Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> But the thing is; our sense of NR_CPUS has shifted, where it used to be 
> ok to do something like:
> 
>   for_each_cpu()
> 
> With preemption disabled; it gets to be less and less sane to do so, 
> simply because 'common' hardware has 256+ CPUs these days. If we cannot 
> rely on preempt disable to exclude hotplug, we must use 
> get_online_cpus(), but get_online_cpus() is global state and thus cannot 
> be used at any sort of frequency.

So ... why not make it _really_ cheap, i.e. the read lock costing nothing, 
and tie CPU hotplug to freezing all tasks in the system?

Actual CPU hot unplugging and repluggin is _ridiculously_ rare in a 
system, I don't understand how we tolerate _any_ overhead from this utter 
slowpath.

Thanks,

	Ingo
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