Commit 7f212e99 authored by Christoph Hellwig's avatar Christoph Hellwig Committed by Jens Axboe
Browse files

virtio_blk: reverse request order in virtio_queue_rqs 



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 virtio_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: 0e9911fa ("virtio-blk: support mq_ops->queue_rqs()")
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241113152050.157179-3-hch@lst.de


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent beadf008

drivers/block/virtio_blk.c

+21 −25
Original line number Diff line number Diff line
@@ -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;

}