Subject: Re: [PATCH 24/33] virtio_net: xsk: stop disable tx napi

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 3 Feb 2023 03:33:41 -0500

On Fri, Feb 03, 2023 at 11:24:42AM +0800, Xuan Zhuo wrote:
> On Thu, 2 Feb 2023 12:25:59 -0500, "Michael S. Tsirkin" <[email-masked]> wrote:
> > On Thu, Feb 02, 2023 at 07:00:49PM +0800, Xuan Zhuo wrote:
> > > Since xsk's TX queue is consumed by TX NAPI, if sq is bound to xsk, then
> > > we must stop tx napi from being disabled.
> > >
> > > Signed-off-by: Xuan Zhuo <[email-masked]>
> > > ---
> > >  drivers/net/virtio/main.c | 9 ++++++++-
> > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/virtio/main.c b/drivers/net/virtio/main.c
> > > index ed79e750bc6c..232cf151abff 100644
> > > --- a/drivers/net/virtio/main.c
> > > +++ b/drivers/net/virtio/main.c
> > > @@ -2728,8 +2728,15 @@ static int virtnet_set_coalesce(struct net_device *dev,
> > >  		return ret;
> > >
> > >  	if (update_napi) {
> > > -		for (i = 0; i < vi->max_queue_pairs; i++)
> > > +		for (i = 0; i < vi->max_queue_pairs; i++) {
> > > +			/* xsk xmit depend on the tx napi. So if xsk is active,
> >
> > depends.
> >
> > > +			 * prevent modifications to tx napi.
> > > +			 */
> > > +			if (rtnl_dereference(vi->sq[i].xsk.pool))
> > > +				continue;
> > > +
> > >  			vi->sq[i].napi.weight = napi_weight;
> >
> > I don't get it.
> > changing napi weight does not work then.
> > why is this ok?
> 
> 
> static void skb_xmit_done(struct virtqueue *vq)
> {
> 	struct virtnet_info *vi = vq->vdev->priv;
> 	struct napi_struct *napi = &vi->sq[vq2txq(vq)].napi;
> 
> 	/* Suppress further interrupts. */
> 	virtqueue_disable_cb(vq);
> 
> 	if (napi->weight)
> 		virtqueue_napi_schedule(napi, vq);
> 	else
> 		/* We were probably waiting for more output buffers. */
> 		netif_wake_subqueue(vi->dev, vq2txq(vq));
> }
> 
> 
> If the weight is 0, tx napi will not be triggered again.
> 
> Thanks.

This needs more thought then. First ignoring what user is requesting is
not nice.  Second what if napi is first disabled and then xsk enabled?

> > > +		}
> > >  	}
> > >  	return ret;
> > > --
> > > 2.32.0.3.g01195cf9f