From: Ming Lei ming.lei@redhat.com
[ 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 --- block/blk-mq-sched.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index 56c493c6cd90..f5745acc2d98 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -339,7 +339,8 @@ 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);