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

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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>
This commit is contained in:
Ming Lei 2018-07-02 17:35:59 +08:00 committed by Jens Axboe
parent 3f0cedc7e9
commit b04f50ab8a
1 changed files with 2 additions and 1 deletions
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3
block/blk-mq-sched.c
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@ -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);