linux-kernel - Attaching a cgroup subsystem to multiple hierarchies

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Message-Id: <050BD595-BE55-401D-9B61-E877FB9F06A8@gopivotal.com>
Date:	Wed, 5 Feb 2014 14:39:52 +0000
From:	Glyn Normington <gnormington@...ivotal.com>
To:	linux-kernel@...r.kernel.org
Subject: Attaching a cgroup subsystem to multiple hierarchies

Reading cgroups.txt and casting around the net leads me to believe that it is possible to attach a cgroup subsystem (e.g. memory) to multiple hierarchies, but this seems to result in “mirrored” hierarchies which are automatically kept in step with each other - essentially it looks like the same hierarchy at multiple file system paths.

Take the following interaction for example:

\begin{verbatim}
$ 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

Is this working as intended? Is there some other way to attach a subsystem to *distinct* hierarchies? Distinct hierarchies would allow distinct cgroups, distinct settings (e.g. memory.limit_in_bytes) and distinct partitions of the tasks in the system.

Note: I don’t have a good use for this function - I’m simply trying to reverse engineer the semantics of cgroups to get a precise understanding.

Glyn--
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