linux-kernel - Re: current linux-2.6.git: cpusets completely broken

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Date:	Mon, 14 Jul 2008 16:05:40 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Paul Menage <menage@...gle.com>,
	Max Krasnyansky <maxk@...lcomm.com>, Paul Jackson <pj@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, miaox@...fujitsu.com,
	rostedt@...dmis.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: current linux-2.6.git: cpusets completely broken

On Tue, 15 Jul 2008, Dmitry Adamushko wrote:
> 
> cpu_clear(cpu, cpu_active_map); _alone_ does not guarantee that after
> its completion, no new tasks can appear on (be migrated to) 'cpu'.

But I think we should make it do that.

I do realize that we "queue" processes, but that's part of the whole 
complexity. More importantly, the people who do that kind of asynchronous 
queueing don't even really care - *if* they cared about the process 
_having_ to show up on the destination core, they'd be waiting 
synchronously and re-trying (which they do).

So by doing the test for cpu_active_map not at queuing time, but at the 
time when we actually try to do the migration, we can now also make that 
cpu_active_map be totally serialized.

(Of course, anybody who clears the bit does need to take the runqueue lock 
of that CPU too, but cpu_down() will have to do that as it does the 
"migrate away live tasks" anyway, so that's not a problem)

		Linus
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