Re: [PATCH v3 1/2] virtio-blk: support polling I/O

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

On Thu, Mar 24, 2022 at 01:58:28PM -0400, Michael S. Tsirkin wrote:
> On Thu, Mar 24, 2022 at 11:04:49PM +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 = 380K, avg latency = 167.87us
> > 	-- numjobs=2 : IOPS = 409K, avg latency = 312.6us
> > 	-- numjobs=4 : IOPS = 413K, avg latency = 619.72us
> > 
> > Reported-by: kernel test robot <lkp@intel.com>
> > Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> > ---
> >  drivers/block/virtio_blk.c | 101 +++++++++++++++++++++++++++++++++++--
> >  1 file changed, 97 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 8c415be86732..3d16f8b753e7 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 num_poll_queues;
> > +module_param(num_poll_queues, uint, 0644);
> > +MODULE_PARM_DESC(num_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 +572,13 @@ 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, num_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;
> > +
> >  	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> >  	if (!vblk->vqs)
> >  		return -ENOMEM;
> > @@ -578,8 +592,13 @@ static int init_vq(struct virtio_blk *vblk)
> >  	}
> >  
> >  	for (i = 0; i < num_vqs; i++) {
> > -		callbacks[i] = virtblk_done;
> > -		snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", 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;
> >  	}
> >  
> > @@ -728,16 +747,87 @@ static const struct attribute_group *virtblk_attr_groups[] = {
> >  static int virtblk_map_queues(struct blk_mq_tag_set *set)
> >  {
> >  	struct virtio_blk *vblk = set->driver_data;
> > +	int i, qoff;
> > +
> > +	for (i = 0, qoff = 0; i < set->nr_maps; i++) {
> > +		struct blk_mq_queue_map *map = &set->map[i];
> > +
> > +		map->nr_queues = vblk->io_queues[i];
> > +		map->queue_offset = qoff;
> > +		qoff += map->nr_queues;
> > +
> > +		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)
> > +			blk_mq_virtio_map_queues(&set->map[i], vblk->vdev, 0);
> > +		else
> > +			blk_mq_map_queues(&set->map[i]);
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static void virtblk_complete_batch(struct io_comp_batch *iob)
> > +{
> > +	struct request *req;
> > +	struct virtblk_req *vbr;
> >  
> > -	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
> > -					vblk->vdev, 0);
> > +	rq_list_for_each(&iob->req_list, req) {
> > +		vbr = blk_mq_rq_to_pdu(req);
> > +		virtblk_unmap_data(req, vbr);
> > +		virtblk_cleanup_cmd(req);
> > +	}
> > +	blk_mq_end_request_batch(iob);
> > +}
> > +
> > +static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
> > +{
> > +	struct virtio_blk_vq *vq = hctx->driver_data;
> > +	struct virtblk_req *vbr;
> > +	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);
> > +
> > +		found++;
> > +		if (!blk_mq_add_to_batch(req, iob, vbr->status,
> > +						virtblk_complete_batch))
> > +			blk_mq_complete_request(req);
> > +	}
> > +
> > +	spin_unlock_irqrestore(&vq->lock, flags);
> > +
> > +	return found;
> > +}
> > +
> > +static int virtblk_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
> > +			  unsigned int hctx_idx)
> > +{
> > +	struct virtio_blk *vblk = data;
> > +	struct virtio_blk_vq *vq = &vblk->vqs[hctx_idx];
> > +
> > +	WARN_ON(vblk->tag_set.tags[hctx_idx] != hctx->tags);
> > +	hctx->driver_data = vq;
> > +	return 0;
> >  }
> >  
> >  static const struct blk_mq_ops virtio_mq_ops = {
> >  	.queue_rq	= virtio_queue_rq,
> >  	.commit_rqs	= virtio_commit_rqs,
> > +	.init_hctx	= virtblk_init_hctx,
> >  	.complete	= virtblk_request_done,
> >  	.map_queues	= virtblk_map_queues,
> > +	.poll		= virtblk_poll,
> >  };
> >  
> >  static unsigned int virtblk_queue_depth;
> > @@ -816,6 +906,9 @@ static int virtblk_probe(struct virtio_device *vdev)
> >  		sizeof(struct scatterlist) * VIRTIO_BLK_INLINE_SG_CNT;
> >  	vblk->tag_set.driver_data = vblk;
> >  	vblk->tag_set.nr_hw_queues = vblk->num_vqs;
> > +	vblk->tag_set.nr_maps = 1;
> > +	if (vblk->io_queues[HCTX_TYPE_POLL])
> > +		vblk->tag_set.nr_maps = 3;
> >  
> >  	err = blk_mq_alloc_tag_set(&vblk->tag_set);
> >  	if (err)
> 
> 
> 
> So wrt cleanup, does something poll for all buffers to be
> used when device is removed?
 
Sorry for late reply.

Maybe below function calls iterate each HW queue and flush requests
before device is removed?

-----
virtblk_remove() -> blk_cleanup_disk()/blk_cleanup_queue() ->
blk_queue_start_drain()/blk_freeze_queue() 
-----

Regards,
Suwan Kim