From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] Update sched_domains_numa_masks when new cpus are
 onlined.
Date: Tue, 25 Sep 2012 13:07:22 +0200
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To: Tang Chen <tangchen@cn.fujitsu.com>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org, x86@kernel.org, linux-numa@vger.kernel.org, wency@cn.fujitsu.com, mingo@kernel.org, Thomas Gleixner <tglx@linutronix.de>

On Tue, 2012-09-25 at 10:39 +0800, Tang Chen wrote:
> > We do this because nr_node_ids changed, right? This means the entire
> > distance table grew/shrunk, which means we should do the level scan
> > again.
> 
> It seems that nr_node_ids will not change once the system is up.
> I'm not quite sure. If I am wrong, please tell me. :) 

Ah, right you are..

So the problem is that cpumask_of_node() doesn't contain offline cpus,
and we might not boot the machine with all cpus present.

In that case I guess the suggested hotplug hooks are fine. Its just that
your changelog was confusing.