[systemd-devel] systemd-nspawn and cgroup hybrid mode

[Antoine Pietri]

Hi,

I have a probably dumb question for which I couldn't find an answer in
the docs. I'm trying to make a program that uses the cgroupv1 API run
into a systemd-nspawn container. In the host, I know that I can just
look at /proc/self/cgroup to see the path of my cgroup and write stuff
there. The legacy cgroup tree is properly created on the host:

$ systemd-run --pty find /sys/fs/cgroup/ | grep 'run-u[0-9]' | grep
group/memory/
/sys/fs/cgroup/memory/system.slice/run-u4161.service/cgroup.procs
/sys/fs/cgroup/memory/system.slice/run-u4161.service/memory.use_hierarchy
/sys/fs/cgroup/memory/system.slice/run-u4161.service/memory.kmem.tcp.usage_in_bytes
/sys/fs/cgroup/memory/system.slice/run-u4161.service/memory.soft_limit_in_bytes
[...]

But when I'm in the container, it doesn't work anymore. Running the
same command returns no results. Now, this is most certainly due to
the fact that in the container, /sys/fs/cgroup is mounted in read-only
so systemd can't create anything in /sys/fs/cgroup/memory... but then,
what is the proper way to write into the legacy cgroups? I also tried
to turn on Delegate=true to see if it would change something, but it
doesn't.

Thanks,

-- 
Antoine Pietri