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

[Oleg Nesterov]

On 10/02, Peter Zijlstra wrote:
>
> On Wed, Oct 02, 2013 at 04:00:20PM +0200, Oleg Nesterov wrote:
> > And again, even
> >
> > 	for (;;) {
> > 		percpu_down_write();
> > 		percpu_up_write();
> > 	}
> >
> > should not completely block the readers.
>
> Sure there's a tiny window, but don't forget that a reader will have to
> wait for the gp_state cacheline to transfer to shared state and the
> per-cpu refcount cachelines to be brought back into exclusive mode and
> the above can be aggressive enough that by that time we'll observe
> state == blocked again.

Sure, but don't forget that other callers of cpu_down() do a lot more
work before/after they actually call cpu_hotplug_begin/end().

> So I'll stick to waitcount -- as you can see in the patches I've just
> posted.

I still do not believe we need this waitcount "in practice" ;)

But even if I am right this is minor and we can reconsider this later,
so please forget.

Oleg.

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