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

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Message-ID: <1490878672.28917.13.camel@redhat.com>
Date:   Thu, 30 Mar 2017 08:57:52 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Thu, 2017-03-30 at 00:54 +0200, Frederic Weisbecker wrote:
> (Adding Thomas in Cc)
> 
> On Wed, Mar 29, 2017 at 04:08:45PM -0400, Rik van Riel wrote:
> > 
> > Frederic, can you think of any reason why
> > the tick on nohz_full CPUs would end up aligned
> > with the tick on cpu0, instead of running at some
> > random offset?
> 
> tick_init_jiffy_update() takes that decision to align all ticks.
> 
> I'm not sure why. 

I don't see why that would matter, either.

> I'm not sure that randomizing the tick start per CPU would be a
> right solution. Somewhere in the world you can be sure the tick
> randomization of some nohz_full CPU will coincide with the tick
> of CPU 0 :o)
> 
> Or we could force that tick on nohz_full CPUs to be far from
> CPU 0's tick... I'm not sure such a solution would be accepted
> though.

I am not sure we would have to force things.

Simply getting rid of tick_init_jiffy_update
and scheduling the next tick for "now + tick
period" might have the same effect, when the
tick gets stopped and restarted on nohz_full
CPUs.