linux-kernel - Re: [3.2.16 -> 3.2.17 regression] High reported CPU load when idle

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Date:	Wed, 23 May 2012 16:53:59 -0500
From:	Jonathan Nieder <jrnieder@...il.com>
To:	Anders Boström <anders@...insight.net>
Cc:	linux-kernel@...r.kernel.org,
	Lesław Kopeć <leslaw.kopec@...za-klasa.pl>,
	Aman Gupta <aman@...1.net>, Doug Smythies <dsmythies@...us.net>
Subject: Re: [3.2.16 -> 3.2.17 regression] High reported CPU load when idle

Hi Anders,

Anders Boström wrote[1]:

> Starting with 3.2.17-1, the CPU load accounting is broken when the
> computer is idle. The CPU load is reported as >0.50 when
> idle. 3.2.16-1 don't suffer from this problem.
>
> Suspected patch is the upstream patch
> "sched: Fix nohz load accounting -- again!"
> commit 5e2d50da11f0e6ec3ce8fe658d7c83b0b4346c68 to 3.2 and
> originating from c308b56b5398779cd3da0f62ab26b0453494c3d4 .
>
> See also:
>
> https://bugs.launchpad.net/unity/+bug/991370
> https://lkml.org/lkml/2012/5/22/310
> https://bugzilla.redhat.com/show_bug.cgi?id=822877
> https://bbs.archlinux.org/viewtopic.php?id=141289

Thanks for writing.

If I understand correctly, the load average calculation both before
and after that commit is broken, in different ways.

I'm cc-ing Lesław Kopeć, Aman Gupta, and Doug Smythies who worked
on the above change[2].  I recommend pulling Thomas Gleixner
<tglx@...utronix.de> into the conversation once you have a better
idea of what's going on or a new change to recommend.  If you'd like
to also track this on a bugtracker, http://bugzilla.kernel.org/,
product Process Management, component Scheduler might be a good place.

Aside from that, I can't really offer much to help you, but others
on linux-kernel might.

Hope that helps,
Jonathan

[1] http://bugs.debian.org/674153
[2] http://thread.gmane.org/gmane.linux.kernel/1249223/focus=1262319
[3] http://thread.gmane.org/gmane.linux.kernel/1291870/focus=1292058
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