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

[Paul E. McKenney]

On Tue, Oct 01, 2013 at 08:07:50PM +0200, Oleg Nesterov wrote:
> On 10/01, Peter Zijlstra wrote:
> >
> > On Tue, Oct 01, 2013 at 07:45:08PM +0200, Oleg Nesterov wrote:
> > >
> > > 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 :-)
> 
> I don't really agree but I won't argue ;)

The authors of arch/x86/kernel/cpu/mcheck/mce.c would seem to be the
guys who would need to complain, given that they seem to have the only
use in 3.11.

							Thanx, Paul

> > > 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.
> 
> > > 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.
> 
> Yes,
> 
> > 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.
> 
> See above... This makes POST_DEAD really "special" compared to other
> CPU_* events.
> 
> And again. Something like a global lock taken by CPU_DOWN_PREPARE and
> released by POST_DEAD or DOWN_FAILED does not look "too wrong" to me.
> 
> But I leave this to you and Srivatsa.
> 
> Oleg.
> 

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