linux-kernel - [PATCH V2 2/5] dm-mpath: return DM_MAPIO_REQUEUE in case of rq allocation failure

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Date:   Mon, 27 Nov 2017 13:07:18 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com
Cc:     Christoph Hellwig <hch@...radead.org>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        linux-kernel@...r.kernel.org, Hannes Reinecke <hare@...e.com>,
        Omar Sandoval <osandov@...com>, Ming Lei <ming.lei@...hat.com>
Subject: [PATCH V2 2/5] dm-mpath: return DM_MAPIO_REQUEUE in case of rq allocation failure

blk-mq will rerun queue via RESTART or dispatch wake after one request
is completed, so not necessary to wait random time for requeuing, we
should trust blk-mq to do it.

More importantly, we need return BLK_STS_RESOURCE to blk-mq so that
dequeue from I/O scheduler can be stopped, then I/O merge gets improved.

Signed-off-by: Ming Lei <ming.lei@...hat.com>
---
 drivers/md/dm-mpath.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index 8fe3f45407ce..1fa1ff5bfe73 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -500,8 +500,20 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
 		if (queue_dying) {
 			atomic_inc(&m->pg_init_in_progress);
 			activate_or_offline_path(pgpath);
+			return DM_MAPIO_DELAY_REQUEUE;
 		}
-		return DM_MAPIO_DELAY_REQUEUE;
+
+		/*
+		 * blk-mq's SCHED_RESTART can cover this requeue, so
+		 * we needn't to deal with it by DELAY_REQUEUE. More
+		 * importantly, we have to return DM_MAPIO_REQUEUE
+		 * so that blk-mq can get the queue busy feedback,
+		 * otherwise I/O merge can be hurt.
+		 */
+		if (q->mq_ops)
+			return DM_MAPIO_REQUEUE;
+		else
+			return DM_MAPIO_DELAY_REQUEUE;
 	}
 	clone->bio = clone->biotail = NULL;
 	clone->rq_disk = bdev->bd_disk;
-- 
2.9.5