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

Christoph Hellwig
2022-04-05T09:23:36Z

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.