Messages in this thread | | | Date | Thu, 10 Oct 2013 18:52:29 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2 |
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* Andrew Morton <akpm@linux-foundation.org> wrote:
> On Thu, 10 Oct 2013 17:26:12 +0200 Oleg Nesterov <oleg@redhat.com> wrote: > > > On 10/10, Ingo Molnar wrote: > > > > > > * Peter Zijlstra <peterz@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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