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

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Message-ID: <20170403144047.GA3820@lerouge>
Date:   Mon, 3 Apr 2017 16:40:49 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Mike Galbraith <efault@....de>
Cc:     Rik van Riel <riel@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Thu, Mar 30, 2017 at 03:35:22PM +0200, Mike Galbraith wrote:
> On Thu, 2017-03-30 at 09:02 -0400, Rik van Riel wrote:
> > On Thu, 2017-03-30 at 14:51 +0200, Frederic Weisbecker wrote:
> 
> > > Also, why does it raise power consumption issues?
> > 
> > On a system without either nohz_full or nohz idle
> > mode, skewed ticks result in CPU cores waking up
> > at different times, and keeping an idle system
> > consuming power for more time than it would if all
> > the ticks happened simultaneously.
> 
> And if your server farm is mostly idle, that power savings may delay
> your bankruptcy proceedings by a whole microsecond ;-)
> 
> Or more seriously, what skew does do on boxen of size X today is
> something for perf to say.  At the time, removal was very bad for my 8
> socket box, and allegedly caused huge SGI beasts in horrific pain.

I see.
Nohz_full is already bad for powersavings anyway. CPU 0 always ticks :-)