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Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>, Mike Galbraith <efault@gmx.de>,
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        Andreas Herrmann <andreas.herrmann3@amd.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Suresh Siddha <suresh.b.siddha@intel.com>
Date: Thu, 27 Sep 2012 10:21:44 +0200
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On Wed, 2012-09-26 at 11:19 -0700, Linus Torvalds wrote:
> 
> For example, it starts with the maximum target scheduling domain, and
> works its way in over the scheduling groups within that domain. What
> the f*ck is the logic of that kind of crazy thing? It never makes
> sense to look at a biggest domain first. 

That's about SMT, it was felt that you don't want SMT siblings first
because typically SMT siblings are somewhat under-powered compared to
actual cores.

Also, the whole scheduler topology thing doesn't have L2/L3 domains, it
only has the LLC domain, if you want more we'll need to fix that. For
now its a fixed:

 SMT
 MC (llc)
 CPU (package/machine-for-!numa)
 NUMA

So in your patch, your for_each_domain() loop will really only do the
SMT/MC levels and prefer an SMT sibling over an idle core.