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 normal (non-reserved) tags
  72  */
  73 static void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags)
  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
  90 /*
  91  * If a previously busy queue goes inactive, potential waiters could now
  92  * be allowed to queue. Wake them up and check.
  93  */
  94 void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
  95 {
  96         struct blk_mq_tags *tags = hctx->tags;
  97
  98         if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
  99                 return;
 100
 101         atomic_dec(&tags->active_queues);
 102
 103         blk_mq_tag_wakeup_all(tags);
 104 }
 105
 106 /*
 107  * For shared tag users, we track the number of currently active users
 108  * and attempt to provide a fair share of the tag depth for each of them.
 109  */
 110 static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
 111                                   struct blk_mq_bitmap_tags *bt)
 112 {
 113         unsigned int depth, users;
 114
 115         if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_SHARED))
 116                 return true;
 117         if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 118                 return true;
 119
 120         /*
 121          * Don't try dividing an ant
 122          */
 123         if (bt->depth == 1)
 124                 return true;
 125
 126         users = atomic_read(&hctx->tags->active_queues);
 127         if (!users)
 128                 return true;
 129
 130         /*
 131          * Allow at least some tags
 132          */
 133         depth = max((bt->depth + users - 1) / users, 4U);
 134         return atomic_read(&hctx->nr_active) < depth;
 135 }
 136
 137 static int __bt_get_word(struct blk_align_bitmap *bm, unsigned int last_tag)
 138 {
 139         int tag, org_last_tag, end;
 140         bool wrap = last_tag != 0;
 141
 142         org_last_tag = last_tag;
 143         end = bm->depth;
 144         do {
 145 restart:
 146                 tag = find_next_zero_bit(&bm->word, end, last_tag);
 147                 if (unlikely(tag >= end)) {
 148                         /*
 149                          * We started with an offset, start from 0 to
 150                          * exhaust the map.
 151                          */
 152                         if (wrap) {
 153                                 wrap = false;
 154                                 end = org_last_tag;
 155                                 last_tag = 0;
 156                                 goto restart;
 157                         }
 158                         return -1;
 159                 }
 160                 last_tag = tag + 1;
 161         } while (test_and_set_bit(tag, &bm->word));
 162
 163         return tag;
 164 }
 165
 166 /*
 167  * Straight forward bitmap tag implementation, where each bit is a tag
 168  * (cleared == free, and set == busy). The small twist is using per-cpu
 169  * last_tag caches, which blk-mq stores in the blk_mq_ctx software queue
 170  * contexts. This enables us to drastically limit the space searched,
 171  * without dirtying an extra shared cacheline like we would if we stored
 172  * the cache value inside the shared blk_mq_bitmap_tags structure. On top
 173  * of that, each word of tags is in a separate cacheline. This means that
 174  * multiple users will tend to stick to different cachelines, at least
 175  * until the map is exhausted.
 176  */
 177 static int __bt_get(struct blk_mq_hw_ctx *hctx, struct blk_mq_bitmap_tags *bt,
 178                     unsigned int *tag_cache)
 179 {
 180         unsigned int last_tag, org_last_tag;
 181         int index, i, tag;
 182
 183         if (!hctx_may_queue(hctx, bt))
 184                 return -1;
 185
 186         last_tag = org_last_tag = *tag_cache;
 187         index = TAG_TO_INDEX(bt, last_tag);
 188
 189         for (i = 0; i < bt->map_nr; i++) {
 190                 tag = __bt_get_word(&bt->map[index], TAG_TO_BIT(bt, last_tag));
 191                 if (tag != -1) {
 192                         tag += (index << bt->bits_per_word);
 193                         goto done;
 194                 }
 195
 196                 last_tag = 0;
 197                 if (++index >= bt->map_nr)
 198                         index = 0;
 199         }
 200
 201         *tag_cache = 0;
 202         return -1;
 203
 204         /*
 205          * Only update the cache from the allocation path, if we ended
 206          * up using the specific cached tag.
 207          */
 208 done:
 209         if (tag == org_last_tag) {
 210                 last_tag = tag + 1;
 211                 if (last_tag >= bt->depth - 1)
 212                         last_tag = 0;
 213
 214                 *tag_cache = last_tag;
 215         }
 216
 217         return tag;
 218 }
 219
 220 static struct bt_wait_state *bt_wait_ptr(struct blk_mq_bitmap_tags *bt,
 221                                          struct blk_mq_hw_ctx *hctx)
 222 {
 223         struct bt_wait_state *bs;
 224         int wait_index;
 225
 226         if (!hctx)
 227                 return &bt->bs[0];
 228
 229         wait_index = atomic_read(&hctx->wait_index);
 230         bs = &bt->bs[wait_index];
 231         bt_index_atomic_inc(&hctx->wait_index);
 232         return bs;
 233 }
 234
 235 static int bt_get(struct blk_mq_alloc_data *data,
 236                 struct blk_mq_bitmap_tags *bt,
 237                 struct blk_mq_hw_ctx *hctx,
 238                 unsigned int *last_tag)
 239 {
 240         struct bt_wait_state *bs;
 241         DEFINE_WAIT(wait);
 242         int tag;
 243
 244         tag = __bt_get(hctx, bt, last_tag);
 245         if (tag != -1)
 246                 return tag;
 247
 248         if (!(data->gfp & __GFP_WAIT))
 249                 return -1;
 250
 251         bs = bt_wait_ptr(bt, hctx);
 252         do {
 253                 prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
 254
 255                 tag = __bt_get(hctx, bt, last_tag);
 256                 if (tag != -1)
 257                         break;
 258
 259                 blk_mq_put_ctx(data->ctx);
 260
 261                 io_schedule();
 262
 263                 data->ctx = blk_mq_get_ctx(data->q);
 264                 data->hctx = data->q->mq_ops->map_queue(data->q,
 265                                 data->ctx->cpu);
 266                 if (data->reserved) {
 267                         bt = &data->hctx->tags->breserved_tags;
 268                 } else {
 269                         last_tag = &data->ctx->last_tag;
 270                         hctx = data->hctx;
 271                         bt = &hctx->tags->bitmap_tags;
 272                 }
 273                 finish_wait(&bs->wait, &wait);
 274                 bs = bt_wait_ptr(bt, hctx);
 275         } while (1);
 276
 277         finish_wait(&bs->wait, &wait);
 278         return tag;
 279 }
 280
 281 static unsigned int __blk_mq_get_tag(struct blk_mq_alloc_data *data)
 282 {
 283         int tag;
 284
 285         tag = bt_get(data, &data->hctx->tags->bitmap_tags, data->hctx,
 286                         &data->ctx->last_tag);
 287         if (tag >= 0)
 288                 return tag + data->hctx->tags->nr_reserved_tags;
 289
 290         return BLK_MQ_TAG_FAIL;
 291 }
 292
 293 static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_alloc_data *data)
 294 {
 295         int tag, zero = 0;
 296
 297         if (unlikely(!data->hctx->tags->nr_reserved_tags)) {
 298                 WARN_ON_ONCE(1);
 299                 return BLK_MQ_TAG_FAIL;
 300         }
 301
 302         tag = bt_get(data, &data->hctx->tags->breserved_tags, NULL, &zero);
 303         if (tag < 0)
 304                 return BLK_MQ_TAG_FAIL;
 305
 306         return tag;
 307 }
 308
 309 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 310 {
 311         if (!data->reserved)
 312                 return __blk_mq_get_tag(data);
 313
 314         return __blk_mq_get_reserved_tag(data);
 315 }
 316
 317 static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt)
 318 {
 319         int i, wake_index;
 320
 321         wake_index = atomic_read(&bt->wake_index);
 322         for (i = 0; i < BT_WAIT_QUEUES; i++) {
 323                 struct bt_wait_state *bs = &bt->bs[wake_index];
 324
 325                 if (waitqueue_active(&bs->wait)) {
 326                         int o = atomic_read(&bt->wake_index);
 327                         if (wake_index != o)
 328                                 atomic_cmpxchg(&bt->wake_index, o, wake_index);
 329
 330                         return bs;
 331                 }
 332
 333                 wake_index = bt_index_inc(wake_index);
 334         }
 335
 336         return NULL;
 337 }
 338
 339 static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
 340 {
 341         const int index = TAG_TO_INDEX(bt, tag);
 342         struct bt_wait_state *bs;
 343         int wait_cnt;
 344
 345         clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word);
 346
 347         /* Ensure that the wait list checks occur after clear_bit(). */
 348         smp_mb();
 349
 350         bs = bt_wake_ptr(bt);
 351         if (!bs)
 352                 return;
 353
 354         wait_cnt = atomic_dec_return(&bs->wait_cnt);
 355         if (unlikely(wait_cnt < 0))
 356                 wait_cnt = atomic_inc_return(&bs->wait_cnt);
 357         if (wait_cnt == 0) {
 358                 atomic_add(bt->wake_cnt, &bs->wait_cnt);
 359                 bt_index_atomic_inc(&bt->wake_index);
 360                 wake_up(&bs->wait);
 361         }
 362 }
 363
 364 static void __blk_mq_put_tag(struct blk_mq_tags *tags, unsigned int tag)
 365 {
 366         BUG_ON(tag >= tags->nr_tags);
 367
 368         bt_clear_tag(&tags->bitmap_tags, tag);
 369 }
 370
 371 static void __blk_mq_put_reserved_tag(struct blk_mq_tags *tags,
 372                                       unsigned int tag)
 373 {
 374         BUG_ON(tag >= tags->nr_reserved_tags);
 375
 376         bt_clear_tag(&tags->breserved_tags, tag);
 377 }
 378
 379 void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, unsigned int tag,
 380                     unsigned int *last_tag)
 381 {
 382         struct blk_mq_tags *tags = hctx->tags;
 383
 384         if (tag >= tags->nr_reserved_tags) {
 385                 const int real_tag = tag - tags->nr_reserved_tags;
 386
 387                 __blk_mq_put_tag(tags, real_tag);
 388                 *last_tag = real_tag;
 389         } else
 390                 __blk_mq_put_reserved_tag(tags, tag);
 391 }
 392
 393 static void bt_for_each(struct blk_mq_hw_ctx *hctx,
 394                 struct blk_mq_bitmap_tags *bt, unsigned int off,
 395                 busy_iter_fn *fn, void *data, bool reserved)
 396 {
 397         struct request *rq;
 398         int bit, i;
 399
 400         for (i = 0; i < bt->map_nr; i++) {
 401                 struct blk_align_bitmap *bm = &bt->map[i];
 402
 403                 for (bit = find_first_bit(&bm->word, bm->depth);
 404                      bit < bm->depth;
 405                      bit = find_next_bit(&bm->word, bm->depth, bit + 1)) {
 406                         rq = hctx->tags->rqs[off + bit];
 407                         if (rq->q == hctx->queue)
 408                                 fn(hctx, rq, data, reserved);
 409                 }
 410
 411                 off += (1 << bt->bits_per_word);
 412         }
 413 }
 414
 415 void blk_mq_tag_busy_iter(struct blk_mq_hw_ctx *hctx, busy_iter_fn *fn,
 416                 void *priv)
 417 {
 418         struct blk_mq_tags *tags = hctx->tags;
 419
 420         if (tags->nr_reserved_tags)
 421                 bt_for_each(hctx, &tags->breserved_tags, 0, fn, priv, true);
 422         bt_for_each(hctx, &tags->bitmap_tags, tags->nr_reserved_tags, fn, priv,
 423                         false);
 424 }
 425 EXPORT_SYMBOL(blk_mq_tag_busy_iter);
 426
 427 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
 428 {
 429         unsigned int i, used;
 430
 431         for (i = 0, used = 0; i < bt->map_nr; i++) {
 432                 struct blk_align_bitmap *bm = &bt->map[i];
 433
 434                 used += bitmap_weight(&bm->word, bm->depth);
 435         }
 436
 437         return bt->depth - used;
 438 }
 439
 440 static void bt_update_count(struct blk_mq_bitmap_tags *bt,
 441                             unsigned int depth)
 442 {
 443         unsigned int tags_per_word = 1U << bt->bits_per_word;
 444         unsigned int map_depth = depth;
 445
 446         if (depth) {
 447                 int i;
 448
 449                 for (i = 0; i < bt->map_nr; i++) {
 450                         bt->map[i].depth = min(map_depth, tags_per_word);
 451                         map_depth -= bt->map[i].depth;
 452                 }
 453         }
 454
 455         bt->wake_cnt = BT_WAIT_BATCH;
 456         if (bt->wake_cnt > depth / BT_WAIT_QUEUES)
 457                 bt->wake_cnt = max(1U, depth / BT_WAIT_QUEUES);
 458
 459         bt->depth = depth;
 460 }
 461
 462 static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
 463                         int node, bool reserved)
 464 {
 465         int i;
 466
 467         bt->bits_per_word = ilog2(BITS_PER_LONG);
 468
 469         /*
 470          * Depth can be zero for reserved tags, that's not a failure
 471          * condition.
 472          */
 473         if (depth) {
 474                 unsigned int nr, tags_per_word;
 475
 476                 tags_per_word = (1 << bt->bits_per_word);
 477
 478                 /*
 479                  * If the tag space is small, shrink the number of tags
 480                  * per word so we spread over a few cachelines, at least.
 481                  * If less than 4 tags, just forget about it, it's not
 482                  * going to work optimally anyway.
 483                  */
 484                 if (depth >= 4) {
 485                         while (tags_per_word * 4 > depth) {
 486                                 bt->bits_per_word--;
 487                                 tags_per_word = (1 << bt->bits_per_word);
 488                         }
 489                 }
 490
 491                 nr = ALIGN(depth, tags_per_word) / tags_per_word;
 492                 bt->map = kzalloc_node(nr * sizeof(struct blk_align_bitmap),
 493                                                 GFP_KERNEL, node);
 494                 if (!bt->map)
 495                         return -ENOMEM;
 496
 497                 bt->map_nr = nr;
 498         }
 499
 500         bt->bs = kzalloc(BT_WAIT_QUEUES * sizeof(*bt->bs), GFP_KERNEL);
 501         if (!bt->bs) {
 502                 kfree(bt->map);
 503                 bt->map = NULL;
 504                 return -ENOMEM;
 505         }
 506
 507         bt_update_count(bt, depth);
 508
 509         for (i = 0; i < BT_WAIT_QUEUES; i++) {
 510                 init_waitqueue_head(&bt->bs[i].wait);
 511                 atomic_set(&bt->bs[i].wait_cnt, bt->wake_cnt);
 512         }
 513
 514         return 0;
 515 }
 516
 517 static void bt_free(struct blk_mq_bitmap_tags *bt)
 518 {
 519         kfree(bt->map);
 520         kfree(bt->bs);
 521 }
 522
 523 static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
 524                                                    int node)
 525 {
 526         unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
 527
 528         if (bt_alloc(&tags->bitmap_tags, depth, node, false))
 529                 goto enomem;
 530         if (bt_alloc(&tags->breserved_tags, tags->nr_reserved_tags, node, true))
 531                 goto enomem;
 532
 533         return tags;
 534 enomem:
 535         bt_free(&tags->bitmap_tags);
 536         kfree(tags);
 537         return NULL;
 538 }
 539
 540 struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
 541                                      unsigned int reserved_tags, int node)
 542 {
 543         struct blk_mq_tags *tags;
 544
 545         if (total_tags > BLK_MQ_TAG_MAX) {
 546                 pr_err("blk-mq: tag depth too large\n");
 547                 return NULL;
 548         }
 549
 550         tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
 551         if (!tags)
 552                 return NULL;
 553
 554         tags->nr_tags = total_tags;
 555         tags->nr_reserved_tags = reserved_tags;
 556
 557         return blk_mq_init_bitmap_tags(tags, node);
 558 }
 559
 560 void blk_mq_free_tags(struct blk_mq_tags *tags)
 561 {
 562         bt_free(&tags->bitmap_tags);
 563         bt_free(&tags->breserved_tags);
 564         kfree(tags);
 565 }
 566
 567 void blk_mq_tag_init_last_tag(struct blk_mq_tags *tags, unsigned int *tag)
 568 {
 569         unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
 570
 571         *tag = prandom_u32() % depth;
 572 }
 573
 574 int blk_mq_tag_update_depth(struct blk_mq_tags *tags, unsigned int tdepth)
 575 {
 576         tdepth -= tags->nr_reserved_tags;
 577         if (tdepth > tags->nr_tags)
 578                 return -EINVAL;
 579
 580         /*
 581          * Don't need (or can't) update reserved tags here, they remain
 582          * static and should never need resizing.
 583          */
 584         bt_update_count(&tags->bitmap_tags, tdepth);
 585         blk_mq_tag_wakeup_all(tags);
 586         return 0;
 587 }
 588
 589 ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)
 590 {
 591         char *orig_page = page;
 592         unsigned int free, res;
 593
 594         if (!tags)
 595                 return 0;
 596
 597         page += sprintf(page, "nr_tags=%u, reserved_tags=%u, "
 598                         "bits_per_word=%u\n",
 599                         tags->nr_tags, tags->nr_reserved_tags,
 600                         tags->bitmap_tags.bits_per_word);
 601
 602         free = bt_unused_tags(&tags->bitmap_tags);
 603         res = bt_unused_tags(&tags->breserved_tags);
 604
 605         page += sprintf(page, "nr_free=%u, nr_reserved=%u\n", free, res);
 606         page += sprintf(page, "active_queues=%u\n", atomic_read(&tags->active_queues));
 607
 608         return page - orig_page;
 609 }