* Andrew Morton <a...@linux-foundation.org> wrote:
> On Thu, 10 Oct 2013 17:26:12 +0200 Oleg Nesterov <o...@redhat.com> wrote:
>
> > On 10/10, Ingo Molnar wrote:
> > >
> > > * 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.
> >
> > Well, iirc Srivatsa (cc'ed) pointed out that some systems do
> > cpu_down/up quite often to save the power.
>
> cpu hotremove already uses stop_machine, so such an approach shouldn't
> actually worsen things (a lot) for them?
Also, using CPU hotremove to save power, instead of implementing proper
power aware scheduling, is very broken to begin with.
Thanks,
Ingo
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