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

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Date:	Mon, 14 Jul 2008 23:47:27 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	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,
	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 Mon, 14 Jul 2008, Linus Torvalds wrote:
> >
> > On Mon, 14 Jul 2008, Linus Torvalds wrote:
> > > 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)
> >
> > Wouldn't simply doing a synchronize_sched() after clearing the bit also
> > make sure that no new task will be scheduled on that CPU?
>
> My point was that it DOESN'T NEED TO DO ANYTHING AT ALL.
>
> It has to get the runqueue lock in order to move the currently executing
> threads off that CPU anyway. The fact that it can (and actually does)
> synchronize with the scheduler in other ways is totally and utterly
> immaterial.
>
> That's what "robust" means. It means that things just work, and there are
> no subtle interactions.
>
> Sure, you can serialize with something complicated and heavy.
>
> But isn't it nice that the absolutely _least_ complicated and heavy
> operation (ie getting the runqueue lock) also serializes sufficiently?
> Isn't it nice how you have to do that in order to do all those other
> things that you obviously have to do?
>
> Please don't argue about how we can add more subtle rules, or how other
> thigns can serialize this thing as well. Isn't it sufficient that the
> _obvious_ things serialize it?

Oh, I'm not arguing. My mind is going off to an even bigger picture, where
something in the future would need to stop migration to a particular CPU,
and that it could simply clear the bit and call synchronize_sched. The run
queue lock is only visible to the scheduler.  Sorry, I may have been day
dreaming out loud ;-)

But for the case at hand, yes I agree, simply grabbing the run queue lock
is a very elegant and simple solution.

-- Steve

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