Re: [PATCH net V2] virtio-net: correctly enable callback during start_xmit

[Jason Wang]

On Thu, Dec 15, 2022 at 5:02 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Thu, Dec 15, 2022 at 11:27:19AM +0800, Jason Wang wrote:
> > Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
> > virtqueue callback via the following statement:
> >
> >         do {
> >            ......
> >       } while (use_napi && kick &&
> >                unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> >
> > When NAPI is used and kick is false, the callback won't be enabled
> > here. And when the virtqueue is about to be full, the tx will be
> > disabled, but we still don't enable tx interrupt which will cause a TX
> > hang. This could be observed when using pktgen with burst enabled.
> >
> > Fixing this by trying to enable tx interrupt after we disable TX when
> > we're not using napi or kick is false.
> >
> > Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> > The patch is needed for -stable.
> > Changes since V1:
> > - enable tx interrupt after we disable tx
> > ---
> >  drivers/net/virtio_net.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 86e52454b5b5..dcf3a536d78a 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1873,7 +1873,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> >        */
> >       if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> >               netif_stop_subqueue(dev, qnum);
> > -             if (!use_napi &&
> > +             if ((!use_napi || !kick) &&
> >                   unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> >                       /* More just got used, free them then recheck. */
> >                       free_old_xmit_skbs(sq, false);
>
> This will work but the following lines are:
>
>                        if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
>                                 netif_start_subqueue(dev, qnum);
>                                 virtqueue_disable_cb(sq->vq);
>                         }
>
>
> and I thought we are supposed to keep callbacks enabled with napi?

This seems to be the opposite logic of commit a7766ef18b33 that
disables callbacks for NAPI.

It said:

    There are currently two cases where we poll TX vq not in response to a
    callback: start xmit and rx napi.  We currently do this with callbacks
    enabled which can cause extra interrupts from the card.  Used not to be
    a big issue as we run with interrupts disabled but that is no longer the
    case, and in some cases the rate of spurious interrupts is so high
    linux detects this and actually kills the interrupt.

My undersatnding is that it tries to disable callbacks on TX.

> One of the ideas of napi is to free on napi callback, not here
> immediately.
>
> I think it is easier to just do a separate branch here. Along the
> lines of:
>
>                 if (use_napi) {
>                         if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
>                                 virtqueue_napi_schedule(napi, vq);

This seems to be a new logic and it causes some delay in processing TX
(unnecessary NAPI).

>                 } else {
>                         ... old code ...
>                 }
>
> also reduces chances of regressions on !napi (which is not well tested)
> and keeps callbacks off while we free skbs.

I think my patch doesn't change the logic of !napi? (It checks !napi || kick).

Thanks

>
> No?
>
>
> > --
> > 2.25.1
>