Re: [PATCH 1/6] hotplug: Optimize {get,put}_online_cpus()

[Oleg Nesterov <oleg@redhat.com>]

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.