linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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Date:	Thu, 10 Oct 2013 09:00:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

On Thu, 10 Oct 2013 17:26:12 +0200 Oleg Nesterov <oleg@...hat.com> wrote:

> On 10/10, Ingo Molnar wrote:
> >
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > > But the thing is; our sense of NR_CPUS has shifted, where it used to be
> > > ok to do something like:
> > >
> > >   for_each_cpu()
> > >
> > > With preemption disabled; it gets to be less and less sane to do so,
> > > simply because 'common' hardware has 256+ CPUs these days. If we cannot
> > > rely on preempt disable to exclude hotplug, we must use
> > > get_online_cpus(), but get_online_cpus() is global state and thus cannot
> > > be used at any sort of frequency.
> >
> > So ... why not make it _really_ cheap, i.e. the read lock costing nothing,
> > and tie CPU hotplug to freezing all tasks in the system?
> >
> > Actual CPU hot unplugging and repluggin is _ridiculously_ rare in a
> > system, I don't understand how we tolerate _any_ overhead from this utter
> > slowpath.
> 
> Well, iirc Srivatsa (cc'ed) pointed out that some systems do cpu_down/up
> quite often to save the power.

cpu hotremove already uses stop_machine, so such an approach shouldn't
actually worsen things (a lot) for them?

It's been ages since I looked at this stuff :( Although it isn't used
much, memory hotplug manages to use stop_machine() on the add/remove
(ie, "writer") side and nothing at all on the "reader" side.  Is there
anything which fundamentally prevents cpu hotplug from doing the same?

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