Re: [PATCH] virtio_ring: Suppress tx interrupt when napi_tx disable

Jason Wang
2023-03-22T02:37:41Z

On Tue, Mar 21, 2023 at 5:00 PM Albert Huang
<huangjie.albert@bytedance.com> wrote:
>
> From: "huangjie.albert" <huangjie.albert@bytedance.com>
>
> fix commit 8d622d21d248 ("virtio: fix up virtio_disable_cb")
>
> if we disable the napi_tx. when we triger a tx interrupt, the

typo should be "trigger"

> vq->event_triggered will be set to true. It will no longer be
> set to false. Unless we explicitly call virtqueue_enable_cb_delayed
> or virtqueue_enable_cb_prepare
>
> if we disable the napi_tx, It will only be called when the tx ring
> buffer is relatively small:
> virtio_net->start_xmit:
> if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> netif_stop_subqueue(dev, qnum);
> if (!use_napi &&
> unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> /* More just got used, free them then recheck. */
> free_old_xmit_skbs(sq, false);
> if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> netif_start_subqueue(dev, qnum);
> virtqueue_disable_cb(sq->vq);
> }

The code example here is out of date, make sure your tree has this:

commit d71ebe8114b4bf622804b810f5e274069060a174
Author: Jason Wang <jasowang@redhat.com>
Date: Tue Jan 17 11:47:07 2023 +0800

virtio-net: correctly enable callback during start_xmit

> }
> }
> Because event_triggered is true.Therefore, VRING_AVAIL_F_NO_INTERRUPT or
> VRING_PACKED_EVENT_FLAG_DISABLE will not be set.So we update
> vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
> every time we call virtqueue_get_buf_ctx.This will bring more interruptions.

Can you please post how to test with the performance numbers?

>
> if event_triggered is set to true, do not update vring_used_event(&vq->split.vring)
> or vq->packed.vring.driver->off_wrap
>
> Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
> ---
> drivers/virtio/virtio_ring.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 307e139cb11d..f486cccadbeb 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -795,7 +795,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> /* If we expect an interrupt for the next entry, tell host
> * by writing event index and flush out the write before
> * the read in the next get_buf call. */
> - if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
> + if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)
> + && (vq->event_triggered == false))

I'm not sure this can work, when event_triggered is true it means
we've got an interrupt, in this case if we want another interrupt for
the next entry, we should update used_event otherwise we will lose
that interrupt?

Thanks

> virtio_store_mb(vq->weak_barriers,
> &vring_used_event(&vq->split.vring),
> cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
> @@ -1529,7 +1530,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> * by writing event index and flush out the write before
> * the read in the next get_buf call.
> */
> - if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC)
> + if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC
> + && (vq->event_triggered == false))
> virtio_store_mb(vq->weak_barriers,
> &vq->packed.vring.driver->off_wrap,
> cpu_to_le16(vq->last_used_idx));
> --
> 2.31.1
>