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

[Jason Wang]

On 04/17/2012 12:57 PM, Michael S. Tsirkin wrote:
> On Tue, Apr 17, 2012 at 11:27:01AM +0800, Jason Wang wrote:
> > On 04/16/2012 09:39 PM, Michael S. Tsirkin wrote:
> > > On Mon, Apr 16, 2012 at 04:28:10PM +0800, Jason Wang wrote:
> > > > >   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.
> > > Does same thing happen if we get an error from copy from user?
> > Right, so do you think it makes sense that we only restart polling
> > on -EAGAIN or -ENOBUFS?
> Sounds OK. BTW how do you test this?

Not very hard, w/o this patch, we can see almost 100% cpu utilization 
for vhost thread if guest hit EFAULT or EINVAL. With this patch, the cpu 
utilization should be very low I think.
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