Re: [V2 PATCH 9/9] vhost: zerocopy: poll vq in zerocopy callback

[Jason Wang]

On 05/17/2012 01:32 AM, Shirley Ma wrote:
> On Wed, 2012-05-16 at 18:14 +0300, Michael S. Tsirkin wrote:
> > On Wed, May 16, 2012 at 08:10:27AM -0700, Shirley Ma wrote:
> > > On Wed, 2012-05-16 at 10:58 +0800, Jason Wang wrote:
> > > > > >    drivers/vhost/vhost.c |    1 +
> > > > > >    1 files changed, 1 insertions(+), 0 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > > > index 947f00d..7b75fdf 100644
> > > > > > --- a/drivers/vhost/vhost.c
> > > > > > +++ b/drivers/vhost/vhost.c
> > > > > > @@ -1604,6 +1604,7 @@ void vhost_zerocopy_callback(void *arg)
> > > > > >           struct vhost_ubuf_ref *ubufs = ubuf->arg;
> > > > > >           struct vhost_virtqueue *vq = ubufs->vq;
> > > > > >
> > > > > > +       vhost_poll_queue(&vq->poll);
> > > > > >           /* set len = 1 to mark this desc buffers done DMA */
> > > > > >           vq->heads[ubuf->desc].len = VHOST_DMA_DONE_LEN;
> > > > > >           kref_put(&ubufs->kref, vhost_zerocopy_done_signal);
> > > > > Doing so, we might have redundant vhost_poll_queue(). Do you
> > know in
> > > > > which scenario there might be missing of adding and signaling
> > during
> > > > > zerocopy?
> > > > Yes, as we only do signaling and adding during tx work, if there's
> > no
> > > > tx
> > > > work when the skb were sent, we may lose the opportunity to let
> > guest
> > > > know about the completion. It's easy to be reproduced with netperf
> > > > test.
> > > The reason which host signals guest is to free guest tx buffers, if
> > > there is no tx work, then it's not necessary to signal the guest
> > unless
> > > guest runs out of memory. The pending buffers will be released
> > > virtio_net device gone.

Looks like we only free the skbs in .ndo_start_xmit().
> > > What's the behavior of netperf test when you hit this situation?
> > >
> > > Thanks
> > > Shirley
> > IIRC guest networking seems to be lost.
> It seems vhost_enable_notify is missing in somewhere else?
>
> Thanks
> Shirley

The problem is we may stop the tx queue when there no enough capacity to 
place packets, at this moment  we depends on the tx interrupt to 
re-enable the tx queue. So if we didn't poll the vhost during callback, 
guest may lose the tx interrupt to re-enable the tx queue which could 
stall the whole tx queue.

Thanks
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