[PATCHv2] blk-mq: reinsert cached request to the list

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

A previous commit removed an optimization out of caution for a scenario
that turns out not to be real: all the "queue_exit" goto's are safe to
reinsert the request into the cached_rq's plug list as they are either
from a non-blocking path, or a successful merge that already holds the
queue reference. This optimization is most needed for small sequential
workloads that successfully merge into larger requests.

Fixes: dc278e9bf2b9 ("blk-mq: pop cached request if it is usable")
Suggested-by: Ming Lei <tom.leiming@gmail.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>
---
v1->v2:

  Actually use the correct rq_list function to return the rq to the
  list.

 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 28c2d931e75ea..a24175441380e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3246,7 +3246,7 @@ void blk_mq_submit_bio(struct bio *bio)
 	if (!rq)
 		blk_queue_exit(q);
 	else
-		blk_mq_free_request(rq);
+		rq_list_add_head(&plug->cached_rqs, rq);
 }
 
 #ifdef CONFIG_BLK_MQ_STACKING
-- 
2.53.0-Meta