linux-kernel - Re: High CPU load when machine is idle (related to PROBLEM: Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)

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Message-ID: <1291030726.32004.4.camel@laptop>
Date:	Mon, 29 Nov 2010 12:38:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Damien Wyart <damien.wyart@...e.fr>
Cc:	tmhikaru@...il.com, Venkatesh Pallipadi <venki@...gle.com>,
	Chase Douglas <chase.douglas@...onical.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Kyle McMartin <kyle@...artin.ca>
Subject: Re: High CPU load when machine is idle (related to PROBLEM:
 Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)

On Sun, 2010-11-28 at 12:40 +0100, Damien Wyart wrote:
> Hi,
> 
> * Peter Zijlstra <peterz@...radead.org> [2010-11-27 21:15]:
> > How does this work for you? Its hideous but lets start simple.
> > [...]
> 
> Doesn't give wrong numbers like initial bug and tentative patches, but
> feels a bit too slow when numbers go up and down. Correct values are
> reached when waiting long enough, but it feels slow.
> 
> As I've tested many combinations, maybe this is an impression because
> I do not remember about "normal" delays for the load to rise and fall,
> but this still feels slow.

You can test this by either booting with nohz=off, or builting with
CONFIG_NO_HZ=n and then comparing the result, something like

make O=defconfig clean; while sleep 10; do uptime >> load.log; done &
make -j32 O=defconfig; kill %1

And comparing the curves between the NO_HZ and !NO_HZ kernels.

I'll try and make the patch less hideous ;-)
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