Messages in this thread | | | Date | Thu, 10 Oct 2013 13:06:16 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/6] hotplug: Optimize {get,put}_online_cpus() |
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On 10/09, Andrew Morton wrote: > > On Tue, 08 Oct 2013 12:25:06 +0200 Peter Zijlstra <peterz@infradead.org> wrote: > > > The current implementation of get_online_cpus() is global of nature > > and thus not suited for any kind of common usage. > > > > Re-implement the current recursive r/w cpu hotplug lock such that the > > read side locks are as light as possible. > > > > The current cpu hotplug lock is entirely reader biased; but since > > readers are expensive there aren't a lot of them about and writer > > starvation isn't a particular problem. > > > > However by making the reader side more usable there is a fair chance > > it will get used more and thus the starvation issue becomes a real > > possibility. > > > > Therefore this new implementation is fair, alternating readers and > > writers; this however requires per-task state to allow the reader > > recursion. > > Obvious question: can't we adapt lglocks for this? It would need the > counter in task_struct to permit reader nesting, but what else is > needed?
Unlikely. If nothing else, get_online_cpus() is might_sleep().
But we can joing this with percpu_rw_semaphore (and I am going to try to do this). Ignoring the counter in task_struct this is the same thing, but get_online_cpus() is also optimized for the case when the writer is pending (percpu_down_read() uses down_read() in this case).
Oleg.
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