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
8 * Uses active queue tracking to support fairer distribution of tags
9 * between multiple submitters when a shared tag map is used.
11 * Copyright (C) 2013-2014 Jens Axboe
13 #include <linux/kernel.h>
14 #include <linux/module.h>
15 #include <linux/random.h>
17 #include <linux/blk-mq.h>
20 #include "blk-mq-tag.h"
22 static bool bt_has_free_tags(struct blk_mq_bitmap_tags *bt)
26 for (i = 0; i < bt->map_nr; i++) {
27 struct blk_align_bitmap *bm = &bt->map[i];
30 ret = find_first_zero_bit(&bm->word, bm->depth);
38 bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
43 return bt_has_free_tags(&tags->bitmap_tags);
46 static inline int bt_index_inc(int index)
48 return (index + 1) & (BT_WAIT_QUEUES - 1);
51 static inline void bt_index_atomic_inc(atomic_t *index)
53 int old = atomic_read(index);
54 int new = bt_index_inc(old);
55 atomic_cmpxchg(index, old, new);
59 * If a previously inactive queue goes active, bump the active user count.
61 bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
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);
71 * Wakeup all potentially sleeping on normal (non-reserved) tags
73 static void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags)
75 struct blk_mq_bitmap_tags *bt;
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];
83 if (waitqueue_active(&bs->wait))
86 wake_index = bt_index_inc(wake_index);
91 * If a previously busy queue goes inactive, potential waiters could now
92 * be allowed to queue. Wake them up and check.
94 void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
96 struct blk_mq_tags *tags = hctx->tags;
98 if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
101 atomic_dec(&tags->active_queues);
103 blk_mq_tag_wakeup_all(tags);
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.
110 static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
111 struct blk_mq_bitmap_tags *bt)
113 unsigned int depth, users;
115 if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_SHARED))
117 if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
121 * Don't try dividing an ant
126 users = atomic_read(&hctx->tags->active_queues);
131 * Allow at least some tags
133 depth = max((bt->depth + users - 1) / users, 4U);
134 return atomic_read(&hctx->nr_active) < depth;
137 static int __bt_get_word(struct blk_align_bitmap *bm, unsigned int last_tag)
139 int tag, org_last_tag, end;
140 bool wrap = last_tag != 0;
142 org_last_tag = last_tag;
146 tag = find_next_zero_bit(&bm->word, end, last_tag);
147 if (unlikely(tag >= end)) {
149 * We started with an offset, start from 0 to
161 } while (test_and_set_bit(tag, &bm->word));
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.
177 static int __bt_get(struct blk_mq_hw_ctx *hctx, struct blk_mq_bitmap_tags *bt,
178 unsigned int *tag_cache)
180 unsigned int last_tag, org_last_tag;
183 if (!hctx_may_queue(hctx, bt))
186 last_tag = org_last_tag = *tag_cache;
187 index = TAG_TO_INDEX(bt, last_tag);
189 for (i = 0; i < bt->map_nr; i++) {
190 tag = __bt_get_word(&bt->map[index], TAG_TO_BIT(bt, last_tag));
192 tag += (index << bt->bits_per_word);
197 if (++index >= bt->map_nr)
205 * Only update the cache from the allocation path, if we ended
206 * up using the specific cached tag.
209 if (tag == org_last_tag) {
211 if (last_tag >= bt->depth - 1)
214 *tag_cache = last_tag;
220 static struct bt_wait_state *bt_wait_ptr(struct blk_mq_bitmap_tags *bt,
221 struct blk_mq_hw_ctx *hctx)
223 struct bt_wait_state *bs;
229 wait_index = atomic_read(&hctx->wait_index);
230 bs = &bt->bs[wait_index];
231 bt_index_atomic_inc(&hctx->wait_index);
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)
240 struct bt_wait_state *bs;
244 tag = __bt_get(hctx, bt, last_tag);
248 if (!(data->gfp & __GFP_WAIT))
252 bs = bt_wait_ptr(bt, hctx);
253 prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
255 tag = __bt_get(hctx, bt, last_tag);
260 * We're out of tags on this hardware queue, kick any
261 * pending IO submits before going to sleep waiting for
264 blk_mq_run_hw_queue(hctx, false);
267 * Retry tag allocation after running the hardware queue,
268 * as running the queue may also have found completions.
270 tag = __bt_get(hctx, bt, last_tag);
274 blk_mq_put_ctx(data->ctx);
278 data->ctx = blk_mq_get_ctx(data->q);
279 data->hctx = data->q->mq_ops->map_queue(data->q,
281 if (data->reserved) {
282 bt = &data->hctx->tags->breserved_tags;
284 last_tag = &data->ctx->last_tag;
286 bt = &hctx->tags->bitmap_tags;
290 finish_wait(&bs->wait, &wait);
294 static unsigned int __blk_mq_get_tag(struct blk_mq_alloc_data *data)
298 tag = bt_get(data, &data->hctx->tags->bitmap_tags, data->hctx,
299 &data->ctx->last_tag);
301 return tag + data->hctx->tags->nr_reserved_tags;
303 return BLK_MQ_TAG_FAIL;
306 static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_alloc_data *data)
310 if (unlikely(!data->hctx->tags->nr_reserved_tags)) {
312 return BLK_MQ_TAG_FAIL;
315 tag = bt_get(data, &data->hctx->tags->breserved_tags, NULL, &zero);
317 return BLK_MQ_TAG_FAIL;
322 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
325 return __blk_mq_get_tag(data);
327 return __blk_mq_get_reserved_tag(data);
330 static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt)
334 wake_index = atomic_read(&bt->wake_index);
335 for (i = 0; i < BT_WAIT_QUEUES; i++) {
336 struct bt_wait_state *bs = &bt->bs[wake_index];
338 if (waitqueue_active(&bs->wait)) {
339 int o = atomic_read(&bt->wake_index);
341 atomic_cmpxchg(&bt->wake_index, o, wake_index);
346 wake_index = bt_index_inc(wake_index);
352 static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
354 const int index = TAG_TO_INDEX(bt, tag);
355 struct bt_wait_state *bs;
358 clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word);
360 /* Ensure that the wait list checks occur after clear_bit(). */
363 bs = bt_wake_ptr(bt);
367 wait_cnt = atomic_dec_return(&bs->wait_cnt);
368 if (unlikely(wait_cnt < 0))
369 wait_cnt = atomic_inc_return(&bs->wait_cnt);
371 atomic_add(bt->wake_cnt, &bs->wait_cnt);
372 bt_index_atomic_inc(&bt->wake_index);
377 void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, unsigned int tag,
378 unsigned int *last_tag)
380 struct blk_mq_tags *tags = hctx->tags;
382 if (tag >= tags->nr_reserved_tags) {
383 const int real_tag = tag - tags->nr_reserved_tags;
385 BUG_ON(real_tag >= tags->nr_tags);
386 bt_clear_tag(&tags->bitmap_tags, real_tag);
387 *last_tag = real_tag;
389 BUG_ON(tag >= tags->nr_reserved_tags);
390 bt_clear_tag(&tags->breserved_tags, tag);
394 static void bt_for_each(struct blk_mq_hw_ctx *hctx,
395 struct blk_mq_bitmap_tags *bt, unsigned int off,
396 busy_iter_fn *fn, void *data, bool reserved)
401 for (i = 0; i < bt->map_nr; i++) {
402 struct blk_align_bitmap *bm = &bt->map[i];
404 for (bit = find_first_bit(&bm->word, bm->depth);
406 bit = find_next_bit(&bm->word, bm->depth, bit + 1)) {
407 rq = blk_mq_tag_to_rq(hctx->tags, off + bit);
408 if (rq->q == hctx->queue)
409 fn(hctx, rq, data, reserved);
412 off += (1 << bt->bits_per_word);
416 void blk_mq_tag_busy_iter(struct blk_mq_hw_ctx *hctx, busy_iter_fn *fn,
419 struct blk_mq_tags *tags = hctx->tags;
421 if (tags->nr_reserved_tags)
422 bt_for_each(hctx, &tags->breserved_tags, 0, fn, priv, true);
423 bt_for_each(hctx, &tags->bitmap_tags, tags->nr_reserved_tags, fn, priv,
426 EXPORT_SYMBOL(blk_mq_tag_busy_iter);
428 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
430 unsigned int i, used;
432 for (i = 0, used = 0; i < bt->map_nr; i++) {
433 struct blk_align_bitmap *bm = &bt->map[i];
435 used += bitmap_weight(&bm->word, bm->depth);
438 return bt->depth - used;
441 static void bt_update_count(struct blk_mq_bitmap_tags *bt,
444 unsigned int tags_per_word = 1U << bt->bits_per_word;
445 unsigned int map_depth = depth;
450 for (i = 0; i < bt->map_nr; i++) {
451 bt->map[i].depth = min(map_depth, tags_per_word);
452 map_depth -= bt->map[i].depth;
456 bt->wake_cnt = BT_WAIT_BATCH;
457 if (bt->wake_cnt > depth / BT_WAIT_QUEUES)
458 bt->wake_cnt = max(1U, depth / BT_WAIT_QUEUES);
463 static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
464 int node, bool reserved)
468 bt->bits_per_word = ilog2(BITS_PER_LONG);
471 * Depth can be zero for reserved tags, that's not a failure
475 unsigned int nr, tags_per_word;
477 tags_per_word = (1 << bt->bits_per_word);
480 * If the tag space is small, shrink the number of tags
481 * per word so we spread over a few cachelines, at least.
482 * If less than 4 tags, just forget about it, it's not
483 * going to work optimally anyway.
486 while (tags_per_word * 4 > depth) {
488 tags_per_word = (1 << bt->bits_per_word);
492 nr = ALIGN(depth, tags_per_word) / tags_per_word;
493 bt->map = kzalloc_node(nr * sizeof(struct blk_align_bitmap),
501 bt->bs = kzalloc(BT_WAIT_QUEUES * sizeof(*bt->bs), GFP_KERNEL);
507 bt_update_count(bt, depth);
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);
517 static void bt_free(struct blk_mq_bitmap_tags *bt)
523 static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
526 unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
528 if (bt_alloc(&tags->bitmap_tags, depth, node, false))
530 if (bt_alloc(&tags->breserved_tags, tags->nr_reserved_tags, node, true))
535 bt_free(&tags->bitmap_tags);
540 struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
541 unsigned int reserved_tags, int node)
543 struct blk_mq_tags *tags;
545 if (total_tags > BLK_MQ_TAG_MAX) {
546 pr_err("blk-mq: tag depth too large\n");
550 tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
554 tags->nr_tags = total_tags;
555 tags->nr_reserved_tags = reserved_tags;
557 return blk_mq_init_bitmap_tags(tags, node);
560 void blk_mq_free_tags(struct blk_mq_tags *tags)
562 bt_free(&tags->bitmap_tags);
563 bt_free(&tags->breserved_tags);
567 void blk_mq_tag_init_last_tag(struct blk_mq_tags *tags, unsigned int *tag)
569 unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
571 *tag = prandom_u32() % depth;
574 int blk_mq_tag_update_depth(struct blk_mq_tags *tags, unsigned int tdepth)
576 tdepth -= tags->nr_reserved_tags;
577 if (tdepth > tags->nr_tags)
581 * Don't need (or can't) update reserved tags here, they remain
582 * static and should never need resizing.
584 bt_update_count(&tags->bitmap_tags, tdepth);
585 blk_mq_tag_wakeup_all(tags);
590 * blk_mq_unique_tag() - return a tag that is unique queue-wide
591 * @rq: request for which to compute a unique tag
593 * The tag field in struct request is unique per hardware queue but not over
594 * all hardware queues. Hence this function that returns a tag with the
595 * hardware context index in the upper bits and the per hardware queue tag in
598 * Note: When called for a request that is queued on a non-multiqueue request
599 * queue, the hardware context index is set to zero.
601 u32 blk_mq_unique_tag(struct request *rq)
603 struct request_queue *q = rq->q;
604 struct blk_mq_hw_ctx *hctx;
608 hctx = q->mq_ops->map_queue(q, rq->mq_ctx->cpu);
609 hwq = hctx->queue_num;
612 return (hwq << BLK_MQ_UNIQUE_TAG_BITS) |
613 (rq->tag & BLK_MQ_UNIQUE_TAG_MASK);
615 EXPORT_SYMBOL(blk_mq_unique_tag);
617 ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)
619 char *orig_page = page;
620 unsigned int free, res;
625 page += sprintf(page, "nr_tags=%u, reserved_tags=%u, "
626 "bits_per_word=%u\n",
627 tags->nr_tags, tags->nr_reserved_tags,
628 tags->bitmap_tags.bits_per_word);
630 free = bt_unused_tags(&tags->bitmap_tags);
631 res = bt_unused_tags(&tags->breserved_tags);
633 page += sprintf(page, "nr_free=%u, nr_reserved=%u\n", free, res);
634 page += sprintf(page, "active_queues=%u\n", atomic_read(&tags->active_queues));
636 return page - orig_page;