linux-kernel - Performance issue since 3.2.6

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Date:	Fri, 18 Jan 2013 22:05:41 +0100
From:	Olivier Doucet <webmaster@...ux.com>
To:	linux-kernel@...r.kernel.org
Subject: Performance issue since 3.2.6

Hello,

I think I found a performance issue in kernel. This problem was
introduced in 3.2.6 and is affecting all version, including 3.7.1
(latest tested).

I measured several kernel builds with a homebrew LAMP platform
benchmark (250 runs, average value kept).

Kernel 3.2.0 was on production so far, and I tried an upgrade to 3.7.1
(latest kernel at that time). No software other than the kernel was
modified. Kernel was built with the same .config file (new options
left with default value).
Difference in performance was quite huge :
Kernel 3.7.1 : ~ 3250 queries / second
Kernel 3.2.0 : ~ 4300 queries / second

Yes, this is a 25% performance drop ...

To narrow things down, I tested several kernels :
3.2.0 to 3.2.5 : OK
3.2.6, 3.2.11, 3.2.28, 3.2.36, 3.7.1 : PERFORMANCE DROP
On faulty kernels, performance drop is always the same.

At this step, I know that bug was introduced in version 3.2.6. I then
used git bisect (amazing tool btw) to find the faulty commit :
f51d67a64f32cd81ea8b67ca964fb7cf7e783b2e	PM / QoS: CPU C-state
breakage with PM Qos change

Next test : I used a 3.2.6 and reverted this patch : performance was
back to normal.
I also reverted this patch on 3.2.36 (latest 3.2.X), and performance
was also OK. I was unable to revert this patch on 3.7.1 (structure of
the source file  /include/linux/pm_qos.h changed too heavily).

Kernel was built on a x86_64 platform with binutils 2.19.1 and gcc 4.4.0.
.config file used : https://gist.github.com/4567342
Benchmark was run on a dual CPU INTEL L5630 with 4GB of RAM

What can I do to resolve this problem ?
Thanks !

Olivier
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