[PATCH 1/4] block: add mq_ops->queue_rqs hook

[Jens Axboe <axboe@kernel.dk>]

If we have a list of requests in our plug list, send it to the driver in
one go, if possible. The driver must set mq_ops->queue_rqs() to support
this, if not the usual one-by-one path is used.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-mq.c         | 17 +++++++++++++++++
 include/linux/blk-mq.h |  8 ++++++++
 2 files changed, 25 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9b4e79e2ac1e..005715206b16 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2208,6 +2208,19 @@ static void blk_mq_plug_issue_direct(struct blk_plug *plug, bool from_schedule)
 	int queued = 0;
 	int errors = 0;
 
+	/*
+	 * Peek first request and see if we have a ->queue_rqs() hook. If we
+	 * do, we can dispatch the whole plug list in one go. We already know
+	 * at this point that all requests belong to the same queue, caller
+	 * must ensure that's the case.
+	 */
+	rq = rq_list_peek(&plug->mq_list);
+	if (rq->q->mq_ops->queue_rqs) {
+		rq->q->mq_ops->queue_rqs(&plug->mq_list);
+		if (rq_list_empty(plug->mq_list))
+			return;
+	}
+
 	while ((rq = rq_list_pop(&plug->mq_list))) {
 		bool last = rq_list_empty(plug->mq_list);
 		blk_status_t ret;
@@ -2256,6 +2269,10 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 
 	if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
 		blk_mq_plug_issue_direct(plug, false);
+		/*
+		 * Expected case, all requests got dispatched. If not, fall
+		 * through to individual dispatch of the remainder.
+		 */
 		if (rq_list_empty(plug->mq_list))
 			return;
 	}
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 3ba1e750067b..897cf475e7eb 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -503,6 +503,14 @@ struct blk_mq_ops {
 	 */
 	void (*commit_rqs)(struct blk_mq_hw_ctx *);
 
+	/**
+	 * @queue_rqs: Queue a list of new requests. Driver is guaranteed
+	 * that each request belongs to the same queue. If the driver doesn't
+	 * empty the @rqlist completely, then the rest will be queued
+	 * individually by the block layer upon return.
+	 */
+	void (*queue_rqs)(struct request **rqlist);
+
 	/**
 	 * @get_budget: Reserve budget before queue request, once .queue_rq is
 	 * run, it is driver's responsibility to release the
-- 
2.33.1