From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT Date: Tue, 17 Apr 2012 09:07:52 +0300 Message-ID: <20120417060752.GB20674@redhat.com> References: <20120416060749.14140.19433.stgit@intel-e5620-16-2.englab.nay.redhat.com> <20120416060833.14140.28139.stgit@intel-e5620-16-2.englab.nay.redhat.com> <20120416071646.GB25396@redhat.com> <4F8BD81A.7010507@redhat.com> <20120416133859.GB13190@redhat.com> <4F8CE305.9090100@redhat.com> <20120417045736.GA31278@redhat.com> <4F8D05AF.4000309@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@vger.kernel.org, xma@us.ibm.com, davem@davemloft.net, linux-kernel@vger.kernel.org, ebiederm@xmission.com To: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: <4F8D05AF.4000309@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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?