blk-mq: only attempt to merge bio if there is rq in sw queue

BugLink: https://bugs.launchpad.net/bugs/1836117
[ 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@redhat.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Tested-by: Kashyap Desai <kashyap.desai@broadcom.com>
Reported-by: Kashyap Desai <kashyap.desai@broadcom.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index c117bd8fd1f6126849472300b3c294e65825a2f7..fc64558241c90eb636866b744421514f4d5af8f7 100644 (file)
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -324,7 +324,8 @@ bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio)
                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);