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

[Peter Zijlstra]

On Tue, Oct 01, 2013 at 08:07:50PM +0200, Oleg Nesterov wrote:
> > > But note that you do not strictly need this change. Just kill cpuhp_waitcount,
> > > then we can change cpu_hotplug_begin/end to use xxx_enter/exit we discuss in
> > > another thread, this should likely "join" all synchronize_sched's.
> >
> > That would still be 4k * sync_sched() == terribly long.
> 
> No? the next xxx_enter() avoids sync_sched() if rcu callback is still
> pending. Unless __cpufreq_remove_dev_finish() is "too slow" of course.

Hmm,. not in the version you posted; there xxx_enter() would only not do
the sync_sched if there's a concurrent 'writer', in which case it will
wait for it.

You only avoid the sync_sched in xxx_exit() and potentially join in the
sync_sched() of a next xxx_begin().

So with that scheme:

  for (i= ; i<4096; i++) {
    xxx_begin();
    xxx_exit();
  }

Will get 4096 sync_sched() calls from the xxx_begin() and all but the
last xxx_exit() will 'drop' the rcu callback.

And given the construct; I'm not entirely sure you can do away with the
sync_sched() in between. While its clear to me you can merge the two
into one; leaving it out entirely doesn't seem right.
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