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

[Peter Zijlstra]

On Tue, Oct 01, 2013 at 07:45:08PM +0200, Oleg Nesterov wrote:
> On 10/01, Peter Zijlstra wrote:
> >
> > On Tue, Oct 01, 2013 at 10:41:15PM +0530, Srivatsa S. Bhat wrote:
> > > However, as Oleg said, its definitely worth considering whether this proposed
> > > change in semantics is going to hurt us in the future. CPU_POST_DEAD has certainly
> > > proved to be very useful in certain challenging situations (commit 1aee40ac9c
> > > explains one such example), so IMHO we should be very careful not to undermine
> > > its utility.
> >
> > Urgh.. crazy things. I've always understood POST_DEAD to mean 'will be
> > called at some time after the unplug' with no further guarantees. And my
> > patch preserves that.
> 
> I tend to agree with Srivatsa... Without a strong reason it would be better
> to preserve the current logic: "some time after" should not be after the
> next CPU_DOWN/UP*. But I won't argue too much.

Nah, I think breaking it is the right thing :-)

> 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.

> Or split cpu_hotplug_begin() into 2 helpers which handle FAST -> SLOW and
> SLOW -> BLOCK transitions, then move the first "FAST -> SLOW" handler outside
> of for_each_online_cpu().

Right, that's more messy but would work if we cannot teach cpufreq (and
possibly others) to not rely on state you shouldn't rely on anyway.

I tihnk the only guarnatee POST_DEAD should have is that it should be
called before UP_PREPARE of the same cpu ;-) Nothing more, nothing less.
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