Messages in this thread | | | Date | Tue, 17 Apr 2012 13:18:20 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT |
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On Tue, Apr 17, 2012 at 02:30:27PM +0800, Jason Wang wrote: > On 04/17/2012 02:07 PM, Michael S. Tsirkin wrote: > >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? > > Looks like we can do this by supplying an invalid hdr_len in vnet > header as tap does the check for this.
Ah so you patched qemu to do this? Cool. Can you post the patch for testing pls?
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