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Date:	Mon, 10 Mar 2014 10:20:14 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Glyn Normington <gnormington@...ivotal.com>
Cc:	linux-kernel@...r.kernel.org, Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH] control groups: documentation improvements

Hey,

On Mon, Mar 10, 2014 at 02:17:21PM +0000, Glyn Normington wrote:
> Then we missed how to create a hierarchy with no associated
> subsystems. The only way I can think of is to use mount, specify no
> subsystems on -o (which defaults to all the subsystems defined in
> the kernel), and run it in a kernel with no subsystems defined
> (which seems unlikely these days).
>
> Is that what you had in mind or is there some other way of creating
> a hierarchy with no subsystems attached?

Hierarchy name should be specified "-o name=" for hierarchies w/o any
controllers.

> >>Clarify that subsystems may be attached to multiple hierarchies,
> >>although this isn't very useful, and explain what happens.
> >And a subsystem may only be attached to a single hierarchy.
>
> Perhaps that's what should happen, but the following experiment
> demonstrates a subsystem being attached to two hierarchies:
> 
> $ pwd
> /home/vagrant
> $ mkdir mem1
> $ mkdir mem2
> $ sudo su
> # mount -t cgroup -o memory none /home/vagrant/mem1
> # mount -t cgroup -o memory none /home/vagrant/mem2
> # cd mem1
> # mkdir inst1
> # ls inst1
> cgroup.clone_children  memory.failcnt ...
> # ls ../mem2
> cgroup.clone_children  inst1 memory.limit_in_bytes ...
> # cd inst1
> # echo 1000000 > memory.limit_in_bytes
> # cat memory.limit_in_bytes
> 1003520
> # cat ../../mem2/inst1/memory.limit_in_bytes
> 1003520
> # echo $$ > tasks
> # cat tasks
> 1365
> 1409
> # cat ../../mem2/inst1/tasks
> 1365
> 1411

You're mounting the same hierarchy twice.  Those are two views into
the same hierarchy.

Thanks.

-- 
tejun
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