Messages in this thread | | | Date | Thu, 10 Oct 2013 07:55:32 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/6] hotplug: Optimize {get,put}_online_cpus() |
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On Thu, Oct 10, 2013 at 01:06:16PM +0200, Oleg Nesterov wrote: > 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).
To Andrew's overall question, I believe that by the time we apply this in the various places where it can help, it will have simplified things a bit -- and made them faster and more scalable.
Thanx, Paul
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