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: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx>
Cc: Laurence Oberman <loberman@xxxxxxxxxx>
Cc: Omar Sandoval <osandov@xxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Bart Van Assche <bart.vanassche@xxxxxxx>
Reported-by: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
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);
--
2.9.5
