linux-kernel - [PATCH -next RFC v2 2/8] blk-mq: call 'bt_wait_ptr()' later in blk_mq_get

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Message-ID: <20220408073916.1428590-3-yukuai3@huawei.com>
Date:   Fri, 8 Apr 2022 15:39:10 +0800
From:   Yu Kuai <yukuai3@...wei.com>
To:     <axboe@...nel.dk>, <yukuai3@...wei.com>,
        <andriy.shevchenko@...ux.intel.com>, <john.garry@...wei.com>,
        <ming.lei@...hat.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: [PATCH -next RFC v2 2/8] blk-mq: call 'bt_wait_ptr()' later in blk_mq_get_tag()

bt_wait_ptr() will increase 'wait_index', however, if blk_mq_get_tag()
get a tag successfully after bt_wait_ptr() is called and before
sbitmap_prepare_to_wait() is called, then the 'ws' is skipped. This
behavior might cause 8 waitqueues to be unbalanced.

Move bt_wait_ptr() later should reduce the problem when the disk is
under high io preesure.

Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 block/blk-mq-tag.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 68ac23d0b640..228a0001694f 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -155,7 +155,6 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 	if (data->flags & BLK_MQ_REQ_NOWAIT)
 		return BLK_MQ_NO_TAG;
 
-	ws = bt_wait_ptr(bt, data->hctx);
 	do {
 		struct sbitmap_queue *bt_prev;
 
@@ -174,6 +173,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		if (tag != BLK_MQ_NO_TAG)
 			break;
 
+		ws = bt_wait_ptr(bt, data->hctx);
 		sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
 
 		tag = __blk_mq_get_tag(data, bt);
@@ -201,8 +201,6 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		 */
 		if (bt != bt_prev)
 			sbitmap_queue_wake_up(bt_prev);
-
-		ws = bt_wait_ptr(bt, data->hctx);
 	} while (1);
 
 	sbitmap_finish_wait(bt, ws, &wait);
-- 
2.31.1