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Date: Fri, 14 Sep 2012 13:39:25 -0700
From: Tejun Heo <tj@kernel.org>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: containers@lists.linux-foundation.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, Li Zefan <lizefan@huawei.com>,
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        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Neil Horman <nhorman@tuxdriver.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>
Subject: Re: [RFC] cgroup TODOs
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Hello, again.

On Fri, Sep 14, 2012 at 12:49:50PM -0700, Tejun Heo wrote:
> That said, if someone can think of a better solution, I'm all ears.
> One thing that *has* to be maintained is that it should be able to tag
> a resource in such way that its associated controllers are
> identifiable regardless of which task is looking at it.

So, I thought about it more.  How about we do "consider / ignore this
node" instead of "(don't) nest beyond this level".  For example, let's
assume a tree like the following.

        R
     /  |  \
    A   B   C
   / \
  AA AB

If we want to differentiate between AA and AB, we'll have to consider
the whole tree with the previous sheme - A needs to nest, so R needs
to nest and we end up with the whole tree.  Instead, if we have honor
/ ignore this node.  We can set the honor bit on A, AA and AB and see
the tree as

        R
     /
    A
   / \
  AA AB

We still see the intermediate A node but can ignore the other
branches.  Implementation and concept-wise, it's fairly simple too.
For any given node and controller, you travel upwards until you meet a
node which has the controller enabled and that's the cgroup the
controller considers.

Thanks.

-- 
tejun