[PATCH 2/6] virtio_blk: reverse request order in virtio_queue_rqs

[Christoph Hellwig <hch@lst.de>]

blk_mq_flush_plug_list submits requests in the reverse order that they
were submitted, which leads to a rather suboptimal I/O pattern especially
in rotational devices.  Fix this by rewriting nvme_queue_rqs so that it
always pops the requests from the passed in request list, and then adds
them to the head of a local submit list.  This actually simplifies the
code a bit as it removes the complicated list splicing, at the cost of
extra updates of the rq_next pointer.  As that should be cache hot
anyway it should be an easy price to pay.

Fixes: 0e9911fa768f ("virtio-blk: support mq_ops->queue_rqs()")
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/block/virtio_blk.c | 46 +++++++++++++++++---------------------
 1 file changed, 21 insertions(+), 25 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 0e99a4714928..b25f7c06a28e 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -471,18 +471,18 @@ static bool virtblk_prep_rq_batch(struct request *req)
 	return virtblk_prep_rq(req->mq_hctx, vblk, req, vbr) == BLK_STS_OK;
 }
 
-static bool virtblk_add_req_batch(struct virtio_blk_vq *vq,
+static void virtblk_add_req_batch(struct virtio_blk_vq *vq,
 					struct request **rqlist)
 {
+	struct request *req;
 	unsigned long flags;
-	int err;
 	bool kick;
 
 	spin_lock_irqsave(&vq->lock, flags);
 
-	while (!rq_list_empty(*rqlist)) {
-		struct request *req = rq_list_pop(rqlist);
+	while ((req = rq_list_pop(rqlist))) {
 		struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
+		int err;
 
 		err = virtblk_add_req(vq->vq, vbr);
 		if (err) {
@@ -495,37 +495,33 @@ static bool virtblk_add_req_batch(struct virtio_blk_vq *vq,
 	kick = virtqueue_kick_prepare(vq->vq);
 	spin_unlock_irqrestore(&vq->lock, flags);
 
-	return kick;
+	if (kick)
+		virtqueue_notify(vq->vq);
 }
 
 static void virtio_queue_rqs(struct request **rqlist)
 {
-	struct request *req, *next, *prev = NULL;
+	struct request *submit_list = NULL;
 	struct request *requeue_list = NULL;
+	struct request **requeue_lastp = &requeue_list;
+	struct virtio_blk_vq *vq = NULL;
+	struct request *req;
 
-	rq_list_for_each_safe(rqlist, req, next) {
-		struct virtio_blk_vq *vq = get_virtio_blk_vq(req->mq_hctx);
-		bool kick;
-
-		if (!virtblk_prep_rq_batch(req)) {
-			rq_list_move(rqlist, &requeue_list, req, prev);
-			req = prev;
-			if (!req)
-				continue;
-		}
+	while ((req = rq_list_pop(rqlist))) {
+		struct virtio_blk_vq *this_vq = get_virtio_blk_vq(req->mq_hctx);
 
-		if (!next || req->mq_hctx != next->mq_hctx) {
-			req->rq_next = NULL;
-			kick = virtblk_add_req_batch(vq, rqlist);
-			if (kick)
-				virtqueue_notify(vq->vq);
+		if (vq && vq != this_vq)
+			virtblk_add_req_batch(vq, &submit_list);
+		vq = this_vq;
 
-			*rqlist = next;
-			prev = NULL;
-		} else
-			prev = req;
+		if (virtblk_prep_rq_batch(req))
+			rq_list_add(&submit_list, req); /* reverse order */
+		else
+			rq_list_add_tail(&requeue_lastp, req);
 	}
 
+	if (vq)
+		virtblk_add_req_batch(vq, &submit_list);
 	*rqlist = requeue_list;
 }
 
-- 
2.45.2