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: <1348503163.11847.97.camel@twins>
Date:	Mon, 24 Sep 2012 18:12:43 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, 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 Mon, 2012-09-24 at 08:52 -0700, Linus Torvalds wrote:
> And the whole "if we find any non-idle cpu, skip the whole domain"
> logic really seems a bit odd (that's not new to your patch, though).
> Can somebody explain what the whole point of that idiotically written
> function is? 

So we're looking for an idle cpu around @target. We prefer a cpu of an
idle core, since SMT-siblings share L[12] cache. The way we do this is
by iterating the topology tree downwards starting at the LLC (L3) cache
level. Its groups are either the SMT-siblings or singleton groups.

In case its the SMT things, we want to skip if there's a non-idle cpu in
that mask to avoid sharing L[12].

If we don't find an empty core, we go down the domain tree, either
finding a NULL domain and terminating, or finding the SMT domain, and
we'll see if there's an idle SMT sibling.

Is it pretty?, not really. Is there a better way of doing it?, possibly,
I just haven't thought of it yet :/
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