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

Michael S. Tsirkin
2023-03-24T06:38:01Z

Hmm I sent a bit too fast, and my testing rig is down now.
So please do send a new version, I sent comments on what to fix
in this one.

On Fri, Mar 24, 2023 at 02:08:55AM -0400, Michael S. Tsirkin wrote:
> Thanks for the patch!
> I picked it up.
> I made small changes, please look at it in my branch,
> both to see what I changed for your next submission,
> and to test that it still addresses the problem for you.
> Waiting for your confirmation to send upstream.
> Thanks!
>
>
> On Tue, Mar 21, 2023 at 04:59:53PM +0800, Albert Huang 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
> > 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);
> > }
> > }
> > }
> > 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.
> >
> > 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))
> > 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