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Date:	Thu, 10 Oct 2013 09:43:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, 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>,
	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 12:36:31 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:

> On Thu, 10 Oct 2013 09:00:44 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > 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?
> 
> 
> I would think that memory hotplug may require stop machine as all CPUs
> may touch that memory.

Sure.

> But we would like to remove stomp machine from
> CPU hotplug.

We do?  That's news.  It wasn't mentioned in the changelog and should
have been.  Why?  

> Why prevent all CPUs from running when we want to remove
> one?

So get_online_cpus() goes away.  Nothing is more scalable than nothing!
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