From: Frederic Weisbecker <fweisbec@gmail.com>
To: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Wanpeng Li <kernellwp@gmail.com>, Mike Galbraith <efault@gmx.de>,
Rik van Riel <riel@redhat.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [BUG nohz]: wrong user and system time accounting
Date: Mon, 3 Apr 2017 17:23:17 +0200 [thread overview]
Message-ID: <20170403152315.GA4221@lerouge> (raw)
In-Reply-To: <20170331231119.2b78eb1f@redhat.com>
On Fri, Mar 31, 2017 at 11:11:19PM -0400, Luiz Capitulino wrote:
> On Sat, 1 Apr 2017 01:24:54 +0200
> Frederic Weisbecker <fweisbec@gmail.com> wrote:
>
> > On Fri, Mar 31, 2017 at 04:09:10PM -0400, Luiz Capitulino wrote:
> > > On Thu, 30 Mar 2017 17:25:46 -0400
> > > Luiz Capitulino <lcapitulino@redhat.com> wrote:
> > >
> > > > On Thu, 30 Mar 2017 16:18:17 +0200
> > > > Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > >
> > > > > On Thu, Mar 30, 2017 at 09:59:54PM +0800, Wanpeng Li wrote:
> > > > > > 2017-03-30 21:38 GMT+08:00 Frederic Weisbecker <fweisbec@gmail.com>:
> > > > > > > If it works, we may want to take that solution, likely less performance sensitive
> > > > > > > than using sched_clock(). In fact sched_clock() is fast, especially as we require it to
> > > > > > > be stable for nohz_full, but using it involves costly conversion back and forth to jiffies.
> > > > > >
> > > > > > So both Rik and you agree with the skew tick solution, I will try it
> > > > > > tomorrow. Btw, if we should just add random offset to the cpu in the
> > > > > > nohz_full mode or add random offset to all cpus like the codes above?
> > > > >
> > > > > Lets just keep it to all CPUs for simplicty.
> > > > > Also please add a comment that explains why we need that skew_tick on nohz_full.
> > > >
> > > > I've tried all the test-cases we discussed in this thread with skew_tick=1
> > > > and it worked as expected in bare-metal and KVM guests.
> > > >
> > > > However, I found a test-case that works in bare-metal but show problems
> > > > in KVM guests. It could something that's KVM specific, or it could be
> > > > something that's harder to reproduce in bare-metal.
> > >
> > > After discussing some findings on this issue with Rik, I realized that
> > > we don't add the skew when restarting the tick in tick_nohz_restart().
> > > Adding the offset there seems to solve this problem.
> >
> > Are you sure? tick_nohz_restart() doesn't seem to override the initial skew. It
> > always forwards the expiration time on top of the last tick.
>
> OK, I'll double check. Without my change the bug triggers almost
> instantly with the described reproducer. With my change it didn't
> trig for several minutes (but it does look wrong looking at it now).
Do you observe aligned ticks with trace events (hrtimer_expire_entry)?
You might want to enforce the global clock to trace that:
echo "global" > /sys/kernel/debug/tracing/trace_clock
next prev parent reply other threads:[~2017-04-03 15:23 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-23 20:55 [BUG nohz]: wrong user and system time accounting Luiz Capitulino
2017-03-24 0:56 ` Rik van Riel
2017-03-24 1:05 ` Luiz Capitulino
2017-03-24 1:08 ` Rik van Riel
2017-03-24 1:39 ` Luiz Capitulino
2017-03-27 5:33 ` lkml
2017-03-24 1:52 ` Wanpeng Li
2017-03-24 3:56 ` Luiz Capitulino
2017-03-27 1:56 ` Wanpeng Li
2017-03-27 17:35 ` Rik van Riel
2017-03-28 7:19 ` Wanpeng Li
[not found] ` <20170328132406.7d23579c@redhat.com>
[not found] ` <20170328161454.4a5d9e8b@redhat.com>
2017-03-28 21:01 ` Rik van Riel
2017-03-28 21:26 ` Luiz Capitulino
2017-03-29 9:56 ` Wanpeng Li
2017-03-29 12:56 ` Frederic Weisbecker
2017-03-28 21:24 ` Rik van Riel
2017-03-28 21:30 ` Luiz Capitulino
[not found] ` <20170329131656.1d6cb743@redhat.com>
2017-03-29 20:08 ` Rik van Riel
2017-03-29 22:54 ` Frederic Weisbecker
2017-03-30 12:57 ` Rik van Riel
2017-03-30 1:58 ` Wanpeng Li
2017-03-30 12:40 ` Frederic Weisbecker
2017-03-30 13:19 ` Mike Galbraith
2017-03-30 4:27 ` Mike Galbraith
2017-03-30 6:47 ` Wanpeng Li
2017-03-30 11:52 ` Wanpeng Li
2017-03-30 12:33 ` Mike Galbraith
2017-03-30 13:38 ` Frederic Weisbecker
2017-03-30 13:59 ` Wanpeng Li
2017-03-30 14:18 ` Frederic Weisbecker
2017-03-30 21:25 ` Luiz Capitulino
2017-03-31 20:09 ` Luiz Capitulino
2017-03-31 23:24 ` Frederic Weisbecker
2017-04-01 3:11 ` Luiz Capitulino
2017-04-03 15:23 ` Frederic Weisbecker [this message]
2017-04-03 19:06 ` Luiz Capitulino
2017-04-04 17:36 ` Luiz Capitulino
2017-04-05 14:26 ` Rik van Riel
2017-04-11 11:03 ` Wanpeng Li
2017-04-11 11:36 ` Peter Zijlstra
2017-04-11 11:43 ` Wanpeng Li
2017-04-11 14:22 ` Thomas Gleixner
2017-04-12 13:18 ` Frederic Weisbecker
2017-04-12 14:57 ` Thomas Gleixner
2017-04-12 15:14 ` Frederic Weisbecker
2017-04-13 4:31 ` Wanpeng Li
2017-04-13 13:32 ` Frederic Weisbecker
2017-05-02 10:01 ` Wanpeng Li
2017-05-15 8:17 ` Wanpeng Li
2017-06-29 17:22 ` Frederic Weisbecker
2017-03-30 12:51 ` Frederic Weisbecker
2017-03-30 13:02 ` Rik van Riel
2017-03-30 13:35 ` Mike Galbraith
2017-04-03 14:40 ` Frederic Weisbecker
2017-04-04 7:32 ` Mike Galbraith
2017-03-30 13:44 ` Frederic Weisbecker
[not found] ` <20170329221700.GB23895@lerouge>
2017-03-29 22:46 ` Wanpeng Li
2017-03-30 2:14 ` Luiz Capitulino
2017-03-30 12:27 ` Wanpeng Li
2017-03-27 18:38 ` Luiz Capitulino
2017-03-28 5:28 ` Wanpeng Li
2017-03-28 13:44 ` Luiz Capitulino
2017-03-29 13:04 ` Frederic Weisbecker
2017-03-29 13:14 ` Rik van Riel
2017-03-29 13:23 ` Luiz Capitulino
2017-03-29 21:12 ` Frederic Weisbecker
2017-03-30 1:48 ` Luiz Capitulino
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