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

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Message-ID: <20170330125157.GB3626@lerouge>
Date:   Thu, 30 Mar 2017 14:51:58 +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 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?

Thanks.