Re: virtio_net hang

[Mark McLoughlin <markmc@redhat.com>]

On Tue, 2008-11-18 at 19:37 +0100, Emmanuel Lacour wrote:
> On Fri, Nov 14, 2008 at 06:26:44PM +0000, Mark McLoughlin wrote:
> > 
> > Right, the tap device tx queue is full because kvm-userspace isn't
> > reading packets from it.
> > 
> > This could be because kvm-userspace has just stopped noticing that
> > there's data available from the tapfd or because virtio_net in the guest
> > has stopped noticing that packets are available in the ring.
> > 
> > One thing you could easily check is whether:
> > 
> >   ip link set eth0 down
> >   ip link set eth0 up
> > 
> > in the host is sufficient to fix it? If it is, then it points to a guest
> > driver issue.
> > 
> 
> I made the test, putting link down then up fix it.

Thanks, that's very interesting.

Since bringing the interface up and down basically just causes the
driver to re-schedule itself with NAPI, all I can see as a possibility
is that we somehow (e.g. a race condition) had gotten ourselves into a
state where we have rx ring interrupts disabled and we're not scheduled
with NAPI.

We synchronise around the NAPI_STATE_SCHED bit with atomic operations
and all the logic looks correct ... so I'm stumped, really.

Is it possible for you to try a newer guest kernel?

Cheers,
Mark.