Re: New NUMA scheduler and hotplug CPU

[Rusty Russell]

In message <368660000.1075217230@[10.10.2.4]> you write:
> > Yeah, I talked it over with Rusty some on IRC. I have more of a feeling
> > why he's trying to do it that way now. 
> 
> BTW, Rusty - what are the locking rules for cpu_online_map under hotplug?
> Is it RCU or something? The sched domains usage of it doesn't seem to take 
> any locks.

The trivial usage is to take the cpucontrol sem (down_cpucontrol()).
There's a grace period between taking the cpu offline and actually
killing it too, so for most usages RCU is sufficient.

Fortunately, I've yet to hit a case where this isn't sufficient.  For
the scheduler there's an explicit "move all tasks off the CPU" call
which takes the tasklist lock and walks the tasks.

Cheers,
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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