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

[Jason Wang]

On 05/17/2012 03:08 AM, Shirley Ma wrote:
> On Wed, 2012-05-16 at 21:36 +0300, Michael S. Tsirkin wrote:
> > On Wed, May 16, 2012 at 10:32:05AM -0700, 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.
> > > > >
> > > > > 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
> > Donnu. I see virtio sending packets but they do not get
> > to tun on host. debugging.
> I looked at the code, if (zerocopy) check is needed for below code:
>
> +	if (zerocopy) {
>                          num_pends = likely(vq->upend_idx>= vq->done_idx) ?
>                                      (vq->upend_idx - vq->done_idx) :
>                                      (vq->upend_idx + UIO_MAXIOV - vq->done_idx);
>                          if (unlikely(num_pends>  VHOST_MAX_PEND)) {
>                                  tx_poll_start(net, sock);
> 				vhost_poll_queue
>                                  set_bit(SOCK_ASYNC_NOSPACE,&sock->flags);
>                                  break;
>                          }
> +	}
>                          if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>                                  vhost_disable_notify(&net->dev, vq);
>                                  continue;
>                          }
>                          break;

No objection to add this but looks no difference to me as we won't touch 
upend_idx and done_idx when zercocopy is not used.

> Second, whether it's possible the problem comes from tx_poll_start()? In
> some situation, vhost_poll_wakeup() is not being called?
>
> Thanks
> Shirley
>
>
>
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