Messages in this thread | | | Date | Thu, 10 Oct 2013 16:57:38 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2 |
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* 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.
Thanks,
Ingo
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