linux-kernel - Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets

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Date:	Fri, 14 Sep 2012 15:20:31 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Borislav Petkov <bp@...en8.de>,
	Nikolay Ulyanitsky <lystor@...il.com>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected

On Fri, Sep 14, 2012 at 3:10 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
> I'm not seeing how it could be. Only ever looking at 1 other cpu
> (regardless which one) cannot be the same as checking 'all' of them.

Oh, you're right, it has that cpu_idle() in the loop too. So yeah, you
can't make it be even remotely equivalent. You can only make it
equivalent for the "all other cpu's are idle" case.

It doesn't even do *that*, though.

In fact, as far as I can tell, it looks like a cpu could be its own
idle_buddy. That whole logic looks very odd.

I vote we just revert it as "insane". The code really doesn't seem to
make any sense.

               Linus
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