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

[Suwan Kim <suwan.kim027@gmail.com>]

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