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

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Message-ID: <20120516151444.GC9934@redhat.com>
Date:	Wed, 16 May 2012 18:14:44 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	Jason Wang <jasowang@...hat.com>, eric.dumazet@...il.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com, davem@...emloft.net
Subject: Re: [V2 PATCH 9/9] vhost: zerocopy: poll vq in zerocopy callback

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.


-- 
MST
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