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Subject: Re: [BUG nohz]: wrong user and system time accounting
From: Rik van Riel <riel@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>,
        Mike Galbraith <efault@gmx.de>
Cc: Luiz Capitulino <lcapitulino@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org
Date: Thu, 30 Mar 2017 09:02:31 -0400
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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.

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.