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

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Date:	Mon, 11 Jun 2012 23:12:04 -0700
From:	"Doug Smythies" <dsmythies@...us.net>
To:	"'Jonathan Nieder'" <jrnieder@...il.com>
Cc:	'Anders Boström' <anders@...insight.net>,
	<linux-kernel@...r.kernel.org>,
	'Lesław Kopeć' <leslaw.kopec@...za-klasa.pl>,
	"'Aman Gupta'" <aman@...1.net>,
	"'Peter Zijlstra'" <a.p.zijlstra@...llo.nl>,
	"'Thomas Gleixner'" <tglx@...utronix.de>,
	"'Charles Wang'" <muming.wq@...il.com>,
	"Doug Smythies" <dsmythies@...us.net>
Subject: RE: [3.2.16 -> 3.2.17 regression] High reported CPU load when idle

>> On 2012.06.08 10:01 Doug Smythies wrote:
>  On 2012.06.10 10:50 Jonathan Nieder wrote:

>> By the way, I found and tested 
>> 5aaa0b7a2ed5b12692c9ffb5222182bd558d3146
>> It is similar (minimally tested).

Which a day later was included in kernel 3.5 RC2, which I also tested for
low load conditions only (i.e. in case I made some mistake with my manual
back edit.)
Herein, the abbreviation "5aaa" means Kernel 3.5 RC2 with
5aaa0b7a2ed5b12692c9ffb5222182bd558d3146 and its predecessors.

> Another load average related patch is being discussed (not meant
> particularly to address the too-low load case, just mentioning it FYI):

>	sched: Folding nohz load accounting more accurate
> [...]
> From [*].
> [*] http://thread.gmane.org/gmane.linux.kernel/1310462

Jonathan: Thanks for the reference.

I also back edited that patch (by Charles Wang) into my working Kernel.
Herein, the abbreviation "Wang" means my working Kernel (3.2.0-24.39
(Ubuntu reference)) with these back edits: The above referenced patch by
Charles Wang; 5aaa0b7a2ed5b12692c9ffb5222182bd558d3146;
556061b00c9f2fd6a5524b6bde823ef12f299ecf;
and c308b56b5398779cd3da0f62ab26b0453494c3d4.

The abbreviation "c308" means my working kernel with only
c308b56b5398779cd3da0f62ab26b0453494c3d4.

The abbreviation "Control" means a tick based kernel compiled with
CONFIG_NO_HZ=no.

See the attached PNG file (and or [1]) for relatively low load test
results.

Summary:

"c308" and "5aaa" are the same, with reported load averages higher than
actual.
"Wang" is worse, with reported load averages in error even higher.
"Control" tends to track, but sometimes reported load averages are
somewhat low.

[1] http://www.smythies.com/~doug/network/load_average/wang_compare.png

Doug Smythies

Download attachment "wang_compare.png" of type "image/png" (78127 bytes)