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

Suwan Kim
2022-04-05T11:48:20Z

On Tue, Apr 05, 2022 at 01:57:06AM -0700, Christoph Hellwig wrote:
> 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.

Ok. I will remove vbr->sg_num and save it to vbr->sg_table.nents.

Regards,
Suwan Kim