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

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Message-ID: <20170330134400.GD3626@lerouge>
Date:   Thu, 30 Mar 2017 15:44:01 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Rik van Riel <riel@...hat.com>
Cc:     Mike Galbraith <efault@....de>,
        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 09:02:31AM -0400, Rik van Riel wrote:
> On Thu, 2017-03-30 at 14:51 +0200, Frederic Weisbecker wrote:
> > On Thu, Mar 30, 2017 at 06:27:31AM +0200, Mike Galbraith wrote:
> > > On Wed, 2017-03-29 at 16:08 -0400, Rik van Riel wrote:
> > > 
> > > > A random offset, or better yet a somewhat randomized
> > > > tick length to make sure that simultaneous ticks are
> > > > fairly rare and the vtime sampling does not end up
> > > > "in phase" with the jiffies incrementing, could make
> > > > the accounting work right again.
> > > 
> > > That improves jitter, especially on big boxen.  I have an 8 socket
> > > box
> > > that thinks it's an extra large PC, there, collision avoidance
> > > matters
> > > hugely.  I couldn't reproduce bean counting woes, no idea if
> > > collision
> > > avoidance will help that.
> > 
> > Out of curiosity, where is the main contention between ticks? I
> > indeed
> > know some locks that can be taken on special cases, such as posix cpu
> > timers.
> > 
> > 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.

Ah fair point!

> 
> This is not a factor at all on systems that switch
> off the tick while idle, since the CPU will be busy
> anyway while the tick is enabled.

I see. Thanks!