linux-kernel - Re: Performance issue since 3.2.6

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Message-ID: <2048745.M7CAe2g1gj@vostro.rjw.lan>
Date:	Tue, 22 Jan 2013 00:09:41 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Olivier Doucet <webmaster@...ux.com>
Cc:	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Subject: Re: Performance issue since 3.2.6

On Monday, January 21, 2013 11:57:06 PM Olivier Doucet wrote:
> Good evening,
> 
> >> I'll try to track this commit.
> >
> > Thanks a lot!
> 
> I failed to find a working version today with CONFIG_PM=y
> (tested 3.2.6, 3.2.5, 3.2.0, 3.0.1, 2.6.32.8). I'll try older kernels tomorrow.

I don't think that's necessary.  It looks like CONFIG_PM=y has never really
worked for you, but you wasn't aware of that.

> I'm very sceptical now : how can a bug with such a big impact (25%
> performance drop) can survive for so many years, with no one seeing it
> ?
> Good point is that benchmark can be easily done (it took less than an
> hour to test a new version). If you have patches to be tested, feel
> free to ask :)
> 
> In the meanwhile, do you think I can compile a working 3.7 kernel ?
> Maybe by activating / deactivating specific options ?

I'm really not sure.  It looks like CONFIG_PM makes PM QoS affect cpuidle
for you in a wrong way, so to speak.

I suppose it's time to look into the code and see what makes the difference.

Thanks,
Rafael

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