blk-mq: apply normal plugging for HDD

Some HDD drive may expose multiple hardware queues, such as MegraRaid.
Let's 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: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5d4bef3c378dbfdf59191816fab5b6c604926fbb..ec791156e9ccd1b17cc9c07b98f55b08f2a705de 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1992,10 +1992,14 @@ 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;