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SubjectRe: current linux-2.6.git: cpusets completely broken


On Mon, 14 Jul 2008, Steven Rostedt 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?

Linus


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