[PATCH 1/2] blk-mq: respect io scheduler

[Ming Lei <ming.lei@redhat.com>]

Now in case of real MQ, io scheduler may be bypassed, and not only this
way may hurt performance for some slow MQ device, but also break zoned
device which depends on mq-deadline for respecting the write order in
one zone.

So don't bypass io scheduler if we have one setup.

This patch can double sequential write performance basically on MQ
scsi_debug when mq-deadline is applied.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 20a49be536b5..d7aed6518e62 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2003,6 +2003,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 		}
 
 		blk_add_rq_to_plug(plug, rq);
+	} else if (q->elevator) {
+		blk_mq_sched_insert_request(rq, false, true, true);
 	} else if (plug && !blk_queue_nomerges(q)) {
 		/*
 		 * We do limited plugging. If the bio can be merged, do that.
@@ -2026,8 +2028,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 			blk_mq_try_issue_directly(data.hctx, same_queue_rq,
 					&cookie);
 		}
-	} else if ((q->nr_hw_queues > 1 && is_sync) || (!q->elevator &&
-			!data.hctx->dispatch_busy)) {
+	} else if ((q->nr_hw_queues > 1 && is_sync) ||
+			!data.hctx->dispatch_busy) {
 		blk_mq_try_issue_directly(data.hctx, rq, &cookie);
 	} else {
 		blk_mq_sched_insert_request(rq, false, true, true);
-- 
2.20.1