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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: Mike Galbraith <efault@gmx.de>
To: Borislav Petkov <bp@alien8.de>
Cc: david@lang.hm, Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Mel Gorman <mgorman@suse.de>,
        Nikolay Ulyanitsky <lystor@gmail.com>, linux-kernel@vger.kernel.org,
        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 15:38:20 +0200
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On Thu, 2012-09-27 at 12:20 +0200, Borislav Petkov wrote: 
> On Thu, Sep 27, 2012 at 12:17:22AM -0700, david@lang.hm wrote:
> > It seems to me that trying to figure out if you are going to
> > overload the L2 is an impossible task, so just assume that it will
> > all fit, and the worst case is you have one balancing cycle where
> > you can't do as much work and then the normal balancing will kick in
> > and move something anyway.
> 
> Right, and this implies that when the load balancer runs, it will
> definitely move the task away from the L2. But what do I do in the cases
> where the two tasks don't overload the L2 and it is actually beneficial
> to keep them there? How does the load balancer know that?

It doesn't, but it has task_hot().  A preempted buddy may be pulled, but
the next wakeup will try to bring buddies back together.

-Mike