From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753313AbcHPXIa (ORCPT ); Tue, 16 Aug 2016 19:08:30 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:35619 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752053AbcHPXI3 (ORCPT ); Tue, 16 Aug 2016 19:08:29 -0400 MIME-Version: 1.0 In-Reply-To: <1471356104.32433.38.camel@redhat.com> References: <1468421405-20056-1-git-send-email-fweisbec@gmail.com> <1468421405-20056-2-git-send-email-fweisbec@gmail.com> <1470751579.13905.77.camel@redhat.com> <20160810125212.78564dc2@annuminas.surriel.com> <1470969892.13905.120.camel@redhat.com> <20160812115803.0f26211c@annuminas.surriel.com> <1471273244.32433.22.camel@redhat.com> <1471313483.32433.33.camel@redhat.com> <1471356104.32433.38.camel@redhat.com> From: Wanpeng Li Date: Wed, 17 Aug 2016 07:08:26 +0800 Message-ID: Subject: Re: [PATCH] time,virt: resync steal time when guest & host lose sync To: Rik van Riel Cc: Frederic Weisbecker , Ingo Molnar , LKML , Paolo Bonzini , Peter Zijlstra , Wanpeng Li , Thomas Gleixner , Radim Krcmar , Mike Galbraith Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2016-08-16 22:01 GMT+08:00 Rik van Riel : > On Tue, 2016-08-16 at 14:54 +0800, Wanpeng Li wrote: >> 2016-08-16 10:11 GMT+08:00 Rik van Riel : >> > On Tue, 2016-08-16 at 09:31 +0800, Wanpeng Li wrote: >> > > 2016-08-15 23:00 GMT+08:00 Rik van Riel : >> > > > On Mon, 2016-08-15 at 16:53 +0800, Wanpeng Li wrote: >> > > > > 2016-08-12 23:58 GMT+08:00 Rik van Riel : >> > > > > [...] >> > > > > > Wanpeng, does the patch below work for you? >> > > > > >> > > > > It will break steal time for full dynticks guest, and there >> > > > > is a >> > > > > calltrace of thread_group_cputime_adjusted call stack, RIP is >> > > > > cputime_adjust+0xff/0x130. >> > > > >> > > > How? This patch is equivalent to passing ULONG_MAX to >> > > > steal_account_process_time, which you tried to no ill >> > > > effect before. >> > > >> > > https://lkml.org/lkml/2016/6/8/404/ Paolo original suggested to >> > > add >> > > the max cputime limit to the vtime, when the cpu is running in >> > > nohz >> > > full mode and stop the tick, jiffies will be updated depends on >> > > clock >> > > source instead of clock event device in >> > > guest(tick_nohz_update_jiffies() callsite, ktime_get()), so it >> > > will >> > > not be affected by lost clock ticks, my patch keeps the limit for >> > > vtime and remove the limit to non-vtime. However, your patch >> > > removes >> > > the limit for both scenarios and results in the below calltrace >> > > for >> > > vtime. >> > >> > I understand what it does. >> > >> > What I would like to understand is WHY enforcing the limit >> > is the right thing when using vtime, and the wrong thing >> > in all other scenarios. >> >> I observed that function get_vtime_delta() underflow which means that >> delta < other when debugging your bugfix patch, I believe that is why >> Paolo suggested to add the max cputime limit to vtime, he also >> pointed >> out the potentional underflow before >> https://lkml.org/lkml/2016/6/8/404/ > > Looking at get_vtime_delta() I can see exactly how the underflow > can happen. The interval returned by account_other_time() is NOT > rounded down to the nearest jiffy, while the base interval it is > subtracted from is. > > Furthermore, even if we did not have that rounding issue, a guest > could get preempted in-between determining delta, and calling > account_other_time(), which could also cause the issue. > > Could you re-send your patch with a comment in get_vtime_delta(), > as well as the changelog, explaining exactly why account_other_time() > should be limited from get_vtime_delta(), but not from the other > three call sites? > > Documentation could save future developers a bunch of debugging > time on this code. Will do. Thanks for bearing with me through such a long discussion, I'm very happy we finally come to an agreement. :) Regards, Wanpeng Li