From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT Date: Tue, 17 Apr 2012 14:30:27 +0800 Message-ID: <4F8D0E03.7030105@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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, xma@us.ibm.com, davem@davemloft.net, linux-kernel@vger.kernel.org, ebiederm@xmission.com To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:22371 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754105Ab2DQGah (ORCPT ); Tue, 17 Apr 2012 02:30:37 -0400 In-Reply-To: <20120417060752.GB20674@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: 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. > -- > 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