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

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>

1 files changed, 2 insertions, 1 deletions

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 4e027f6108ae..f3b4b5ceb4d1 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -326,7 +326,8 @@ 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);