[cgmanager-devel] cgmanager fails to start on Fedora (or cgmanager cannot start on distros which run systemd)

[David Shwatrz]

Hello, cgmanager-devel,

When I start cgmanager (built latest from git) on Fedora I get the
following error:
cgmanager: Failed mounting cpuset: Device or resource busy
cgmanager: Error mounting subsystem cpuset
cgmanager: Failed to set up cgroup mounts

There is no such problem (or any other problem in fact) to run
cgmanager (which is again built latest from git) on Ubuntu.

I traced down the error to the do_mount_subsys() method in fs.c (of
cgmanager tree), which tries to mount the cpu
cgroup controller.

The problem is, AFAIK, that cgmanger tries to mount the cpu
controller, which is mounted in the following way
 by systemd (as cat /proc/mounts show):
...
cgroup /sys/fs/cgroup/cpu,cpuacct cgroup
rw,nosuid,nodev,noexec,relatime,cpuacct,cpu 0 0
...
Now, since the cpu is part of a mask of cgroup
controllers(cpu,cpuacct), as can be seen above, mounting fails, AFAIK.
The
cgroup subsystem does not let mount a controller which is mounted as
part of a mask of controllers on a different path.

What I mean is that if you look at any other controller which is
mounted as a single controller (not
part of a mask), and try to mount on another path, it will work; for
example, the memory controller
is mounted thus by systemd:
cgroup on /sys/fs/cgroup/memory type cgroup
(rw,nosuid,nodev,noexec,relatime,memory)

And trying to mount the memory controller under a different path works
successfully:
mount -t cgroup -o memory test /cgroup/memtest/

>From what I understand in the following article,
http://lwn.net/Articles/575672/ ("Another daemon for managing control
groups"),
it seems that systemd does not intend to use cgmanager.

So the question is (assuming I was correct in what described here
earlier): how can cgmanger run on Fedora and other distros which run
Systemd (in the future, Debian and Ubuntu are also going to use
Systemd).

Regards.
DavidS