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: Mon, 16 Apr 2012 16:39:00 +0300 Message-ID: <20120416133859.GB13190@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> 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: <4F8BD81A.7010507@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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? > Another possible solution is don't discard the descriptor. > > > >>Fix this by > >>restart the tx polling only when sendmsg() returns value other than > >>-EFAULT. > >> > >>Signed-off-by: Jason Wang > >>--- > >> drivers/vhost/net.c | 3 ++- > >> 1 files changed, 2 insertions(+), 1 deletions(-) > >> > >>diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > >>index 29abd65..035fa95 100644 > >>--- a/drivers/vhost/net.c > >>+++ b/drivers/vhost/net.c > >>@@ -262,7 +262,8 @@ static void handle_tx(struct vhost_net *net) > >> UIO_MAXIOV; > >> } > >> vhost_discard_vq_desc(vq, 1); > >>- tx_poll_start(net, sock); > >>+ if (err != -EFAULT) > >>+ tx_poll_start(net, sock); > >> break; > >> } > >> if (err != len) > >-- > >To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > >the body of a message to majordomo@vger.kernel.org > >More majordomo info at http://vger.kernel.org/majordomo-info.html > >Please read the FAQ at http://www.tux.org/lkml/