Re: [PATCH v4 2/2] virtio-blk: support mq_ops->queue_rqs()

Christoph Hellwig
2022-04-05T09:50:55Z

On Tue, Apr 05, 2022 at 02:31:22PM +0900, Suwan Kim wrote:
> This patch supports mq_ops->queue_rqs() hook. It has an advantage of
> batch submission to virtio-blk driver. It also helps polling I/O because
> polling uses batched completion of block layer. Batch submission in
> queue_rqs() can boost polling performance.
>
> In queue_rqs(), it iterates plug->mq_list, collects requests that
> belong to same HW queue until it encounters a request from other
> HW queue or sees the end of the list.
> Then, virtio-blk adds requests into virtqueue and kicks virtqueue
> to submit requests.
>
> If there is an error, it inserts error request to requeue_list and
> passes it to ordinary block layer path.
>
> For verification, I did fio test.
> (io_uring, randread, direct=1, bs=4K, iodepth=64 numjobs=N)
> I set 4 vcpu and 2 virtio-blk queues for VM and run fio test 5 times.
> It shows about 2% improvement.
>
> | numjobs=2 | numjobs=4
> -----------------------------------------------------------
> fio without queue_rqs() | 291K IOPS | 238K IOPS
> -----------------------------------------------------------
> fio with queue_rqs() | 295K IOPS | 243K IOPS
>
> For polling I/O performance, I also did fio test as below.
> (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=4)
> I set 4 vcpu and 2 poll queues for VM.
> It shows about 2% improvement in polling I/O.
>
> | IOPS | avg latency
> -----------------------------------------------------------
> fio poll without queue_rqs() | 424K | 613.05 usec
> -----------------------------------------------------------
> fio poll with queue_rqs() | 435K | 601.01 usec
>
> Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> ---
> drivers/block/virtio_blk.c | 110 +++++++++++++++++++++++++++++++++----
> 1 file changed, 99 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 712579dcd3cc..a091034bc551 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -92,6 +92,7 @@ struct virtio_blk {
> struct virtblk_req {
> struct virtio_blk_outhdr out_hdr;
> u8 status;
> + int sg_num;
> struct sg_table sg_table;
> struct scatterlist sg[];
> };
> @@ -311,18 +312,13 @@ static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
> virtqueue_notify(vq->vq);
> }
>
> -static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
> - const struct blk_mq_queue_data *bd)
> +static blk_status_t virtblk_prep_rq(struct blk_mq_hw_ctx *hctx,
> + struct virtio_blk *vblk,
> + struct request *req,
> + struct virtblk_req *vbr)
> {
> - struct virtio_blk *vblk = hctx->queue->queuedata;
> - struct request *req = bd->rq;
> - struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
> - unsigned long flags;
> - int num;
> - int qid = hctx->queue_num;
> - bool notify = false;
> blk_status_t status;
> - int err;
> + int num;
>
> status = virtblk_setup_cmd(vblk->vdev, req, vbr);
> if (unlikely(status))
> @@ -335,9 +331,30 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
> virtblk_cleanup_cmd(req);
> return BLK_STS_RESOURCE;
> }
> + vbr->sg_num = num;

This can go into the nents field of vbr->sg_table.

> + int err;
> +
> + status = virtblk_prep_rq(hctx, vblk, req, vbr);
> + if (unlikely(status))
> + return status;
>
> spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
> - err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg_table.sgl, num);
> + err = virtblk_add_req(vblk->vqs[qid].vq, vbr,
> + vbr->sg_table.sgl, vbr->sg_num);

And while we're at it - virtblk_add_req can lose the data_sg and
have_data arguments as they can be derived from vbr.