linux-kernel - Re: Performance issue since 3.2.6

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Message-ID: <2125649.yvtIPMVHz8@vostro.rjw.lan>
Date:	Fri, 18 Jan 2013 22:46:32 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Borislav Petkov <bp@...en8.de>,
	Olivier Doucet <webmaster@...ux.com>
Cc:	linux-kernel@...r.kernel.org,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Re: Performance issue since 3.2.6

On Friday, January 18, 2013 10:24:48 PM Borislav Petkov wrote:
> Good description.
> 
> I'm leaving the whole email in for reference, see below:
> 
> On Fri, Jan 18, 2013 at 10:05:41PM +0100, Olivier Doucet wrote:
> > 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 ?
> 
> Btw, the commit-id you've given is the stable commit-id but this is
> still ok, the mainline commit is mentioned in the commit message and is:
> 
> commit d020283dc694c9ec31b410f522252f7a8397e67d
> Author: Venkatesh Pallipadi <venki@...gle.com>
> Date:   Fri Feb 3 22:22:25 2012 +0100
> 
>     PM / QoS: CPU C-state breakage with PM Qos change
> 
> So, let's invite the parties from the commit to CC, see what they have
> to say.

Thanks Boris!

Olivier, may I see the kernel .config file?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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