linux-kernel - Re: [PATCH RFC v2 2/4] virtio

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Date:   Tue, 13 Apr 2021 10:08:34 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>, Wei Wang <weiwan@...gle.com>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC v2 2/4] virtio_net: disable cb aggressively

On Tue, Apr 13, 2021 at 4:53 AM Jason Wang <jasowang@...hat.com> wrote:
>
>
> 在 2021/4/13 下午1:47, Michael S. Tsirkin 写道:
> > 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.
> >
> > Fix up by disabling the callbacks before polling the tx vq.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >   drivers/net/virtio_net.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 82e520d2cb12..16d5abed582c 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1429,6 +1429,7 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
> >               return;
> >
> >       if (__netif_tx_trylock(txq)) {
> > +             virtqueue_disable_cb(sq->vq);
> >               free_old_xmit_skbs(sq, true);
> >               __netif_tx_unlock(txq);
>
>
> Any reason that we don't need to enable the cb here?

This is an opportunistic clean outside the normal tx-napi path, so if
disabling the tx interrupt here, it won't be reenabled based on
napi_complete_done.

I think that means that it stays disabled until the following start_xmit:

        if (use_napi && kick)
                virtqueue_enable_cb_delayed(sq->vq);

But that seems sufficient.