linux-kernel - [PATCH -next] blk-mq: Fix wrong wakeup batch configuration which will cause hang

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Date:   Thu, 27 Jan 2022 18:00:47 +0800
From:   Laibin Qiu <qiulaibin@...wei.com>
To:     <axboe@...nel.dk>, <andriy.shevchenko@...ux.intel.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <alex_y_xu@...oo.ca>
Subject: [PATCH -next] blk-mq: Fix wrong wakeup batch configuration which will cause hang

Commit 180dccb0dba4f ("blk-mq: fix tag_get wait task can't be
awakened") will recalculating wake_batch when inc or dec active_queues
to avoid wake_batch is > hctx_max_depth. At the same time, in order to
not affect performance as much as possible, the minimum wakeup batch is
set to 4. But when the QD is small (such as QD=1), if inc or dec
active_queues will increase wakeup batch, which will lead to hang.

Fix this problem with the following strategies:
QD          :  >= 32 | < 32
---------------------------------
wakeup batch:  8~4   | 3~1

Fixes: 180dccb0dba4f ("blk-mq: fix tag_get wait task can't be awakened")
Link: https://lore.kernel.org/linux-block/78cafe94-a787-e006-8851-69906f0c2128@huawei.com/T/#t
Reported-by: Alex Xu (Hello71) <alex_y_xu@...oo.ca>
Signed-off-by: Laibin Qiu <qiulaibin@...wei.com>
---
 lib/sbitmap.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 6220fa67fb7e..09d293c30fd2 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -488,9 +488,13 @@ void sbitmap_queue_recalculate_wake_batch(struct sbitmap_queue *sbq,
 					    unsigned int users)
 {
 	unsigned int wake_batch;
+	unsigned int min_batch;
+	unsigned int depth = (sbq->sb.depth + users - 1) / users;
 
-	wake_batch = clamp_val((sbq->sb.depth + users - 1) /
-			users, 4, SBQ_WAKE_BATCH);
+	min_batch = sbq->sb.depth >= (4 * SBQ_WAIT_QUEUES) ? 4 : 1;
+
+	wake_batch = clamp_val(depth / SBQ_WAIT_QUEUES,
+			min_batch, SBQ_WAKE_BATCH);
 	__sbitmap_queue_update_wake_batch(sbq, wake_batch);
 }
 EXPORT_SYMBOL_GPL(sbitmap_queue_recalculate_wake_batch);
-- 
2.22.0