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Date: Thu, 10 Oct 2013 09:43:55 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
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On Thu, 10 Oct 2013 12:36:31 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 10 Oct 2013 09:00:44 -0700
> Andrew Morton <akpm@linux-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!