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

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Message-ID: <20120417060752.GB20674@redhat.com>
Date:	Tue, 17 Apr 2012 09:07:52 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	netdev@...r.kernel.org, xma@...ibm.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com
Subject: Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT

On Tue, Apr 17, 2012 at 01:54:55PM +0800, Jason Wang wrote:
> 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.

Yes but do you have a test that makes guest hit EFAULT or EINVAL?

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