linux-kernel - Re: [BUG nohz]: wrong user and system time accounting

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Message-ID: <20170329125635.GA8306@lerouge>
Date:   Wed, 29 Mar 2017 14:56:37 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Wed, Mar 29, 2017 at 05:56:30PM +0800, Wanpeng Li wrote:
> 2017-03-29 5:26 GMT+08:00 Luiz Capitulino <lcapitulino@...hat.com>:
> > On Tue, 28 Mar 2017 17:01:52 -0400
> > Rik van Riel <riel@...hat.com> wrote:
> >
> >> On Tue, 2017-03-28 at 16:14 -0400, Luiz Capitulino wrote:
> >> > On Tue, 28 Mar 2017 13:24:06 -0400
> >> > Luiz Capitulino <lcapitulino@...hat.com> wrote:
> >> > > I'm starting to suspect that the nohz code may be programming
> >> > > the tick period to be shorter than 1ms when it re-activates
> >> > > the tick.
> >> >
> >> > And I think I was right, it looks like the nohz code is programming
> >> > the tick period incorrectly when restarting the tick. The patch below
> >> > fixes things for me, but I still have some homework todo and more
> >> > testing before posting a patch for inclusion. Could you guys test it?
> >>
> >> Your patch seems to work. I don't claim to understand why
> >> your patch makes a difference, but for this particular test
> >> case, on this particular setup, it seems to work...
> >
> > I don't fully understand why either yet. I was looking for places
> > where nohz might be programming the tick period incorrectly and
> 
> The bug is still present when I config CONTEXT_TRACKING_FORCE and
> nohz=off in the boot parameter.

Indeed I saw something similar a few days ago with:

    !CONFIG_NO_HZ_FULL && CONFIG_VIRT_CPU_ACCOUNTING_GEN && CONTEXT_TRACKING_FORCE

And it disappeared with CONFIG_NO_HZ_FULL=y so I didn't care much because that setting
isn't used in production and in fact I intend to remove CONTEXT_TRACKING_FORCE. But
it could be the sign of something important.

It might be different than Luiz's bug because I can't reproduce his bug yet even with
his config.