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
))
251 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
;
288 finish_wait(&bs
->wait
, &wait
);
289 bs
= bt_wait_ptr(bt
, hctx
);
292 finish_wait(&bs
->wait
, &wait
);
296 static unsigned int __blk_mq_get_tag(struct blk_mq_alloc_data
*data
)
300 tag
= bt_get(data
, &data
->hctx
->tags
->bitmap_tags
, data
->hctx
,
301 &data
->ctx
->last_tag
);
303 return tag
+ data
->hctx
->tags
->nr_reserved_tags
;
305 return BLK_MQ_TAG_FAIL
;
308 static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_alloc_data
*data
)
312 if (unlikely(!data
->hctx
->tags
->nr_reserved_tags
)) {
314 return BLK_MQ_TAG_FAIL
;
317 tag
= bt_get(data
, &data
->hctx
->tags
->breserved_tags
, NULL
, &zero
);
319 return BLK_MQ_TAG_FAIL
;
324 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data
*data
)
327 return __blk_mq_get_tag(data
);
329 return __blk_mq_get_reserved_tag(data
);
332 static struct bt_wait_state
*bt_wake_ptr(struct blk_mq_bitmap_tags
*bt
)
336 wake_index
= atomic_read(&bt
->wake_index
);
337 for (i
= 0; i
< BT_WAIT_QUEUES
; i
++) {
338 struct bt_wait_state
*bs
= &bt
->bs
[wake_index
];
340 if (waitqueue_active(&bs
->wait
)) {
341 int o
= atomic_read(&bt
->wake_index
);
343 atomic_cmpxchg(&bt
->wake_index
, o
, wake_index
);
348 wake_index
= bt_index_inc(wake_index
);
354 static void bt_clear_tag(struct blk_mq_bitmap_tags
*bt
, unsigned int tag
)
356 const int index
= TAG_TO_INDEX(bt
, tag
);
357 struct bt_wait_state
*bs
;
360 clear_bit(TAG_TO_BIT(bt
, tag
), &bt
->map
[index
].word
);
362 /* Ensure that the wait list checks occur after clear_bit(). */
365 bs
= bt_wake_ptr(bt
);
369 wait_cnt
= atomic_dec_return(&bs
->wait_cnt
);
370 if (unlikely(wait_cnt
< 0))
371 wait_cnt
= atomic_inc_return(&bs
->wait_cnt
);
373 atomic_add(bt
->wake_cnt
, &bs
->wait_cnt
);
374 bt_index_atomic_inc(&bt
->wake_index
);
379 void blk_mq_put_tag(struct blk_mq_hw_ctx
*hctx
, unsigned int tag
,
380 unsigned int *last_tag
)
382 struct blk_mq_tags
*tags
= hctx
->tags
;
384 if (tag
>= tags
->nr_reserved_tags
) {
385 const int real_tag
= tag
- tags
->nr_reserved_tags
;
387 BUG_ON(real_tag
>= tags
->nr_tags
);
388 bt_clear_tag(&tags
->bitmap_tags
, real_tag
);
389 *last_tag
= real_tag
;
391 BUG_ON(tag
>= tags
->nr_reserved_tags
);
392 bt_clear_tag(&tags
->breserved_tags
, tag
);
396 static void bt_for_each(struct blk_mq_hw_ctx
*hctx
,
397 struct blk_mq_bitmap_tags
*bt
, unsigned int off
,
398 busy_iter_fn
*fn
, void *data
, bool reserved
)
403 for (i
= 0; i
< bt
->map_nr
; i
++) {
404 struct blk_align_bitmap
*bm
= &bt
->map
[i
];
406 for (bit
= find_first_bit(&bm
->word
, bm
->depth
);
408 bit
= find_next_bit(&bm
->word
, bm
->depth
, bit
+ 1)) {
409 rq
= blk_mq_tag_to_rq(hctx
->tags
, off
+ bit
);
410 if (rq
->q
== hctx
->queue
)
411 fn(hctx
, rq
, data
, reserved
);
414 off
+= (1 << bt
->bits_per_word
);
418 void blk_mq_tag_busy_iter(struct blk_mq_hw_ctx
*hctx
, busy_iter_fn
*fn
,
421 struct blk_mq_tags
*tags
= hctx
->tags
;
423 if (tags
->nr_reserved_tags
)
424 bt_for_each(hctx
, &tags
->breserved_tags
, 0, fn
, priv
, true);
425 bt_for_each(hctx
, &tags
->bitmap_tags
, tags
->nr_reserved_tags
, fn
, priv
,
428 EXPORT_SYMBOL(blk_mq_tag_busy_iter
);
430 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags
*bt
)
432 unsigned int i
, used
;
434 for (i
= 0, used
= 0; i
< bt
->map_nr
; i
++) {
435 struct blk_align_bitmap
*bm
= &bt
->map
[i
];
437 used
+= bitmap_weight(&bm
->word
, bm
->depth
);
440 return bt
->depth
- used
;
443 static void bt_update_count(struct blk_mq_bitmap_tags
*bt
,
446 unsigned int tags_per_word
= 1U << bt
->bits_per_word
;
447 unsigned int map_depth
= depth
;
452 for (i
= 0; i
< bt
->map_nr
; i
++) {
453 bt
->map
[i
].depth
= min(map_depth
, tags_per_word
);
454 map_depth
-= bt
->map
[i
].depth
;
458 bt
->wake_cnt
= BT_WAIT_BATCH
;
459 if (bt
->wake_cnt
> depth
/ BT_WAIT_QUEUES
)
460 bt
->wake_cnt
= max(1U, depth
/ BT_WAIT_QUEUES
);
465 static int bt_alloc(struct blk_mq_bitmap_tags
*bt
, unsigned int depth
,
466 int node
, bool reserved
)
470 bt
->bits_per_word
= ilog2(BITS_PER_LONG
);
473 * Depth can be zero for reserved tags, that's not a failure
477 unsigned int nr
, tags_per_word
;
479 tags_per_word
= (1 << bt
->bits_per_word
);
482 * If the tag space is small, shrink the number of tags
483 * per word so we spread over a few cachelines, at least.
484 * If less than 4 tags, just forget about it, it's not
485 * going to work optimally anyway.
488 while (tags_per_word
* 4 > depth
) {
490 tags_per_word
= (1 << bt
->bits_per_word
);
494 nr
= ALIGN(depth
, tags_per_word
) / tags_per_word
;
495 bt
->map
= kzalloc_node(nr
* sizeof(struct blk_align_bitmap
),
503 bt
->bs
= kzalloc(BT_WAIT_QUEUES
* sizeof(*bt
->bs
), GFP_KERNEL
);
509 bt_update_count(bt
, depth
);
511 for (i
= 0; i
< BT_WAIT_QUEUES
; i
++) {
512 init_waitqueue_head(&bt
->bs
[i
].wait
);
513 atomic_set(&bt
->bs
[i
].wait_cnt
, bt
->wake_cnt
);
519 static void bt_free(struct blk_mq_bitmap_tags
*bt
)
525 static struct blk_mq_tags
*blk_mq_init_bitmap_tags(struct blk_mq_tags
*tags
,
528 unsigned int depth
= tags
->nr_tags
- tags
->nr_reserved_tags
;
530 if (bt_alloc(&tags
->bitmap_tags
, depth
, node
, false))
532 if (bt_alloc(&tags
->breserved_tags
, tags
->nr_reserved_tags
, node
, true))
537 bt_free(&tags
->bitmap_tags
);
542 struct blk_mq_tags
*blk_mq_init_tags(unsigned int total_tags
,
543 unsigned int reserved_tags
, int node
)
545 struct blk_mq_tags
*tags
;
547 if (total_tags
> BLK_MQ_TAG_MAX
) {
548 pr_err("blk-mq: tag depth too large\n");
552 tags
= kzalloc_node(sizeof(*tags
), GFP_KERNEL
, node
);
556 tags
->nr_tags
= total_tags
;
557 tags
->nr_reserved_tags
= reserved_tags
;
559 return blk_mq_init_bitmap_tags(tags
, node
);
562 void blk_mq_free_tags(struct blk_mq_tags
*tags
)
564 bt_free(&tags
->bitmap_tags
);
565 bt_free(&tags
->breserved_tags
);
569 void blk_mq_tag_init_last_tag(struct blk_mq_tags
*tags
, unsigned int *tag
)
571 unsigned int depth
= tags
->nr_tags
- tags
->nr_reserved_tags
;
573 *tag
= prandom_u32() % depth
;
576 int blk_mq_tag_update_depth(struct blk_mq_tags
*tags
, unsigned int tdepth
)
578 tdepth
-= tags
->nr_reserved_tags
;
579 if (tdepth
> tags
->nr_tags
)
583 * Don't need (or can't) update reserved tags here, they remain
584 * static and should never need resizing.
586 bt_update_count(&tags
->bitmap_tags
, tdepth
);
587 blk_mq_tag_wakeup_all(tags
);
592 * blk_mq_unique_tag() - return a tag that is unique queue-wide
593 * @rq: request for which to compute a unique tag
595 * The tag field in struct request is unique per hardware queue but not over
596 * all hardware queues. Hence this function that returns a tag with the
597 * hardware context index in the upper bits and the per hardware queue tag in
600 * Note: When called for a request that is queued on a non-multiqueue request
601 * queue, the hardware context index is set to zero.
603 u32
blk_mq_unique_tag(struct request
*rq
)
605 struct request_queue
*q
= rq
->q
;
606 struct blk_mq_hw_ctx
*hctx
;
610 hctx
= q
->mq_ops
->map_queue(q
, rq
->mq_ctx
->cpu
);
611 hwq
= hctx
->queue_num
;
614 return (hwq
<< BLK_MQ_UNIQUE_TAG_BITS
) |
615 (rq
->tag
& BLK_MQ_UNIQUE_TAG_MASK
);
617 EXPORT_SYMBOL(blk_mq_unique_tag
);
619 ssize_t
blk_mq_tag_sysfs_show(struct blk_mq_tags
*tags
, char *page
)
621 char *orig_page
= page
;
622 unsigned int free
, res
;
627 page
+= sprintf(page
, "nr_tags=%u, reserved_tags=%u, "
628 "bits_per_word=%u\n",
629 tags
->nr_tags
, tags
->nr_reserved_tags
,
630 tags
->bitmap_tags
.bits_per_word
);
632 free
= bt_unused_tags(&tags
->bitmap_tags
);
633 res
= bt_unused_tags(&tags
->breserved_tags
);
635 page
+= sprintf(page
, "nr_free=%u, nr_reserved=%u\n", free
, res
);
636 page
+= sprintf(page
, "active_queues=%u\n", atomic_read(&tags
->active_queues
));
638 return page
- orig_page
;
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