From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754117Ab2DQKqv (ORCPT ); Tue, 17 Apr 2012 06:46:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:1025 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753468Ab2DQKqt (ORCPT ); Tue, 17 Apr 2012 06:46:49 -0400 Message-ID: <4F8D4A01.9040901@redhat.com> Date: Tue, 17 Apr 2012 18:46:25 +0800 From: Jason Wang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1 MIME-Version: 1.0 To: "Michael S. Tsirkin" 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 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> <20120417101820.GB20905@redhat.com> In-Reply-To: <20120417101820.GB20905@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/17/2012 06:18 PM, Michael S. Tsirkin wrote: > 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? > No, I mean patch the guest driver like this: diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 019da01..6e2f487 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -582,7 +582,7 @@ static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb) } if (skb_is_gso(skb)) { - hdr->hdr.hdr_len = skb_headlen(skb); + hdr->hdr.hdr_len = 65535; hdr->hdr.gso_size = skb_shinfo(skb)->gso_size; if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) hdr->hdr.gso_type = VIRTIO_NET_HDR_GSO_TCPV4; btw, If we still choose to drop the packet, we can hit -EFAULT by send a descriptor with a large number of pages.