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 void bt_index_inc(unsigned int *index)
  47 {
  48         *index = (*index + 1) & (BT_WAIT_QUEUES - 1);
  49 }
  50
  51 /*
  52  * If a previously inactive queue goes active, bump the active user count.
  53  */
  54 bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
  55 {
  56         if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
  57             !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
  58                 atomic_inc(&hctx->tags->active_queues);
  59
  60         return true;
  61 }
  62
  63 /*
  64  * Wakeup all potentially sleeping on normal (non-reserved) tags
  65  */
  66 static void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags)
  67 {
  68         struct blk_mq_bitmap_tags *bt;
  69         int i, wake_index;
  70
  71         bt = &tags->bitmap_tags;
  72         wake_index = bt->wake_index;
  73         for (i = 0; i < BT_WAIT_QUEUES; i++) {
  74                 struct bt_wait_state *bs = &bt->bs[wake_index];
  75
  76                 if (waitqueue_active(&bs->wait))
  77                         wake_up(&bs->wait);
  78
  79                 bt_index_inc(&wake_index);
  80         }
  81 }
  82
  83 /*
  84  * If a previously busy queue goes inactive, potential waiters could now
  85  * be allowed to queue. Wake them up and check.
  86  */
  87 void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
  88 {
  89         struct blk_mq_tags *tags = hctx->tags;
  90
  91         if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
  92                 return;
  93
  94         atomic_dec(&tags->active_queues);
  95
  96         blk_mq_tag_wakeup_all(tags);
  97 }
  98
  99 /*
 100  * For shared tag users, we track the number of currently active users
 101  * and attempt to provide a fair share of the tag depth for each of them.
 102  */
 103 static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
 104                                   struct blk_mq_bitmap_tags *bt)
 105 {
 106         unsigned int depth, users;
 107
 108         if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_SHARED))
 109                 return true;
 110         if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 111                 return true;
 112
 113         /*
 114          * Don't try dividing an ant
 115          */
 116         if (bt->depth == 1)
 117                 return true;
 118
 119         users = atomic_read(&hctx->tags->active_queues);
 120         if (!users)
 121                 return true;
 122
 123         /*
 124          * Allow at least some tags
 125          */
 126         depth = max((bt->depth + users - 1) / users, 4U);
 127         return atomic_read(&hctx->nr_active) < depth;
 128 }
 129
 130 static int __bt_get_word(struct blk_align_bitmap *bm, unsigned int last_tag)
 131 {
 132         int tag, org_last_tag, end;
 133
 134         org_last_tag = last_tag;
 135         end = bm->depth;
 136         do {
 137 restart:
 138                 tag = find_next_zero_bit(&bm->word, end, last_tag);
 139                 if (unlikely(tag >= end)) {
 140                         /*
 141                          * We started with an offset, start from 0 to
 142                          * exhaust the map.
 143                          */
 144                         if (org_last_tag && last_tag) {
 145                                 end = last_tag;
 146                                 last_tag = 0;
 147                                 goto restart;
 148                         }
 149                         return -1;
 150                 }
 151                 last_tag = tag + 1;
 152         } while (test_and_set_bit_lock(tag, &bm->word));
 153
 154         return tag;
 155 }
 156
 157 /*
 158  * Straight forward bitmap tag implementation, where each bit is a tag
 159  * (cleared == free, and set == busy). The small twist is using per-cpu
 160  * last_tag caches, which blk-mq stores in the blk_mq_ctx software queue
 161  * contexts. This enables us to drastically limit the space searched,
 162  * without dirtying an extra shared cacheline like we would if we stored
 163  * the cache value inside the shared blk_mq_bitmap_tags structure. On top
 164  * of that, each word of tags is in a separate cacheline. This means that
 165  * multiple users will tend to stick to different cachelines, at least
 166  * until the map is exhausted.
 167  */
 168 static int __bt_get(struct blk_mq_hw_ctx *hctx, struct blk_mq_bitmap_tags *bt,
 169                     unsigned int *tag_cache)
 170 {
 171         unsigned int last_tag, org_last_tag;
 172         int index, i, tag;
 173
 174         if (!hctx_may_queue(hctx, bt))
 175                 return -1;
 176
 177         last_tag = org_last_tag = *tag_cache;
 178         index = TAG_TO_INDEX(bt, last_tag);
 179
 180         for (i = 0; i < bt->map_nr; i++) {
 181                 tag = __bt_get_word(&bt->map[index], TAG_TO_BIT(bt, last_tag));
 182                 if (tag != -1) {
 183                         tag += (index << bt->bits_per_word);
 184                         goto done;
 185                 }
 186
 187                 last_tag = 0;
 188                 if (++index >= bt->map_nr)
 189                         index = 0;
 190         }
 191
 192         *tag_cache = 0;
 193         return -1;
 194
 195         /*
 196          * Only update the cache from the allocation path, if we ended
 197          * up using the specific cached tag.
 198          */
 199 done:
 200         if (tag == org_last_tag) {
 201                 last_tag = tag + 1;
 202                 if (last_tag >= bt->depth - 1)
 203                         last_tag = 0;
 204
 205                 *tag_cache = last_tag;
 206         }
 207
 208         return tag;
 209 }
 210
 211 static struct bt_wait_state *bt_wait_ptr(struct blk_mq_bitmap_tags *bt,
 212                                          struct blk_mq_hw_ctx *hctx)
 213 {
 214         struct bt_wait_state *bs;
 215
 216         if (!hctx)
 217                 return &bt->bs[0];
 218
 219         bs = &bt->bs[hctx->wait_index];
 220         bt_index_inc(&hctx->wait_index);
 221         return bs;
 222 }
 223
 224 static int bt_get(struct blk_mq_alloc_data *data,
 225                 struct blk_mq_bitmap_tags *bt,
 226                 struct blk_mq_hw_ctx *hctx,
 227                 unsigned int *last_tag)
 228 {
 229         struct bt_wait_state *bs;
 230         DEFINE_WAIT(wait);
 231         int tag;
 232
 233         tag = __bt_get(hctx, bt, last_tag);
 234         if (tag != -1)
 235                 return tag;
 236
 237         if (!(data->gfp & __GFP_WAIT))
 238                 return -1;
 239
 240         bs = bt_wait_ptr(bt, hctx);
 241         do {
 242                 bool was_empty;
 243
 244                 was_empty = list_empty(&wait.task_list);
 245                 prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
 246
 247                 tag = __bt_get(hctx, bt, last_tag);
 248                 if (tag != -1)
 249                         break;
 250
 251                 if (was_empty)
 252                         atomic_set(&bs->wait_cnt, bt->wake_cnt);
 253
 254                 blk_mq_put_ctx(data->ctx);
 255
 256                 io_schedule();
 257
 258                 data->ctx = blk_mq_get_ctx(data->q);
 259                 data->hctx = data->q->mq_ops->map_queue(data->q,
 260                                 data->ctx->cpu);
 261                 if (data->reserved) {
 262                         bt = &data->hctx->tags->breserved_tags;
 263                 } else {
 264                         last_tag = &data->ctx->last_tag;
 265                         hctx = data->hctx;
 266                         bt = &hctx->tags->bitmap_tags;
 267                 }
 268                 finish_wait(&bs->wait, &wait);
 269                 bs = bt_wait_ptr(bt, hctx);
 270         } while (1);
 271
 272         finish_wait(&bs->wait, &wait);
 273         return tag;
 274 }
 275
 276 static unsigned int __blk_mq_get_tag(struct blk_mq_alloc_data *data)
 277 {
 278         int tag;
 279
 280         tag = bt_get(data, &data->hctx->tags->bitmap_tags, data->hctx,
 281                         &data->ctx->last_tag);
 282         if (tag >= 0)
 283                 return tag + data->hctx->tags->nr_reserved_tags;
 284
 285         return BLK_MQ_TAG_FAIL;
 286 }
 287
 288 static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_alloc_data *data)
 289 {
 290         int tag, zero = 0;
 291
 292         if (unlikely(!data->hctx->tags->nr_reserved_tags)) {
 293                 WARN_ON_ONCE(1);
 294                 return BLK_MQ_TAG_FAIL;
 295         }
 296
 297         tag = bt_get(data, &data->hctx->tags->breserved_tags, NULL, &zero);
 298         if (tag < 0)
 299                 return BLK_MQ_TAG_FAIL;
 300
 301         return tag;
 302 }
 303
 304 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 305 {
 306         if (!data->reserved)
 307                 return __blk_mq_get_tag(data);
 308
 309         return __blk_mq_get_reserved_tag(data);
 310 }
 311
 312 static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt)
 313 {
 314         int i, wake_index;
 315
 316         wake_index = bt->wake_index;
 317         for (i = 0; i < BT_WAIT_QUEUES; i++) {
 318                 struct bt_wait_state *bs = &bt->bs[wake_index];
 319
 320                 if (waitqueue_active(&bs->wait)) {
 321                         if (wake_index != bt->wake_index)
 322                                 bt->wake_index = wake_index;
 323
 324                         return bs;
 325                 }
 326
 327                 bt_index_inc(&wake_index);
 328         }
 329
 330         return NULL;
 331 }
 332
 333 static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
 334 {
 335         const int index = TAG_TO_INDEX(bt, tag);
 336         struct bt_wait_state *bs;
 337
 338         /*
 339          * The unlock memory barrier need to order access to req in free
 340          * path and clearing tag bit
 341          */
 342         clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word);
 343
 344         bs = bt_wake_ptr(bt);
 345         if (bs && atomic_dec_and_test(&bs->wait_cnt)) {
 346                 atomic_set(&bs->wait_cnt, bt->wake_cnt);
 347                 bt_index_inc(&bt->wake_index);
 348                 wake_up(&bs->wait);
 349         }
 350 }
 351
 352 static void __blk_mq_put_tag(struct blk_mq_tags *tags, unsigned int tag)
 353 {
 354         BUG_ON(tag >= tags->nr_tags);
 355
 356         bt_clear_tag(&tags->bitmap_tags, tag);
 357 }
 358
 359 static void __blk_mq_put_reserved_tag(struct blk_mq_tags *tags,
 360                                       unsigned int tag)
 361 {
 362         BUG_ON(tag >= tags->nr_reserved_tags);
 363
 364         bt_clear_tag(&tags->breserved_tags, tag);
 365 }
 366
 367 void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, unsigned int tag,
 368                     unsigned int *last_tag)
 369 {
 370         struct blk_mq_tags *tags = hctx->tags;
 371
 372         if (tag >= tags->nr_reserved_tags) {
 373                 const int real_tag = tag - tags->nr_reserved_tags;
 374
 375                 __blk_mq_put_tag(tags, real_tag);
 376                 *last_tag = real_tag;
 377         } else
 378                 __blk_mq_put_reserved_tag(tags, tag);
 379 }
 380
 381 static void bt_for_each_free(struct blk_mq_bitmap_tags *bt,
 382                              unsigned long *free_map, unsigned int off)
 383 {
 384         int i;
 385
 386         for (i = 0; i < bt->map_nr; i++) {
 387                 struct blk_align_bitmap *bm = &bt->map[i];
 388                 int bit = 0;
 389
 390                 do {
 391                         bit = find_next_zero_bit(&bm->word, bm->depth, bit);
 392                         if (bit >= bm->depth)
 393                                 break;
 394
 395                         __set_bit(bit + off, free_map);
 396                         bit++;
 397                 } while (1);
 398
 399                 off += (1 << bt->bits_per_word);
 400         }
 401 }
 402
 403 void blk_mq_tag_busy_iter(struct blk_mq_tags *tags,
 404                           void (*fn)(void *, unsigned long *), void *data)
 405 {
 406         unsigned long *tag_map;
 407         size_t map_size;
 408
 409         map_size = ALIGN(tags->nr_tags, BITS_PER_LONG) / BITS_PER_LONG;
 410         tag_map = kzalloc(map_size * sizeof(unsigned long), GFP_ATOMIC);
 411         if (!tag_map)
 412                 return;
 413
 414         bt_for_each_free(&tags->bitmap_tags, tag_map, tags->nr_reserved_tags);
 415         if (tags->nr_reserved_tags)
 416                 bt_for_each_free(&tags->breserved_tags, tag_map, 0);
 417
 418         fn(data, tag_map);
 419         kfree(tag_map);
 420 }
 421 EXPORT_SYMBOL(blk_mq_tag_busy_iter);
 422
 423 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
 424 {
 425         unsigned int i, used;
 426
 427         for (i = 0, used = 0; i < bt->map_nr; i++) {
 428                 struct blk_align_bitmap *bm = &bt->map[i];
 429
 430                 used += bitmap_weight(&bm->word, bm->depth);
 431         }
 432
 433         return bt->depth - used;
 434 }
 435
 436 static void bt_update_count(struct blk_mq_bitmap_tags *bt,
 437                             unsigned int depth)
 438 {
 439         unsigned int tags_per_word = 1U << bt->bits_per_word;
 440         unsigned int map_depth = depth;
 441
 442         if (depth) {
 443                 int i;
 444
 445                 for (i = 0; i < bt->map_nr; i++) {
 446                         bt->map[i].depth = min(map_depth, tags_per_word);
 447                         map_depth -= bt->map[i].depth;
 448                 }
 449         }
 450
 451         bt->wake_cnt = BT_WAIT_BATCH;
 452         if (bt->wake_cnt > depth / 4)
 453                 bt->wake_cnt = max(1U, depth / 4);
 454
 455         bt->depth = depth;
 456 }
 457
 458 static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
 459                         int node, bool reserved)
 460 {
 461         int i;
 462
 463         bt->bits_per_word = ilog2(BITS_PER_LONG);
 464
 465         /*
 466          * Depth can be zero for reserved tags, that's not a failure
 467          * condition.
 468          */
 469         if (depth) {
 470                 unsigned int nr, tags_per_word;
 471
 472                 tags_per_word = (1 << bt->bits_per_word);
 473
 474                 /*
 475                  * If the tag space is small, shrink the number of tags
 476                  * per word so we spread over a few cachelines, at least.
 477                  * If less than 4 tags, just forget about it, it's not
 478                  * going to work optimally anyway.
 479                  */
 480                 if (depth >= 4) {
 481                         while (tags_per_word * 4 > depth) {
 482                                 bt->bits_per_word--;
 483                                 tags_per_word = (1 << bt->bits_per_word);
 484                         }
 485                 }
 486
 487                 nr = ALIGN(depth, tags_per_word) / tags_per_word;
 488                 bt->map = kzalloc_node(nr * sizeof(struct blk_align_bitmap),
 489                                                 GFP_KERNEL, node);
 490                 if (!bt->map)
 491                         return -ENOMEM;
 492
 493                 bt->map_nr = nr;
 494         }
 495
 496         bt->bs = kzalloc(BT_WAIT_QUEUES * sizeof(*bt->bs), GFP_KERNEL);
 497         if (!bt->bs) {
 498                 kfree(bt->map);
 499                 return -ENOMEM;
 500         }
 501
 502         for (i = 0; i < BT_WAIT_QUEUES; i++)
 503                 init_waitqueue_head(&bt->bs[i].wait);
 504
 505         bt_update_count(bt, depth);
 506         return 0;
 507 }
 508
 509 static void bt_free(struct blk_mq_bitmap_tags *bt)
 510 {
 511         kfree(bt->map);
 512         kfree(bt->bs);
 513 }
 514
 515 static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
 516                                                    int node)
 517 {
 518         unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
 519
 520         if (bt_alloc(&tags->bitmap_tags, depth, node, false))
 521                 goto enomem;
 522         if (bt_alloc(&tags->breserved_tags, tags->nr_reserved_tags, node, true))
 523                 goto enomem;
 524
 525         return tags;
 526 enomem:
 527         bt_free(&tags->bitmap_tags);
 528         kfree(tags);
 529         return NULL;
 530 }
 531
 532 struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
 533                                      unsigned int reserved_tags, int node)
 534 {
 535         struct blk_mq_tags *tags;
 536
 537         if (total_tags > BLK_MQ_TAG_MAX) {
 538                 pr_err("blk-mq: tag depth too large\n");
 539                 return NULL;
 540         }
 541
 542         tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
 543         if (!tags)
 544                 return NULL;
 545
 546         tags->nr_tags = total_tags;
 547         tags->nr_reserved_tags = reserved_tags;
 548
 549         return blk_mq_init_bitmap_tags(tags, node);
 550 }
 551
 552 void blk_mq_free_tags(struct blk_mq_tags *tags)
 553 {
 554         bt_free(&tags->bitmap_tags);
 555         bt_free(&tags->breserved_tags);
 556         kfree(tags);
 557 }
 558
 559 void blk_mq_tag_init_last_tag(struct blk_mq_tags *tags, unsigned int *tag)
 560 {
 561         unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
 562
 563         *tag = prandom_u32() % depth;
 564 }
 565
 566 int blk_mq_tag_update_depth(struct blk_mq_tags *tags, unsigned int tdepth)
 567 {
 568         tdepth -= tags->nr_reserved_tags;
 569         if (tdepth > tags->nr_tags)
 570                 return -EINVAL;
 571
 572         /*
 573          * Don't need (or can't) update reserved tags here, they remain
 574          * static and should never need resizing.
 575          */
 576         bt_update_count(&tags->bitmap_tags, tdepth);
 577         blk_mq_tag_wakeup_all(tags);
 578         return 0;
 579 }
 580
 581 ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)
 582 {
 583         char *orig_page = page;
 584         unsigned int free, res;
 585
 586         if (!tags)
 587                 return 0;
 588
 589         page += sprintf(page, "nr_tags=%u, reserved_tags=%u, "
 590                         "bits_per_word=%u\n",
 591                         tags->nr_tags, tags->nr_reserved_tags,
 592                         tags->bitmap_tags.bits_per_word);
 593
 594         free = bt_unused_tags(&tags->bitmap_tags);
 595         res = bt_unused_tags(&tags->breserved_tags);
 596
 597         page += sprintf(page, "nr_free=%u, nr_reserved=%u\n", free, res);
 598         page += sprintf(page, "active_queues=%u\n", atomic_read(&tags->active_queues));
 599
 600         return page - orig_page;
 601 }