From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750861AbcB2FQD (ORCPT ); Mon, 29 Feb 2016 00:16:03 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40556 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750704AbcB2FQA (ORCPT ); Mon, 29 Feb 2016 00:16:00 -0500 Subject: Re: [PATCH V3 3/3] vhost_net: basic polling support To: "Michael S. Tsirkin" References: <1456476164-17242-1-git-send-email-jasowang@redhat.com> <1456476164-17242-4-git-send-email-jasowang@redhat.com> <20160228140937.GA8855@redhat.com> Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, RAPOPORT@il.ibm.com, yang.zhang.wz@gmail.com From: Jason Wang Message-ID: <56D3D404.6080600@redhat.com> Date: Mon, 29 Feb 2016 13:15:48 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <20160228140937.GA8855@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/28/2016 10:09 PM, Michael S. Tsirkin wrote: > On Fri, Feb 26, 2016 at 04:42:44PM +0800, Jason Wang wrote: >> > This patch tries to poll for new added tx buffer or socket receive >> > queue for a while at the end of tx/rx processing. The maximum time >> > spent on polling were specified through a new kind of vring ioctl. >> > >> > Signed-off-by: Jason Wang > Looks good overall, but I still see one problem. > >> > --- >> > drivers/vhost/net.c | 79 +++++++++++++++++++++++++++++++++++++++++++--- >> > drivers/vhost/vhost.c | 14 ++++++++ >> > drivers/vhost/vhost.h | 1 + >> > include/uapi/linux/vhost.h | 6 ++++ >> > 4 files changed, 95 insertions(+), 5 deletions(-) >> > >> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c >> > index 9eda69e..c91af93 100644 >> > --- a/drivers/vhost/net.c >> > +++ b/drivers/vhost/net.c >> > @@ -287,6 +287,44 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success) >> > rcu_read_unlock_bh(); >> > } >> > >> > +static inline unsigned long busy_clock(void) >> > +{ >> > + return local_clock() >> 10; >> > +} >> > + >> > +static bool vhost_can_busy_poll(struct vhost_dev *dev, >> > + unsigned long endtime) >> > +{ >> > + return likely(!need_resched()) && >> > + likely(!time_after(busy_clock(), endtime)) && >> > + likely(!signal_pending(current)) && >> > + !vhost_has_work(dev) && >> > + single_task_running(); > So I find it quite unfortunate that this still uses single_task_running. > This means that for example a SCHED_IDLE task will prevent polling from > becoming active, and that seems like a bug, or at least > an undocumented feature :). Yes, it may need more thoughts. > > Unfortunately this logic affects the behaviour as observed > by userspace, so we can't merge it like this and tune > afterwards, since otherwise mangement tools will start > depending on this logic. > > How about remove single_task_running() first here and optimize on top? We probably need something like this to handle overcommitment.