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

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Date:	Thu, 24 May 2012 16:45:17 -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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [3.2.16 -> 3.2.17 regression] High reported CPU load when idle

(cc-ing Peter and Thomas because there is a nice graph)
> 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

I just found [1] from [2] which seems to describe the symptoms pretty
well.  Peter, Thomas, advice?

Anders et al: does 556061b00c9f ("sched/nohz: Fix rq->cpu_load[]
calculations", 2012-05-11) change anything?

Thanks,
Jonathan

[1] https://launchpadlibrarian.net/105809696/commit_low_load_rev2.png
[2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/838811
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