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

[Linus Torvalds <torvalds@linux-foundation.org>]

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