Refactor cgroup interface in preparation for Systemd NWO.

In coming releases cgroups will no longer have it's own interface, all interactions will go through systemd's DBUS interface:

http://www.freedesktop.org/wiki/Software/systemd/ControlGroupInterface/

This ticket is to track and allow the refactoring and migration that will be required in order to support.

----- Original Message -----
> From: "Lennart Poettering" <mzerqung@0pointer.de>
> To: "Development discussions related to Fedora" <devel@lists.fedoraproject.org>
> Cc: "Fedora Big Data SIG" <bigdata@lists.fedoraproject.org>
> Sent: Monday, March 17, 2014 8:18:37 PM
> Subject: Re: Systemd & cgroups & NWO
>
> Well, the nebulous choice of words is intended, since we don't want to
> make specific promises on time-frames...
>
> The APIs described (tersely) at the end of the wiki page describe the
> status quo with systemd 211.
>
> The "single-writer" cgroup tree stuff Tejun has been working on for the
> kernel is now working on his machine, but it's not pushed upstream and
> will take a while before it will hit Fedora.
>
> At this point in time you hence still may create cgroups directly
> yourself (but only if you follow the pax cgroup document), however, we
> strongly encourage you to instead use scopes/slices to create them, as
> discussed on the wiki page. This way the cgroups transition will be
> abstracted away from you. You have control of a number of knobs that
> systemd will expose for you, such as CPUShares=, BlockIOWeight= and so
> on, but this is not complete, and primarily so because it's not clear
> that those other properties will continue to exist the way they are in
> the kernel. To read statistics data or to write knobs that systemd
> doesn't cover you need to go directly to the cgroupfs. For that, simply
> read /proc/self/cgroup to find out your own cgroup, and then operate on
> that. However, as during the single-writer cgroup transition the kernel
> interface how we set things up will change, be prepared that things
> might break...
>
> Lennart
>
> –
> Lennart Poettering, Red Hat