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

Ming Lei · axboe · commit b04f50ab8a74 · 2018-07-09T09:07:53.000-06:00
Only attempt to merge bio iff the ctx-&gt;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-&gt;rq_list, so don't try to merge over
sw queue if it is empty, then we can save one acquiring of ctx-&gt;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 &lt;loberman@redhat.com&gt;
Cc: Omar Sandoval &lt;osandov@fb.com&gt;
Cc: Bart Van Assche &lt;bart.vanassche@wdc.com&gt;
Tested-by: Kashyap Desai &lt;kashyap.desai@broadcom.com&gt;
Reported-by: Kashyap Desai &lt;kashyap.desai@broadcom.com&gt;
Reviewed-by: Christoph Hellwig &lt;hch@lst.de&gt;
Signed-off-by: Ming Lei &lt;ming.lei@redhat.com&gt;
Signed-off-by: Jens Axboe &lt;axboe@kernel.dk&gt;
diff --git 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);

Original file line number	Diff line number	Diff line change
`@@ -326,7 +326,8 @@ bool __blk_mq_sched_bio_merge(struct request_queue q, struct bio bio)`
`326`	`326`	`return e->type->ops.mq.bio_merge(hctx, bio);`
`327`	`327`	`}`
`328`	`328`
`329`		`- if (hctx->flags & BLK_MQ_F_SHOULD_MERGE) {`
	`329`	`+ if ((hctx->flags & BLK_MQ_F_SHOULD_MERGE) &&`
	`330`	`+ !list_empty_careful(&ctx->rq_list)) {`
`330`	`331`	`/* default per sw-queue merge */`
`331`	`332`	`spin_lock(&ctx->lock);`
`332`	`333`	`ret = blk_mq_attempt_merge(q, ctx, bio);`