Commit f906a6a0 authored by Jens Axboe's avatar Jens Axboe
Browse files

blk-mq: improve tag waiting setup for non-shared tags 



If we run out of driver tags, we currently treat shared and non-shared
tags the same - both cases hook into the tag waitqueue. This is a bit
more costly than it needs to be on unshared tags, since we have to both
grab the hctx lock, and the waitqueue lock (and disable interrupts).
For the non-shared case, we can simply mark the queue as needing a
restart.

Split blk_mq_dispatch_wait_add() to account for both cases, and
rename it to blk_mq_mark_tag_wait() to better reflect what it
does now.

Without this patch, shared and non-shared performance is about the same
with 4 fio thread hammering on a single null_blk device (~410K, at 75%
sys). With the patch, the shared case is the same, but the non-shared
tags case runs at 431K at 71% sys.

Reviewed-by: default avatarOmar Sandoval <osandov@fb.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 15f7b41f

block/blk-mq.c

+55 −26
Original line number Diff line number Diff line
@@ -1006,13 +1006,26 @@ static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode, 
	return 1;

}



static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx,

/*

 * Mark us waiting for a tag. For shared tags, this involves hooking us into

 * the tag wakeups. For non-shared tags, we can simply mark us nedeing a

 * restart. For both caes, take care to check the condition again after

 * marking us as waiting.

 */

static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,

				 struct request *rq)

{

	struct blk_mq_hw_ctx *this_hctx = *hctx;

	wait_queue_entry_t *wait = &this_hctx->dispatch_wait;

	bool shared_tags = (this_hctx->flags & BLK_MQ_F_TAG_SHARED) != 0;

	struct sbq_wait_state *ws;

	wait_queue_entry_t *wait;

	bool ret;



	if (!shared_tags) {

		if (!test_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state))

			set_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state);

	} else {

		wait = &this_hctx->dispatch_wait;

		if (!list_empty_careful(&wait->entry))

			return false;
@@ -1024,13 +1037,23 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx, 


		ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);

		add_wait_queue(&ws->wait, wait);

	}



	/*

	 * It's possible that a tag was freed in the window between the

	 * allocation failure and adding the hardware queue to the wait

	 * queue.

	 */

	if (!blk_mq_get_driver_tag(rq, hctx, false)) {

	ret = blk_mq_get_driver_tag(rq, hctx, false);



	if (!shared_tags) {

		/*

		 * Don't clear RESTART here, someone else could have set it.

		 * At most this will cost an extra queue run.

		 */

		return ret;

	} else {

		if (!ret) {

			spin_unlock(&this_hctx->lock);

			return false;

		}
@@ -1045,6 +1068,7 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx, 
		spin_unlock(&this_hctx->lock);

		return true;

	}

}



bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,

			     bool got_budget)
@@ -1076,9 +1100,14 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, 
			 * before we add this entry back on the dispatch list,

			 * we'll re-run it below.

			 */

			if (!blk_mq_dispatch_wait_add(&hctx, rq)) {

			if (!blk_mq_mark_tag_wait(&hctx, rq)) {

				if (got_budget)

					blk_mq_put_dispatch_budget(hctx);

				/*

				 * For non-shared tags, the RESTART check

				 * will suffice.

				 */

				if (hctx->flags & BLK_MQ_F_TAG_SHARED)

					no_tag = true;

				break;

			}

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