blk-mq: use plug for devices that implement ->commits_rqs()

If we have that hook, we know the driver handles bd->last == true in
a smart fashion. If it does, even for multiple hardware queues, it's
a good idea to flush batches of requests to the device, if we have
batches of requests from the submitter.

Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 14e31e93a950f6c1ab7997e65e4daae7155e2cf5..7dcef565dc0fba5dcccbf8059b7c89c31a922701 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1945,7 +1945,11 @@ 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) {
+       } else if (plug && (q->nr_hw_queues == 1 || q->mq_ops->commit_rqs)) {
+               /*
+                * Use plugging if we have a ->commit_rqs() hook as well, as
+                * we know the driver uses bd->last in a smart fashion.
+                */
                unsigned int request_count = plug->rq_count;
                struct request *last = NULL;