Commit b04f50ab authored Jul 02, 2018 by Ming Lei Committed by Jens Axboe Jul 09, 2018

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>

parent 3f0cedc7

block/blk-mq-sched.c

+2 −1

Original line number	Original line	Diff line number	Diff line
	@@ -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);		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 */		/* default per sw-queue merge */
	spin_lock(&ctx->lock);		spin_lock(&ctx->lock);
	ret = blk_mq_attempt_merge(q, ctx, bio);		ret = blk_mq_attempt_merge(q, ctx, bio);

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