* Peter Zijlstra <pet...@infradead.org> 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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