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

Michael S. Tsirkin
2023-03-24T06:00:46Z

On Fri, Mar 24, 2023 at 11:41:12AM +0800, Jason Wang wrote:
> On Thu, Mar 23, 2023 at 4:01 PM 黄杰 <huangjie.albert@bytedance.com> wrote:
> >
> > Jason Wang <jasowang@redhat.com> 于2023年3月22日周三 10:37写道：
> > >
> > > 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"
> > >
> >
> > OK, thanks for this. I will correct it in the next version
> >
> > > > 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 The code example here is out of date, make sure your tree has this:
> >
> > also, I will correct it in the next version，this is from kernel 5.15.
> >
> > >
> > > 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?
> > >
> >
> > iperf3 tcp stream:
> > vm1 -----------------> vm2
> > vm2 just receive tcp data stream from vm1, and send the ack to vm1,
> > there are so
> > many tx interruptions in vm2.
> >
> > but without event_triggered there are just a few tx 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))
> > >
> > > 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
> >
> > Normally, if we receive an interrupt, we should disable the interrupt
> > in the interrupt callback handler.
>
> So the problem is:
>
> 1) event_triggered was set to true in vring_interrupt()
>
> 2) after this nothing will happen for virtqueue_disable_cb() so
> VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
> 3) virtqueue_get_buf_ctx_split() will still think the cb is enabled
> then it tries to publish new event

Oh. Good point! I think when I wrote up
8d622d21d248 ("virtio: fix up virtio_disable_cb")
I missed this corner case.

> This makes me think about whether or not we really need
> event_triggered. The assumption in the virtqueue_disable_cb() seems
> wrong:
>
> /* If device triggered an event already it won't trigger one again:
> * no need to disable.
> */
> if (vq->event_triggered)
> return;
>
> This is wrong if there's no event index support.

I don't get it. how does this get triggered?

You are talking about device without event index?
Here's code from vring_interrupt():

/* Just a hint for performance: so it's ok that this can be racy! */
if (vq->event)
vq->event_triggered = true;

> And the
> event_triggered is somehow duplicated with the
> VRING_AVAIL_F_NO_INTERRUPT in the case of event index. The correct fix
> might be:
>
> 1) remove event_triggered
> 2) set VRING_AVAIL_F_NO_INTERRUPT in avail_flags_shadow in
> vring_interrrupt if event index is supported
>
> ?
>
> Thanks

I am not sure all this is right and I'd rather we focused
performance/correctness and cleanups separately.

>
> > But because of the introduction of event_triggered, here,
> > virtqueue_get_buf_ctx_split cannot be recognized
> > that the interrupt has been turned off.
> >
> > if we want another interrupt for the next entry, We should probably
> > call virtqueue_enable_cb？
> >
> > 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
> > > >
> > >
> >