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

[Jens Axboe]

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 at kernel.dk>
---
 block/blk-mq.c         | 26 +++++++++++++++++++++++---
 include/linux/blk-mq.h |  8 ++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e02e7017db03..f24394cb2004 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2512,6 +2512,7 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
 	struct blk_mq_hw_ctx *this_hctx;
 	struct blk_mq_ctx *this_ctx;
+	struct request *rq;
 	unsigned int depth;
 	LIST_HEAD(list);
 
@@ -2520,7 +2521,28 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	plug->rq_count = 0;
 
 	if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
-		struct request_queue *q = rq_list_peek(&plug->mq_list)->q;
+		struct request_queue *q;
+
+		rq = rq_list_peek(&plug->mq_list);
+		q = rq->q;
+
+		/*
+		 * 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.
+		 *
+		 * Since we pass off the full list to the driver at this point,
+		 * we do not increment the active request count for the queue.
+		 * Bypass shared tags for now because of that.
+		 */
+		if (q->mq_ops->queue_rqs &&
+		    !(rq->mq_hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {
+			blk_mq_run_dispatch_ops(q,
+				q->mq_ops->queue_rqs(&plug->mq_list));
+			if (rq_list_empty(plug->mq_list))
+				return;
+		}
 
 		blk_mq_run_dispatch_ops(q,
 				blk_mq_plug_issue_direct(plug, false));
@@ -2532,8 +2554,6 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	this_ctx = NULL;
 	depth = 0;
 	do {
-		struct request *rq;
-
 		rq = rq_list_pop(&plug->mq_list);
 
 		if (!this_hctx) {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 6f858e05781e..1e1cd9cfbbea 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -493,6 +493,14 @@ 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
-- 
2.34.1