Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754161AbdDKLDX (ORCPT ); Tue, 11 Apr 2017 07:03:23 -0400 Received: from mail-oi0-f67.google.com ([209.85.218.67]:33398 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753077AbdDKLDT (ORCPT ); Tue, 11 Apr 2017 07:03:19 -0400 MIME-Version: 1.0 In-Reply-To: <20170330133802.GC3626@lerouge> References: <20170323165512.60945ac6@redhat.com> <1490636129.8850.76.camel@redhat.com> <20170328132406.7d23579c@redhat.com> <20170329131656.1d6cb743@redhat.com> <1490818125.28917.11.camel@redhat.com> <1490848051.4167.57.camel@gmx.de> <20170330133802.GC3626@lerouge> From: Wanpeng Li Date: Tue, 11 Apr 2017 19:03:17 +0800 Message-ID: Subject: Re: [BUG nohz]: wrong user and system time accounting To: Frederic Weisbecker Cc: Mike Galbraith , Rik van Riel , Luiz Capitulino , "linux-kernel@vger.kernel.org" , Peter Zijlstra , Thomas Gleixner Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1873 Lines: 46 2017-03-30 21:38 GMT+08:00 Frederic Weisbecker : > On Thu, Mar 30, 2017 at 02:47:11PM +0800, Wanpeng Li wrote: [...] > >> >> -------------------------------------->8----------------------------------------------------- >> >> use nanosecond granularity to check deltas but only perform an actual >> cputime update when that delta >= TICK_NSEC. >> >> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c >> index f3778e2b..f1ee393 100644 >> --- a/kernel/sched/cputime.c >> +++ b/kernel/sched/cputime.c >> @@ -676,18 +676,21 @@ void thread_group_cputime_adjusted(struct >> task_struct *p, u64 *ut, u64 *st) >> #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN >> static u64 vtime_delta(struct task_struct *tsk) >> { >> - unsigned long now = READ_ONCE(jiffies); >> + u64 now = local_clock(); > > I fear we need a global clock, because the reader (task_cputime()) needs > to compute the delta and therefore use the same clock from any CPU. > > Or we can use the local_clock() but the reader must access the same. > > So there would be vtime_delta_writer() which uses local_clock and stores > the current CPU to tsk->vtime_cpu (under the vtime_seqcount). And then > vtime_delta_reader() which calls sched_clock_cpu(tsk->vtime_cpu) which > is protected by vtime_seqcount as well. > > Although those sched_clock_cpu() things seem to only matter when the > sched_clock() is unstable. And that stability is a condition for nohz_full > to work anyway. So probably sched_clock() alone would be enough. I observed ~60% user time and ~40% sys time when replace local_clock() above by sched_clock()(two cpu hogs on the cpu in nohz_full mode). In addition, Luiz's testcast ./acct-bug 1 995 will show 100% idle time. If keep local_clock() in vtime_delta(), cpu hogs testcase will success. However, Luiz's testcase still show 100% idle time. Regards, Wanpeng Li