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

[Oleg Nesterov]

On 10/10, Ingo Molnar wrote:
>
> * 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.

Well, iirc Srivatsa (cc'ed) pointed out that some systems do cpu_down/up
quite often to save the power.

Oleg.

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