linux-kernel - Re: [PATCH] cpuset: hotunplug cpus and mems in all cpusets

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Date:	Mon, 28 Aug 2006 23:19:17 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Paul Jackson <pj@....com>
Cc:	nathanl@...tin.ibm.com, Simon.Derr@...l.net,
	linux-kernel@...r.kernel.org, ntl@...ox.com, y-goto@...fujitsu.com,
	Anton Blanchard <anton@...ba.org>,
	Dave Hansen <haveblue@...ibm.com>,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH] cpuset: hotunplug cpus and mems in all cpusets

On Mon, 28 Aug 2006 23:08:24 -0700
Paul Jackson <pj@....com> wrote:

> The cpuset code handling hot unplug of CPUs or Memory Nodes
> was incorrect - it could remove a CPU or Node from the top
> cpuset, while leaving it still in some child cpusets.
> 
> One basic rule of cpusets is that each cpusets cpus and mems
> are subsets of its parents.  The cpuset hot unplug code
> violated this rule.
> 
> So the cpuset hotunplug handler must walk down the tree,
> removing any removed CPU or Node from all cpusets.
> 
> However, it is not allowed to make a cpusets cpus or mems
> become empty.  They can only transition from empty to non-empty,
> not back.
> 
> So if the last CPU or Node would be removed from a cpuset by
> the above walk, we scan back up the cpuset hierarchy, finding
> the nearest ancestor that still has something online, and copy
> its CPU or Memory placement.

Did you consider failing the hotremove request instead?
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