Subject: Re: [PATCH RFC v2 2/4] virtio_net: disable cb aggressively

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 13 Apr 2021 10:33:36 -0400

On Tue, Apr 13, 2021 at 04:53:32PM +0800, Jason Wang 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 <[email-masked]>
> > ---
> >   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?

Good point ... probably only if the vq isn't empty ...

> And as we discussed in the past, it's probably the time to have a single
> NAPI for both tx and rx?
> 
> Thanks

Donnu. I'd like to get a minimal bugfix in, refactoring on top ...

> 
> >   	}
> > @@ -1582,6 +1583,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> >   	bool use_napi = sq->napi.weight;
> >   	/* Free up any pending old buffers before queueing new ones. */
> > +	virtqueue_disable_cb(sq->vq);
> >   	free_old_xmit_skbs(sq, false);
> >   	if (use_napi && kick)