54         int new = bt_index_inc(old);
  55         atomic_cmpxchg(index, old, new);
  56 }
  57
  58 /*
  59  * If a previously inactive queue goes active, bump the active user count.
  60  */
  61 bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
  62 {
  63         if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
  64             !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
  65                 atomic_inc(&hctx->tags->active_queues);
  66
  67         return true;
  68 }
  69
  70 /*
  71  * Wakeup all potentially sleeping on tags
  72  */
  73 void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool include_reserve)
  74 {
  75         struct blk_mq_bitmap_tags *bt;
  76         int i, wake_index;
  77
  78         bt = &tags->bitmap_tags;
  79         wake_index = atomic_read(&bt->wake_index);
  80         for (i = 0; i < BT_WAIT_QUEUES; i++) {
  81                 struct bt_wait_state *bs = &bt->bs[wake_index];
  82
  83                 if (waitqueue_active(&bs->wait))
  84                         wake_up(&bs->wait);
  85
  86                 wake_index = bt_index_inc(wake_index);
  87         }
  88
  89         if (include_reserve) {
  90                 bt = &tags->breserved_tags;
  91                 if (waitqueue_active(&bt->bs[0].wait))
  92                         wake_up(&bt->bs[0].wait);
  93         }
  94 }
  95
  96 /*
  97  * If a previously busy queue goes inactive, potential waiters could now
  98  * be allowed to queue. Wake them up and check.
  99  */
 100 void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
 101 {
 102         struct blk_mq_tags *tags = hctx->tags;
 103
 104         if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 105                 return;
 106
 107         atomic_dec(&tags->active_queues);
 108
 109         blk_mq_tag_wakeup_all(tags, false);
 110 }
 111
 112 /*
 113  * For shared tag users, we track the number of currently active users
 114  * and attempt to provide a fair share of the tag depth for each of them.
 115  */
 116 static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
 117                                   struct blk_mq_bitmap_tags *bt)
 118 {
 119         unsigned int depth, users;
 120
 121         if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_SHARED))
 122                 return true;
 123         if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 124                 return true;
 125
 126         /*
 127          * Don't try dividing an ant
 128          */
 129         if (bt->depth == 1)
 130                 return true;
 131
 132         users = atomic_read(&hctx->tags->active_queues);
 133         if (!users)
 134                 return true;
 135
 136         /*
 137          * Allow at least some tags
 138          */
 139         depth = max((bt->depth + users - 1) / users, 4U);
 140         return atomic_read(&hctx->nr_active) < depth;
 141 }
 142
 143 static int __bt_get_word(struct blk_align_bitmap *bm, unsigned int last_tag,
 144                          bool nowrap)
 145 {
 146         int tag, org_last_tag = last_tag;
 147
 148         while (1) {
 149                 tag = find_next_zero_bit(&bm->word, bm->depth, last_tag);
 150                 if (unlikely(tag >= bm->depth)) {
 151                         /*
 152                          * We started with an offset, and we didn't reset the
 153                          * offset to 0 in a failure case, so start from 0 to
 154                          * exhaust the map.
 155                          */
 156                         if (org_last_tag && last_tag && !nowrap) {
 157                                 last_tag = org_last_tag = 0;
 158                                 continue;
 159                         }
 160                         return -1;
 161                 }
 162
 163                 if (!test_and_set_bit(tag, &bm->word))
 164                         break;
 165
 166                 last_tag = tag + 1;
 167                 if (last_tag >= bm->depth - 1)
 168                         last_tag = 0;
 169         }
 170
 171         return tag;
 172 }
 173
 174 #define BT_ALLOC_RR(tags) (tags->alloc_policy == BLK_TAG_ALLOC_RR)
 175
 176 /*
 177  * Straight forward bitmap tag implementation, where each bit is a tag
 178  * (cleared == free, and set == busy). The small twist is using per-cpu
 179  * last_tag caches, which blk-mq stores in the blk_mq_ctx software queue
 180  * contexts. This enables us to drastically limit the space searched,
 181  * without dirtying an extra shared cacheline like we would if we stored
 182  * the cache value inside the shared blk_mq_bitmap_tags structure. On top
 183  * of that, each word of tags is in a separate cacheline. This means that
 184  * multiple users will tend to stick to different cachelines, at least
 185  * until the map is exhausted.
 186  */
 187 static int __bt_get(struct blk_mq_hw_ctx *hctx, struct blk_mq_bitmap_tags *bt,
 188                     unsigned int *tag_cache, struct blk_mq_tags *tags)
 189 {
 190         unsigned int last_tag, org_last_tag;
 191         int index, i, tag;
 192
 193         if (!hctx_may_queue(hctx, bt))
 194                 return -1;
 195
 196         last_tag = org_last_tag = *tag_cache;
 197         index = TAG_TO_INDEX(bt, last_tag);
 198
 199         for (i = 0; i < bt->map_nr; i++) {
 200                 tag = __bt_get_word(&bt->map[index], TAG_TO_BIT(bt, last_tag),
 201                                     BT_ALLOC_RR(tags));
 202                 if (tag != -1) {
 203                         tag += (index << bt->bits_per_word);
 204                         goto done;
 205                 }
 206
 207                 /*
 208                  * Jump to next index, and reset the last tag to be the
 209                  * first tag of that index
 210                  */
 211                 index++;
 212                 last_tag = (index << bt->bits_per_word);
 213
 214                 if (index >= bt->map_nr) {
 215                         index = 0;
 216                         last_tag = 0;
 217                 }
 218         }
 219
 220         *tag_cache = 0;
 221         return -1;
 222
 223         /*
 224          * Only update the cache from the allocation path, if we ended
 225          * up using the specific cached tag.
 226          */
 227 done:
 228         if (tag == org_last_tag || unlikely(BT_ALLOC_RR(tags))) {
 229                 last_tag = tag + 1;
 230                 if (last_tag >= bt->depth - 1)
 231                         last_tag = 0;
 232
 233                 *tag_cache = last_tag;
 234         }
 235
 236         return tag;
 237 }
 238
 239 static struct bt_wait_state *bt_wait_ptr(struct blk_mq_bitmap_tags *bt,
 240                                          struct blk_mq_hw_ctx *hctx)
 241 {
 242         struct bt_wait_state *bs;
 243         int wait_index;
 244
 245         if (!hctx)
 246                 return &bt->bs[0];
 247
 248         wait_index = atomic_read(&hctx->wait_index);
 249         bs = &bt->bs[wait_index];
 250         bt_index_atomic_inc(&hctx->wait_index);
 251         return bs;
 252 }
 253
 254 static int bt_get(struct blk_mq_alloc_data *data,
 255                 struct blk_mq_bitmap_tags *bt,
 256                 struct blk_mq_hw_ctx *hctx,
 257                 unsigned int *last_tag, struct blk_mq_tags *tags)
 258 {
 259         struct bt_wait_state *bs;
 260         DEFINE_WAIT(wait);
 261         int tag;
 262
 263         tag = __bt_get(hctx, bt, last_tag, tags);
 264         if (tag != -1)
 265                 return tag;
 266
 267         if (!(data->gfp & __GFP_WAIT))
 268                 return -1;
 269
 270         bs = bt_wait_ptr(bt, hctx);
 271         do {
 272                 prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
 273
 274                 tag = __bt_get(hctx, bt, last_tag, tags);
 275                 if (tag != -1)
 276                         break;
 277
 278                 /*
 279                  * We're out of tags on this hardware queue, kick any
 280                  * pending IO submits before going to sleep waiting for
 281                  * some to complete. Note that hctx can be NULL here for
 282                  * reserved tag allocation.
 283                  */
 284                 if (hctx)
 285                         blk_mq_run_hw_queue(hctx, false);
 286
 287                 /*
 288                  * Retry tag allocation after running the hardware queue,
 289                  * as running the queue may also have found completions.
 290                  */
 291                 tag = __bt_get(hctx, bt, last_tag, tags);
 292                 if (tag != -1)
 293                         break;
 294
 295                 blk_mq_put_ctx(data->ctx);
 296
 297                 io_schedule();
 298
 299                 data->ctx = blk_mq_get_ctx(data->q);
 300                 data->hctx = data->q->mq_ops->map_queue(data->q,
 301                                 data->ctx->cpu);
 302                 if (data->reserved) {
 303                         bt = &data->hctx->tags->breserved_tags;
 304                 } else {
 305                         last_tag = &data->ctx->last_tag;
 306                         hctx = data->hctx;
 307                         bt = &hctx->tags->bitmap_tags;
 308                 }
 309                 finish_wait(&bs->wait, &wait);
 310                 bs = bt_wait_ptr(bt, hctx);
 311         } while (1);
 312
 313         finish_wait(&bs->wait, &wait);
 314         return tag;
 315 }
 316
 317 static unsigned int __blk_mq_get_tag(struct blk_mq_alloc_data *data)
 318 {
 319         int tag;
 320
 321         tag = bt_get(data, &data->hctx->tags->bitmap_tags, data->hctx,
 322                         &data->ctx->last_tag, data->hctx->tags);
 323         if (tag >= 0)
 324                 return tag + data->hctx->tags->nr_reserved_tags;
 325
 326         return BLK_MQ_TAG_FAIL;
 327 }
 328
 329 static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_alloc_data *data)
 330 {
 331         int tag, zero = 0;
 332
 333         if (unlikely(!data->hctx->tags->nr_reserved_tags)) {
 334                 WARN_ON_ONCE(1);
 335                 return BLK_MQ_TAG_FAIL;
 336         }
 337
 338         tag = bt_get(data, &data->hctx->tags->breserved_tags, NULL, &zero,
 339                 data->hctx->tags);
 340         if (tag < 0)
 341                 return BLK_MQ_TAG_FAIL;
 342
 343         return tag;
 344 }
 345
 346 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 347 {
 348         if (!data->reserved)
 349                 return __blk_mq_get_tag(data);
 350
 351         return __blk_mq_get_reserved_tag(data);
 352 }
 353
 354 static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt)
 355 {
 356         int i, wake_index;
 357
 358         wake_index = atomic_read(&bt->wake_index);
 359         for (i = 0; i < BT_WAIT_QUEUES; i++) {
 360                 struct bt_wait_state *bs = &bt->bs[wake_index];
 361
 362                 if (waitqueue_active(&bs->wait)) {
 363                         int o = atomic_read(&bt->wake_index);
 364                         if (wake_index != o)
 365                                 atomic_cmpxchg(&bt->wake_index, o, wake_index);
 366
 367                         return bs;
 368                 }
 369
 370                 wake_index = bt_index_inc(wake_index);
 371         }
 372
 373         return NULL;
 374 }
 375
 376 static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
 377 {
 378         const int index = TAG_TO_INDEX(bt, tag);
 379         struct bt_wait_state *bs;
 380         int wait_cnt;
 381
 382         clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word);
 383
 384         /* Ensure that the wait list checks occur after clear_bit(). */
 385         smp_mb();
 386
 387         bs = bt_wake_ptr(bt);
 388         if (!bs)
 389                 return;
 390
 391         wait_cnt = atomic_dec_return(&bs->wait_cnt);
 392         if (unlikely(wait_cnt < 0))
 393                 wait_cnt = atomic_inc_return(&bs->wait_cnt);
 394         if (wait_cnt == 0) {
 395                 atomic_add(bt->wake_cnt, &bs->wait_cnt);
 396                 bt_index_atomic_inc(&bt->wake_index);
 397                 wake_up(&bs->wait);
 398         }
 399 }
 400
 401 void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, unsigned int tag,
 402                     unsigned int *last_tag)
 403 {
 404         struct blk_mq_tags *tags = hctx->tags;
 405
 406         if (tag >= tags->nr_reserved_tags) {
 407                 const int real_tag = tag - tags->nr_reserved_tags;
 408
 409                 BUG_ON(real_tag >= tags->nr_tags);
 410                 bt_clear_tag(&tags->bitmap_tags, real_tag);
 411                 if (likely(tags->alloc_policy == BLK_TAG_ALLOC_FIFO))
 412                         *last_tag = real_tag;
 413         } else {
 414                 BUG_ON(tag >= tags->nr_reserved_tags);
 415                 bt_clear_tag(&tags->breserved_tags, tag);
 416         }
 417 }
 418
 419 static void bt_for_each(struct blk_mq_hw_ctx *hctx,
 420                 struct blk_mq_bitmap_tags *bt, unsigned int off,
 421                 busy_iter_fn *fn, void *data, bool reserved)
 422 {
 423         struct request *rq;
 424         int bit, i;
 425
 426         for (i = 0; i < bt->map_nr; i++) {
 427                 struct blk_align_bitmap *bm = &bt->map[i];
 428
 429                 for (bit = find_first_bit(&bm->word, bm->depth);
 430                      bit < bm->depth;
 431                      bit = find_next_bit(&bm->word, bm->depth, bit + 1)) {
 432                         rq = blk_mq_tag_to_rq(hctx->tags, off + bit);
 433                         if (rq->q == hctx->queue)
 434                                 fn(hctx, rq, data, reserved);
 435                 }
 436
 437                 off += (1 << bt->bits_per_word);
 438         }
 439 }
 440
 441 void blk_mq_tag_busy_iter(struct blk_mq_hw_ctx *hctx, busy_iter_fn *fn,
 442                 void *priv)
 443 {
 444         struct blk_mq_tags *tags = hctx->tags;
 445
 446         if (tags->nr_reserved_tags)
 447                 bt_for_each(hctx, &tags->breserved_tags, 0, fn, priv, true);
 448         bt_for_each(hctx, &tags->bitmap_tags, tags->nr_reserved_tags, fn, priv,
 449                         false);
 450 }
 451 EXPORT_SYMBOL(blk_mq_tag_busy_iter);
 452
 453 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
 454 {
 455         unsigned int i, used;
 456
 457         for (i = 0, used = 0; i < bt->map_nr; i++) {
 458                 struct blk_align_bitmap *bm = &bt->map[i];
 459
 460                 used += bitmap_weight(&bm->word, bm->depth);
 461         }
 462
 463         return bt->depth - used;
 464 }
 465
 466 static void bt_update_count(struct blk_mq_bitmap_tags *bt,
 467                             unsigned int depth)
 468 {
 469         unsigned int tags_per_word = 1U << bt->bits_per_word;
 470         unsigned int map_depth = depth;
 471
 472         if (depth) {
 473                 int i;
 474
 475                 for (i = 0; i < bt->map_nr; i++) {
 476                         bt->map[i].depth = min(map_depth, tags_per_word);
 477                         map_depth -= bt->map[i].depth;
 478                 }
 479         }
 480
 481         bt->wake_cnt = BT_WAIT_BATCH;
 482         if (bt->wake_cnt > depth / BT_WAIT_QUEUES)
 483                 bt->wake_cnt = max(1U, depth / BT_WAIT_QUEUES);
 484
 485         bt->depth = depth;
 486 }
 487
 488 static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
 489                         int node, bool reserved)
 490 {
 491         int i;
 492
 493         bt->bits_per_word = ilog2(BITS_PER_LONG);
 494
 495         /*
 496          * Depth can be zero for reserved tags, that's not a failure
 497          * condition.
 498          */
 499         if (depth) {
 500                 unsigned int nr, tags_per_word;
 501
 502                 tags_per_word = (1 << bt->bits_per_word);
 503
 504                 /*
 505                  * If the tag space is small, shrink the number of tags
 506                  * per word so we spread over a few cachelines, at least.
 507                  * If less than 4 tags, just forget about it, it's not
 508                  * going to work optimally anyway.
 509                  */
 510                 if (depth >= 4) {
 511                         while (tags_per_word * 4 > depth) {
 512                                 bt->bits_per_word--;
 513                                 tags_per_word = (1 << bt->bits_per_word);
 514                         }
 515                 }
 516
 517                 nr = ALIGN(depth, tags_per_word) / tags_per_word;
 518                 bt->map = kzalloc_node(nr * sizeof(struct blk_align_bitmap),
 519                                                 GFP_KERNEL, node);
 520                 if (!bt->map)
 521                         return -ENOMEM;
 522
 523                 bt->map_nr = nr;
 524         }
 525
 526         bt->bs = kzalloc(BT_WAIT_QUEUES * sizeof(*bt->bs), GFP_KERNEL);
 527         if (!bt->bs) {
 528                 kfree(bt->map);
 529                 bt->map = NULL;
 530                 return -ENOMEM;
 531         }
 532
 533         bt_update_count(bt, depth);
 534
 535         for (i = 0; i < BT_WAIT_QUEUES; i++) {
 536                 init_waitqueue_head(&bt->bs[i].wait);
 537                 atomic_set(&bt->bs[i].wait_cnt, bt->wake_cnt);
 538         }
 539
 540         return 0;
 541 }
 542
 543 static void bt_free(struct blk_mq_bitmap_tags *bt)
 544 {
 545         kfree(bt->map);
 546         kfree(bt->bs);
 547 }
 548
 549 static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
 550                                                    int node, int alloc_policy)
 551 {
 552         unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
 553
 554         tags->alloc_policy = alloc_policy;
 555
 556         if (bt_alloc(&tags->bitmap_tags, depth, node, false))
 557                 goto enomem;
 558         if (bt_alloc(&tags->breserved_tags, tags->nr_reserved_tags, node, true))
 559                 goto enomem;
 560
 561         return tags;
 562 enomem:
 563         bt_free(&tags->bitmap_tags);
 564         kfree(tags);
 565         return NULL;
 566 }
 567
 568 struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
 569                                      unsigned int reserved_tags,
 570                                      int node, int alloc_policy)
 571 {
 572         struct blk_mq_tags *tags;
 573
 574         if (total_tags > BLK_MQ_TAG_MAX) {
 575                 pr_err("blk-mq: tag depth too large\n");
 576                 return NULL;
 577         }
 578
 579         tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
 580         if (!tags)
 581                 return NULL;
 582
 583         tags->nr_tags = total_tags;
 584         tags->nr_reserved_tags = reserved_tags;
 585
 586         return blk_mq_init_bitmap_tags(tags, node, alloc_policy);
 587 }
 588
 589 void blk_mq_free_tags(struct blk_mq_tags *tags)
 590 {
 591         bt_free(&tags->bitmap_tags);
 592         bt_free(&tags->breserved_tags);
 593         kfree(tags);
 594 }
 595
 596 void blk_mq_tag_init_last_tag(struct blk_mq_tags *tags, unsigned int *tag)
 597 {
 598         unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
 599
 600         *tag = prandom_u32() % depth;
 601 }
 602
 603 int blk_mq_tag_update_depth(struct blk_mq_tags *tags, unsigned int tdepth)
 604 {
 605         tdepth -= tags->nr_reserved_tags;
 606         if (tdepth > tags->nr_tags)
 607                 return -EINVAL;
 608
 609         /*
 610          * Don't need (or can't) update reserved tags here, they remain
 611          * static and should never need resizing.
 612          */
 613         bt_update_count(&tags->bitmap_tags, tdepth);
 614         blk_mq_tag_wakeup_all(tags, false);
 615         return 0;
 616 }
 617
 618 /**
 619  * blk_mq_unique_tag() - return a tag that is unique queue-wide
 620  * @rq: request for which to compute a unique tag
 621  *
 622  * The tag field in struct request is unique per hardware queue but not over
 623  * all hardware queues. Hence this function that returns a tag with the
 624  * hardware context index in the upper bits and the per hardware queue tag in
 625  * the lower bits.
 626  *
 627  * Note: When called for a request that is queued on a non-multiqueue request
 628  * queue, the hardware context index is set to zero.
 629  */
 630 u32 blk_mq_unique_tag(struct request *rq)
 631 {
 632         struct request_queue *q = rq->q;
 633         struct blk_mq_hw_ctx *hctx;
 634         int hwq = 0;
 635
 636         if (q->mq_ops) {
 637                 hctx = q->mq_ops->map_queue(q, rq->mq_ctx->cpu);
 638                 hwq = hctx->queue_num;
 639         }
 640
 641         return (hwq << BLK_MQ_UNIQUE_TAG_BITS) |
 642                 (rq->tag & BLK_MQ_UNIQUE_TAG_MASK);
 643 }
 644 EXPORT_SYMBOL(blk_mq_unique_tag);
 645
 646 ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)
 647 {
 648         char *orig_page = page;
 649         unsigned int free, res;
 650
 651         if (!tags)
 652                 return 0;
 653
 654         page += sprintf(page, "nr_tags=%u, reserved_tags=%u, "
 655                         "bits_per_word=%u\n",
 656                         tags->nr_tags, tags->nr_reserved_tags,
 657                         tags->bitmap_tags.bits_per_word);
 658
 659         free = bt_unused_tags(&tags->bitmap_tags);
 660         res = bt_unused_tags(&tags->breserved_tags);
 661
 662         page += sprintf(page, "nr_free=%u, nr_reserved=%u\n", free, res);
 663         page += sprintf(page, "active_queues=%u\n", atomic_read(&tags->active_queues));
 664
 665         return page - orig_page;
 666 }