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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 13, 2024 at 04:20:42PM +0100, Christoph Hellwig wrote:
> 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;
>  }



looks ok from virtio POV

Acked-by: Michael S. Tsirkin <mst@redhat.com>


> -- 
> 2.45.2