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 void bt_index_inc(unsigned int *index
)
48 *index
= (*index
+ 1) & (BT_WAIT_QUEUES
- 1);
52 * If a previously inactive queue goes active, bump the active user count.
54 bool __blk_mq_tag_busy(struct blk_mq_hw_ctx
*hctx
)
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
);
64 * Wakeup all potentially sleeping on normal (non-reserved) tags
66 static void blk_mq_tag_wakeup_all(struct blk_mq_tags
*tags
)
68 struct blk_mq_bitmap_tags
*bt
;
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
];
76 if (waitqueue_active(&bs
->wait
))
79 bt_index_inc(&wake_index
);
84 * If a previously busy queue goes inactive, potential waiters could now
85 * be allowed to queue. Wake them up and check.
87 void __blk_mq_tag_idle(struct blk_mq_hw_ctx
*hctx
)
89 struct blk_mq_tags
*tags
= hctx
->tags
;
91 if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE
, &hctx
->state
))
94 atomic_dec(&tags
->active_queues
);
96 blk_mq_tag_wakeup_all(tags
);
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.
103 static inline bool hctx_may_queue(struct blk_mq_hw_ctx
*hctx
,
104 struct blk_mq_bitmap_tags
*bt
)
106 unsigned int depth
, users
;
108 if (!hctx
|| !(hctx
->flags
& BLK_MQ_F_TAG_SHARED
))
110 if (!test_bit(BLK_MQ_S_TAG_ACTIVE
, &hctx
->state
))
114 * Don't try dividing an ant
119 users
= atomic_read(&hctx
->tags
->active_queues
);
124 * Allow at least some tags
126 depth
= max((bt
->depth
+ users
- 1) / users
, 4U);
127 return atomic_read(&hctx
->nr_active
) < depth
;
130 static int __bt_get_word(struct blk_align_bitmap
*bm
, unsigned int last_tag
)
132 int tag
, org_last_tag
, end
;
134 org_last_tag
= last_tag
;
138 tag
= find_next_zero_bit(&bm
->word
, end
, last_tag
);
139 if (unlikely(tag
>= end
)) {
141 * We started with an offset, start from 0 to
144 if (org_last_tag
&& last_tag
) {
152 } while (test_and_set_bit_lock(tag
, &bm
->word
));
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.
168 static int __bt_get(struct blk_mq_hw_ctx
*hctx
, struct blk_mq_bitmap_tags
*bt
,
169 unsigned int *tag_cache
)
171 unsigned int last_tag
, org_last_tag
;
174 if (!hctx_may_queue(hctx
, bt
))
177 last_tag
= org_last_tag
= *tag_cache
;
178 index
= TAG_TO_INDEX(bt
, last_tag
);
180 for (i
= 0; i
< bt
->map_nr
; i
++) {
181 tag
= __bt_get_word(&bt
->map
[index
], TAG_TO_BIT(bt
, last_tag
));
183 tag
+= (index
<< bt
->bits_per_word
);
188 if (++index
>= bt
->map_nr
)
196 * Only update the cache from the allocation path, if we ended
197 * up using the specific cached tag.
200 if (tag
== org_last_tag
) {
202 if (last_tag
>= bt
->depth
- 1)
205 *tag_cache
= last_tag
;
211 static struct bt_wait_state
*bt_wait_ptr(struct blk_mq_bitmap_tags
*bt
,
212 struct blk_mq_hw_ctx
*hctx
)
214 struct bt_wait_state
*bs
;
219 bs
= &bt
->bs
[hctx
->wait_index
];
220 bt_index_inc(&hctx
->wait_index
);
224 static int bt_get(struct blk_mq_bitmap_tags
*bt
, struct blk_mq_hw_ctx
*hctx
,
225 unsigned int *last_tag
, gfp_t gfp
)
227 struct bt_wait_state
*bs
;
231 tag
= __bt_get(hctx
, bt
, last_tag
);
235 if (!(gfp
& __GFP_WAIT
))
238 bs
= bt_wait_ptr(bt
, hctx
);
242 was_empty
= list_empty(&wait
.task_list
);
243 prepare_to_wait(&bs
->wait
, &wait
, TASK_UNINTERRUPTIBLE
);
245 tag
= __bt_get(hctx
, bt
, last_tag
);
250 atomic_set(&bs
->wait_cnt
, bt
->wake_cnt
);
255 finish_wait(&bs
->wait
, &wait
);
259 static unsigned int __blk_mq_get_tag(struct blk_mq_tags
*tags
,
260 struct blk_mq_hw_ctx
*hctx
,
261 unsigned int *last_tag
, gfp_t gfp
)
265 tag
= bt_get(&tags
->bitmap_tags
, hctx
, last_tag
, gfp
);
267 return tag
+ tags
->nr_reserved_tags
;
269 return BLK_MQ_TAG_FAIL
;
272 static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_tags
*tags
,
277 if (unlikely(!tags
->nr_reserved_tags
)) {
279 return BLK_MQ_TAG_FAIL
;
282 tag
= bt_get(&tags
->breserved_tags
, NULL
, &zero
, gfp
);
284 return BLK_MQ_TAG_FAIL
;
289 unsigned int blk_mq_get_tag(struct blk_mq_hw_ctx
*hctx
, unsigned int *last_tag
,
290 gfp_t gfp
, bool reserved
)
293 return __blk_mq_get_tag(hctx
->tags
, hctx
, last_tag
, gfp
);
295 return __blk_mq_get_reserved_tag(hctx
->tags
, gfp
);
298 static struct bt_wait_state
*bt_wake_ptr(struct blk_mq_bitmap_tags
*bt
)
302 wake_index
= bt
->wake_index
;
303 for (i
= 0; i
< BT_WAIT_QUEUES
; i
++) {
304 struct bt_wait_state
*bs
= &bt
->bs
[wake_index
];
306 if (waitqueue_active(&bs
->wait
)) {
307 if (wake_index
!= bt
->wake_index
)
308 bt
->wake_index
= wake_index
;
313 bt_index_inc(&wake_index
);
319 static void bt_clear_tag(struct blk_mq_bitmap_tags
*bt
, unsigned int tag
)
321 const int index
= TAG_TO_INDEX(bt
, tag
);
322 struct bt_wait_state
*bs
;
325 * The unlock memory barrier need to order access to req in free
326 * path and clearing tag bit
328 clear_bit_unlock(TAG_TO_BIT(bt
, tag
), &bt
->map
[index
].word
);
330 bs
= bt_wake_ptr(bt
);
331 if (bs
&& atomic_dec_and_test(&bs
->wait_cnt
)) {
332 atomic_set(&bs
->wait_cnt
, bt
->wake_cnt
);
333 bt_index_inc(&bt
->wake_index
);
338 static void __blk_mq_put_tag(struct blk_mq_tags
*tags
, unsigned int tag
)
340 BUG_ON(tag
>= tags
->nr_tags
);
342 bt_clear_tag(&tags
->bitmap_tags
, tag
);
345 static void __blk_mq_put_reserved_tag(struct blk_mq_tags
*tags
,
348 BUG_ON(tag
>= tags
->nr_reserved_tags
);
350 bt_clear_tag(&tags
->breserved_tags
, tag
);
353 void blk_mq_put_tag(struct blk_mq_hw_ctx
*hctx
, unsigned int tag
,
354 unsigned int *last_tag
)
356 struct blk_mq_tags
*tags
= hctx
->tags
;
358 if (tag
>= tags
->nr_reserved_tags
) {
359 const int real_tag
= tag
- tags
->nr_reserved_tags
;
361 __blk_mq_put_tag(tags
, real_tag
);
362 *last_tag
= real_tag
;
364 __blk_mq_put_reserved_tag(tags
, tag
);
367 static void bt_for_each_free(struct blk_mq_bitmap_tags
*bt
,
368 unsigned long *free_map
, unsigned int off
)
372 for (i
= 0; i
< bt
->map_nr
; i
++) {
373 struct blk_align_bitmap
*bm
= &bt
->map
[i
];
377 bit
= find_next_zero_bit(&bm
->word
, bm
->depth
, bit
);
378 if (bit
>= bm
->depth
)
381 __set_bit(bit
+ off
, free_map
);
385 off
+= (1 << bt
->bits_per_word
);
389 void blk_mq_tag_busy_iter(struct blk_mq_tags
*tags
,
390 void (*fn
)(void *, unsigned long *), void *data
)
392 unsigned long *tag_map
;
395 map_size
= ALIGN(tags
->nr_tags
, BITS_PER_LONG
) / BITS_PER_LONG
;
396 tag_map
= kzalloc(map_size
* sizeof(unsigned long), GFP_ATOMIC
);
400 bt_for_each_free(&tags
->bitmap_tags
, tag_map
, tags
->nr_reserved_tags
);
401 if (tags
->nr_reserved_tags
)
402 bt_for_each_free(&tags
->breserved_tags
, tag_map
, 0);
407 EXPORT_SYMBOL(blk_mq_tag_busy_iter
);
409 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags
*bt
)
411 unsigned int i
, used
;
413 for (i
= 0, used
= 0; i
< bt
->map_nr
; i
++) {
414 struct blk_align_bitmap
*bm
= &bt
->map
[i
];
416 used
+= bitmap_weight(&bm
->word
, bm
->depth
);
419 return bt
->depth
- used
;
422 static void bt_update_count(struct blk_mq_bitmap_tags
*bt
,
425 unsigned int tags_per_word
= 1U << bt
->bits_per_word
;
426 unsigned int map_depth
= depth
;
431 for (i
= 0; i
< bt
->map_nr
; i
++) {
432 bt
->map
[i
].depth
= min(map_depth
, tags_per_word
);
433 map_depth
-= bt
->map
[i
].depth
;
437 bt
->wake_cnt
= BT_WAIT_BATCH
;
438 if (bt
->wake_cnt
> depth
/ 4)
439 bt
->wake_cnt
= max(1U, depth
/ 4);
444 static int bt_alloc(struct blk_mq_bitmap_tags
*bt
, unsigned int depth
,
445 int node
, bool reserved
)
449 bt
->bits_per_word
= ilog2(BITS_PER_LONG
);
452 * Depth can be zero for reserved tags, that's not a failure
456 unsigned int nr
, tags_per_word
;
458 tags_per_word
= (1 << bt
->bits_per_word
);
461 * If the tag space is small, shrink the number of tags
462 * per word so we spread over a few cachelines, at least.
463 * If less than 4 tags, just forget about it, it's not
464 * going to work optimally anyway.
467 while (tags_per_word
* 4 > depth
) {
469 tags_per_word
= (1 << bt
->bits_per_word
);
473 nr
= ALIGN(depth
, tags_per_word
) / tags_per_word
;
474 bt
->map
= kzalloc_node(nr
* sizeof(struct blk_align_bitmap
),
482 bt
->bs
= kzalloc(BT_WAIT_QUEUES
* sizeof(*bt
->bs
), GFP_KERNEL
);
488 for (i
= 0; i
< BT_WAIT_QUEUES
; i
++)
489 init_waitqueue_head(&bt
->bs
[i
].wait
);
491 bt_update_count(bt
, depth
);
495 static void bt_free(struct blk_mq_bitmap_tags
*bt
)
501 static struct blk_mq_tags
*blk_mq_init_bitmap_tags(struct blk_mq_tags
*tags
,
504 unsigned int depth
= tags
->nr_tags
- tags
->nr_reserved_tags
;
506 if (bt_alloc(&tags
->bitmap_tags
, depth
, node
, false))
508 if (bt_alloc(&tags
->breserved_tags
, tags
->nr_reserved_tags
, node
, true))
513 bt_free(&tags
->bitmap_tags
);
518 struct blk_mq_tags
*blk_mq_init_tags(unsigned int total_tags
,
519 unsigned int reserved_tags
, int node
)
521 struct blk_mq_tags
*tags
;
523 if (total_tags
> BLK_MQ_TAG_MAX
) {
524 pr_err("blk-mq: tag depth too large\n");
528 tags
= kzalloc_node(sizeof(*tags
), GFP_KERNEL
, node
);
532 tags
->nr_tags
= total_tags
;
533 tags
->nr_reserved_tags
= reserved_tags
;
535 return blk_mq_init_bitmap_tags(tags
, node
);
538 void blk_mq_free_tags(struct blk_mq_tags
*tags
)
540 bt_free(&tags
->bitmap_tags
);
541 bt_free(&tags
->breserved_tags
);
545 void blk_mq_tag_init_last_tag(struct blk_mq_tags
*tags
, unsigned int *tag
)
547 unsigned int depth
= tags
->nr_tags
- tags
->nr_reserved_tags
;
549 *tag
= prandom_u32() % depth
;
552 int blk_mq_tag_update_depth(struct blk_mq_tags
*tags
, unsigned int tdepth
)
554 tdepth
-= tags
->nr_reserved_tags
;
555 if (tdepth
> tags
->nr_tags
)
559 * Don't need (or can't) update reserved tags here, they remain
560 * static and should never need resizing.
562 bt_update_count(&tags
->bitmap_tags
, tdepth
);
563 blk_mq_tag_wakeup_all(tags
);
567 ssize_t
blk_mq_tag_sysfs_show(struct blk_mq_tags
*tags
, char *page
)
569 char *orig_page
= page
;
570 unsigned int free
, res
;
575 page
+= sprintf(page
, "nr_tags=%u, reserved_tags=%u, "
576 "bits_per_word=%u\n",
577 tags
->nr_tags
, tags
->nr_reserved_tags
,
578 tags
->bitmap_tags
.bits_per_word
);
580 free
= bt_unused_tags(&tags
->bitmap_tags
);
581 res
= bt_unused_tags(&tags
->breserved_tags
);
583 page
+= sprintf(page
, "nr_free=%u, nr_reserved=%u\n", free
, res
);
584 page
+= sprintf(page
, "active_queues=%u\n", atomic_read(&tags
->active_queues
));
586 return page
- orig_page
;
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