Subject: Re: [V2 PATCH 9/9] vhost: zerocopy: poll vq in zerocopy callback
From: Shirley Ma <mashirle@us.ibm.com>
To: Jason Wang <jasowang@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, eric.dumazet@gmail.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        ebiederm@xmission.com, davem@davemloft.net
In-Reply-To: <4FB61D57.4030103@redhat.com>
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Date: Fri, 18 May 2012 08:29:34 -0700
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On Fri, 2012-05-18 at 17:58 +0800, Jason Wang wrote:
> On 05/17/2012 11:34 PM, Shirley Ma wrote:
> > On Thu, 2012-05-17 at 10:50 +0800, Jason Wang wrote:
> >> 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.
> > VHOST_MAX_PEND should handle the capacity.
> >
> > Hasn't the above situation been handled in handle_tx() code?:
> > ...
> >                          if (unlikely(num_pends>  VHOST_MAX_PEND)) {
> >                                  tx_poll_start(net, sock);
> >
> set_bit(SOCK_ASYNC_NOSPACE,&sock->flags);
> >                                  break;
> >                          }
> > ...
> >
> > Thanks
> > Shirley
> 
> It may not help in because:
> 
> - tx polling depends on skb_orphan() which is often called by device 
> driver when it place the packet into the queue of the devices instead 
> of  when the packets were sent. So it was too early for vhost to be 
> notified.
Then do you think it's better to replace with vhost_poll_queue here
instead?

> - it only works when the pending DMAs exceeds VHOST_MAX_PEND, it's 
> highly possible that guest needs to be notified when the pending
> packets 
> isn't so much.
In which situation the guest needs to be notified when there is no TX
besides buffers run out?

> So this piece of code may not help and could be removed and we need
> to 
> poll the virt-queue during zerocopy callback ( through it could be 
> further optimized but may not be easy). 

Thanks
Shirley

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