[PATCH 2/2] blk-mq: apply normal plugging for HDD

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

Some HDD drive may expose multiple hw queue, such as MegraRaid, so
still apply the normal plugging for such devices because sequential IO
may benefit a lot from plug merging.

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, 5 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d7aed6518e62..969dfe02fa7c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1983,10 +1983,14 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 		/* bypass scheduler for flush rq */
 		blk_insert_flush(rq);
 		blk_mq_run_hw_queue(data.hctx, true);
-	} else if (plug && (q->nr_hw_queues == 1 || q->mq_ops->commit_rqs)) {
+	} else if (plug && (q->nr_hw_queues == 1 || q->mq_ops->commit_rqs ||
+				!blk_queue_nonrot(q))) {
 		/*
 		 * Use plugging if we have a ->commit_rqs() hook as well, as
 		 * we know the driver uses bd->last in a smart fashion.
+		 *
+		 * Use normal plugging if this disk is slow HDD, as sequential
+		 * IO may benefit a lot from plug merging.
 		 */
 		unsigned int request_count = plug->rq_count;
 		struct request *last = NULL;
-- 
2.20.1