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Date: Tue, 13 Jun 2006 04:45:32 -0700
From: Andrew Morton <akpm@osdl.org>
To: Keith Owens <kaos@sgi.com>
Cc: ak@suse.de, mingo@elte.hu, michal.k.k.piotrowski@gmail.com,
       linux-kernel@vger.kernel.org
Subject: Re: 2.6.16-rc6-mm2
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On Tue, 13 Jun 2006 15:08:40 +1000
Keith Owens <kaos@sgi.com> wrote:

> Andi Kleen (on Tue, 13 Jun 2006 06:56:45 +0200) wrote:
> >
> >> I have previously suggested a lightweight solution that pins a process
> >> to a cpu 
> >
> >That is preempt_disable()/preempt_enable() effectively
> >It's also light weight as much as these things can be.
> 
> The difference being that preempt_disable() does not allow the code to
> sleep.  There are some places where we want to use cpu local data and
> the code can tolerate preemption and even sleeping, as long as the
> process schedules back on the same cpu.

It would be easy to use this mechanism wrongly:

	thread 1 on CPU N		thread 2 on CPU N

	foo = per_cpu(...)
	<preempt>
					foo = per_cpu(...);
					foo++;
					per_cpu(...) = foo;
					<unpreempt>
	foo++;
	per_cpu(...) = foo;	// whoops


In which scenarios did you envisage it being used?