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

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Date:	Tue, 05 Jun 2012 17:35:23 +0200
From:	Lesław Kopeć <leslaw.kopec@...za-klasa.pl>
To:	Anders Boström <anders@...insight.net>
CC:	dsmythies@...us.net, jrnieder@...il.com,
	linux-kernel@...r.kernel.org, aman@...1.net,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de
Subject: Re: [3.2.16 -> 3.2.17 regression] High reported CPU load when idle

On 05/30/2012 04:54 PM, Anders Boström wrote:

>  DS> This statement: "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."
>  DS> In my opinion has the following mistakes:
>  DS> . The computer is not actually idle. If it was actually idle the reported load average would be 0.
> 
> Well, I tested in single user mode, with very few processes running,
> mostly init, getty, bash and top (+ a lot of kernel threads). And
> 3.2.17 reported a load of >0.5 . Under the same conditions 3.2.16
> typically reports 0.01 or 0.00 .

I've tried to reproduce the problem, but haven't had much luck. I've
tested vanilla and Debian kernels versions 3.2.16 and 3.2.17. Load on an
idle or slightly busy system is the same across all versions.

vanilla 3.2.16		0.15 0.07 0.06
vanilla 3.2.17		0.17 0.11 0.13
Debian 3.2.16-1		0.13 0.07 0.05
Debian 3.2.17-1		0.10 0.09 0.11

When the system is completely idle load drops to 0. I've also tried
3.2.17 with 556061b00c9f, but it makes no difference and in comparison
to plain 3.2.17 load is the same even on a busy system.

I can't explain why we're getting different results on the same kernels.
If you'd like more details just ask.

-- 
Lesław Kopeć

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