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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Message-ID: <CA+55aFwKEjsPKYNFkkNfrr9V=vyEO6M9o_48be4y9w6=h-06UQ@mail.gmail.com>
Date:	Fri, 14 Sep 2012 15:01:44 -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 2:56 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
> Both things change semantics, not looking at the entire package is new
> too.

Well, the "idle_buddy" thing on its own could be considered to be
purely a caching thing.

Sure, it doesn't take tsk_cpus_allowed() into account while setting up
the cache (since it's not dynamic enough), but *assuming* the common
case is that people let threads be on any of the cores of a package,
it should be possible to make the cache 100% equivalent with no
semantic change. No?

The code doesn't even try to do that kind of "don't change semantics",
though, and makes the idle-buddy thing entirely different.

            Linus
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