[PATCH 4.18 228/235] blk-mq: only attempt to merge bio if there is rq in sw queue

[Greg Kroah-Hartman]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ming Lei <ming.lei@xxxxxxxxxx>

[ Upstream commit b04f50ab8a74129b3041a2836c33c916be3c6667 ]

Only attempt to merge bio iff the ctx->rq_list isn't empty, because:

1) for high-performance SSD, most of times dispatch may succeed, then
there may be nothing left in ctx->rq_list, so don't try to merge over
sw queue if it is empty, then we can save one acquiring of ctx->lock

2) we can't expect good merge performance on per-cpu sw queue, and missing
one merge on sw queue won't be a big deal since tasks can be scheduled from
one CPU to another.

Cc: Laurence Oberman <loberman@xxxxxxxxxx>
Cc: Omar Sandoval <osandov@xxxxxx>
Cc: Bart Van Assche <bart.vanassche@xxxxxxx>
Tested-by: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx>
Reported-by: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 block/blk-mq-sched.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -339,7 +339,8 @@ bool __blk_mq_sched_bio_merge(struct req
 		return e->type->ops.mq.bio_merge(hctx, bio);
 	}
 
-	if (hctx->flags & BLK_MQ_F_SHOULD_MERGE) {
+	if ((hctx->flags & BLK_MQ_F_SHOULD_MERGE) &&
+			!list_empty_careful(&ctx->rq_list)) {
 		/* default per sw-queue merge */
 		spin_lock(&ctx->lock);
 		ret = blk_mq_attempt_merge(q, ctx, bio);