Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT

[Jason Wang <jasowang@xxxxxxxxxx>]

On 04/16/2012 03:16 PM, Michael S. Tsirkin wrote:
> On Mon, Apr 16, 2012 at 02:08:33PM +0800, Jason Wang wrote:
> > Currently, we restart tx polling unconditionally when sendmsg()
> > fails. This would cause unnecessary wakeups of vhost wokers as it's
> > only needed when the socket send buffer were exceeded.
> Why is this a problem?

This issue is when guest driver is able to hit the -EFAULT, vhost 
discard the the descriptor and restart the polling. This would wake 
vhost thread and repeat the loop again which waste cpu.

Another possible solution is don't discard the descriptor.
> > Fix this by
> > restart the tx polling only when sendmsg() returns value other than
> > -EFAULT.
> >
> > Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > ---
> >   drivers/vhost/net.c |    3 ++-
> >   1 files changed, 2 insertions(+), 1 deletions(-)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 29abd65..035fa95 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -262,7 +262,8 @@ static void handle_tx(struct vhost_net *net)
> >   					UIO_MAXIOV;
> >   			}
> >   			vhost_discard_vq_desc(vq, 1);
> > -			tx_poll_start(net, sock);
> > +			if (err != -EFAULT)
> > +				tx_poll_start(net, sock);
> >   			break;
> >   		}
> >   		if (err != len)
> --
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