Bug#803013: systemd should not destroy application created cgroups

[Julian Andres Klode]

On Fri, Nov 13, 2015 at 11:36:30AM +1100, paul.szabo at sydney.edu.au wrote:
> Progress? For my efforts upstream, I got the comment:
> 
> > Sorry, but systemd implements a single-writer cgroup logic (as
> > requested by the kernel maintainers), and hence takes possesion of the
> > whole tree. ...
> 
> I observe it only uses the /sys/fs/cgroup/systemd tree.
> (I wonder about the "req by kernel" comment.)

See the end of the email.

> 
> > ... If you want your own cgroup tree to manage, use the "Delegate=yes"
> > feature in a service or scope, but otherwise systemd is in exclusive
> > control.
> 
> Do we have that? Can we have it everywhere? Can we have it by default,
> should not it be so?

No. You set Delegate=yes for the unit which manages its own cgroups
hierarchy beneath the one designated by systemd.

> 
> > Sorry, but multiple access to the cgroup tree is simply not supported.
> 
> Not if we let systemd take over the world.

This is not related to systemd. The kernel cgroups implementation moved
or is moving to a single-writer, single-hierarchy implementation with
a user space daemon arbiter. systemd implements such an arbiter.

http://www.linuxfoundation.org/news-media/blogs/browse/2013/08/all-about-linux-kernel-cgroup%E2%80%99s-redesign
https://wiki.freedesktop.org/www/Software/systemd/ControlGroupInterface/
https://lwn.net/Articles/555920/

While the kernel probably still allows for multiple hierarchies in order
to not break the user space interface, they should not be used anymore.

The single hierarchy changes were discussed in 2012:
  https://lwn.net/Articles/484251/ 
and introduced between 2012 and 2013. Complaining about
that 2 years later is a bit late, although it would not
have changed anything back then either, as the multiple-writer
approach is fundamentally broken.

-- 
Julian Andres Klode  - Debian Developer, Ubuntu Member

See http://wiki.debian.org/JulianAndresKlode and http://jak-linux.org/.

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