From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752302Ab2DQKSN (ORCPT ); Tue, 17 Apr 2012 06:18:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:7078 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750909Ab2DQKSL (ORCPT ); Tue, 17 Apr 2012 06:18:11 -0400 Date: Tue, 17 Apr 2012 13:18:20 +0300 From: "Michael S. Tsirkin" To: Jason Wang Cc: netdev@vger.kernel.org, xma@us.ibm.com, davem@davemloft.net, linux-kernel@vger.kernel.org, ebiederm@xmission.com Subject: Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT Message-ID: <20120417101820.GB20905@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> <20120417060752.GB20674@redhat.com> <4F8D0E03.7030105@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4F8D0E03.7030105@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? > >-- > >To unsubscribe from this list: send the line "unsubscribe netdev" in > >the body of a message to majordomo@vger.kernel.org > >More majordomo info at http://vger.kernel.org/majordomo-info.html