Re: [PATCH v4 1/2] virtio-blk: support polling I/O

Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Stefan Hajnoczi]

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On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> +static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
> +{
> +	struct virtio_blk *vblk = hctx->queue->queuedata;
> +	struct virtio_blk_vq *vq = hctx->driver_data;
> +	struct virtblk_req *vbr;
> +	bool req_done = false;
> +	unsigned long flags;
> +	unsigned int len;
> +	int found = 0;
> +
> +	spin_lock_irqsave(&vq->lock, flags);
> +
> +	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
> +		struct request *req = blk_mq_rq_from_pdu(vbr);
>  
> -	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
> -					vblk->vdev, 0);
> +		found++;
> +		if (!blk_mq_add_to_batch(req, iob, vbr->status,
> +						virtblk_complete_batch))
> +			blk_mq_complete_request(req);
> +		req_done = true;
> +	}
> +
> +	if (req_done)

Minor nit: req_done can be replaced with found > 0.

> +		blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
> +
> +	spin_unlock_irqrestore(&vq->lock, flags);
> +
> +	return found;
> +}

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Re: [PATCH v4 1/2] virtio-blk: support polling I/O

[Christoph Hellwig]

On Tue, Apr 05, 2022 at 02:31:21PM +0900, Suwan Kim wrote:
> This patch supports polling I/O via virtio-blk driver. Polling
> feature is enabled by module parameter "num_poll_queues" and it
> sets dedicated polling queues for virtio-blk. This patch improves
> the polling I/O throughput and latency.
> 
> The virtio-blk driver doesn't not have a poll function and a poll
> queue and it has been operating in interrupt driven method even if
> the polling function is called in the upper layer.
> 
> virtio-blk polling is implemented upon 'batched completion' of block
> layer. virtblk_poll() queues completed request to io_comp_batch->req_list
> and later, virtblk_complete_batch() calls unmap function and ends
> the requests in batch.
> 
> virtio-blk reads the number of poll queues from module parameter
> "num_poll_queues". If VM sets queue parameter as below,
> ("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
> It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
> as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
> queues, the poll queues have no callback function.
> 
> Regarding HW-SW queue mapping, the default queue mapping uses the
> existing method that condsiders MSI irq vector. But the poll queue
> doesn't have an irq, so it uses the regular blk-mq cpu mapping.
> 
> For verifying the improvement, I did Fio polling I/O performance test
> with io_uring engine with the options below.
> (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
> I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
> queues for VM.
> 
> As a result, IOPS and average latency improved about 10%.
> 
> Test result:
> 
> - Fio io_uring poll without virtio-blk poll support
> 	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
> 	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
> 	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us
> 
> - Fio io_uring poll with virtio-blk poll support
> 	-- numjobs=1 : IOPS = 385K, avg latency = 165.94us
> 	-- numjobs=2 : IOPS = 408K, avg latency = 313.28us
> 	-- numjobs=4 : IOPS = 424K, avg latency = 613.05us
> 
> Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> ---
>  drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 108 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 8c415be86732..712579dcd3cc 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
>  		 "0 for no limit. "
>  		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
>  
> +static unsigned int poll_queues;
> +module_param(poll_queues, uint, 0644);
> +MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
> +
>  static int major;
>  static DEFINE_IDA(vd_index_ida);
>  
> @@ -81,6 +85,7 @@ struct virtio_blk {
>  
>  	/* num of vqs */
>  	int num_vqs;
> +	int io_queues[HCTX_MAX_TYPES];
>  	struct virtio_blk_vq *vqs;
>  };
>  
> @@ -548,6 +553,7 @@ static int init_vq(struct virtio_blk *vblk)
>  	const char **names;
>  	struct virtqueue **vqs;
>  	unsigned short num_vqs;
> +	unsigned int num_poll_vqs;
>  	struct virtio_device *vdev = vblk->vdev;
>  	struct irq_affinity desc = { 0, };
>  
> @@ -556,6 +562,7 @@ static int init_vq(struct virtio_blk *vblk)
>  				   &num_vqs);
>  	if (err)
>  		num_vqs = 1;
> +
>  	if (!err && !num_vqs) {
>  		dev_err(&vdev->dev, "MQ advertised but zero queues reported\n");
>  		return -EINVAL;
> @@ -565,6 +572,18 @@ static int init_vq(struct virtio_blk *vblk)
>  			min_not_zero(num_request_queues, nr_cpu_ids),
>  			num_vqs);
>  
> +	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
> +
> +	memset(vblk->io_queues, 0, sizeof(int) * HCTX_MAX_TYPES);
> +	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
> +	vblk->io_queues[HCTX_TYPE_READ] = 0;
> +	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
> +
> +	dev_info(&vdev->dev, "%d/%d/%d default/read/poll queues\n",
> +				vblk->io_queues[HCTX_TYPE_DEFAULT],
> +				vblk->io_queues[HCTX_TYPE_READ],
> +				vblk->io_queues[HCTX_TYPE_POLL]);
> +
>  	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>  	if (!vblk->vqs)
>  		return -ENOMEM;
> @@ -578,8 +597,13 @@ static int init_vq(struct virtio_blk *vblk)
>  	}
>  
>  	for (i = 0; i < num_vqs; i++) {
> +		if (i < num_vqs - num_poll_vqs) {
> +			callbacks[i] = virtblk_done;
> +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
> +		} else {
> +			callbacks[i] = NULL;
> +			snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> +		}
>  		names[i] = vblk->vqs[i].name;

This would look a little cleaner with two loops:

 	for (i = 0; i < num_vqs - num_poll_vqs; i++) {
		callbacks[i] = virtblk_done;
		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
 		names[i] = vblk->vqs[i].name;
	}
 	for (; i < num_vqs; i++) {
		callbacks[i] = NULL;
		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
		names[i] = vblk->vqs[i].name;
	}

> +
> +		if (map->nr_queues == 0)
> +			continue;
> +
> +		/*
> +		 * Regular queues have interrupts and hence CPU affinity is
> +		 * defined by the core virtio code, but polling queues have
> +		 * no interrupts so we let the block layer assign CPU affinity.
> +		 */
> +		if (i == HCTX_TYPE_DEFAULT)

I'd check for
		i != HCTX_TYPE_POLL

here instead to make the check a little more explicit and future proof
for the potential addition of read queues (which would be a Linux only
change without hypervisor or spec changes).  In fact you might as well
add that support now as doing it is completely trivial once a driver
supports multiple map types.

> +static void virtblk_complete_batch(struct io_comp_batch *iob)
> +{
> +	struct request *req;
> +	struct virtblk_req *vbr;
> +
> +	rq_list_for_each(&iob->req_list, req) {
> +		vbr = blk_mq_rq_to_pdu(req);
> +		virtblk_unmap_data(req, vbr);
> +		virtblk_cleanup_cmd(req);

vbr is only used ones, so why not just:

		virtblk_unmap_data(req, blk_mq_rq_to_pdu);
?

Or even better add a cleanup patch to just remove the vbr argument to
virtblk_unmap_data as it is not needed at all.
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Re: [PATCH v4 2/2] virtio-blk: support mq_ops->queue_rqs()

[Christoph Hellwig]

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

[Stefan Hajnoczi]

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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(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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