linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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Date:	Tue, 1 Oct 2013 20:07:50 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

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 ;)

> > 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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