Support unified cgroup hierarchy

[wentasah]

Currently, when demos runs on a system using unified cgroup hierarchy, it fails, because the layout of cgroup filesystems is different than what is expected. Demos should detect this situation (see #27) and configure/create cgroups accordingly. Issue #31 is also related to this problem.

[MatejKafka]

What's the current state of support for unified cgroup hierarchy? I know my current WSL2 kernel still defaults to hybrid mode, and I'd assume other distros do the same. Can we reasonably support both?

[wentasah]

On Wed, Apr 28 2021, Matej Kafka wrote: What's the current state of support for unified cgroup hierarchy?

Fedora seems to started using it more than a year ago (https://medium.com/nttlabs/cgroup-v2-596d035be4d7). I don't have exact picture about other distros, but I see various gossip that they are switching as well. As far as I know, one big blocker for switching was Docker, but it now supports unified hierarchy as well.

I know my current WSL2 kernel still defaults to hybrid mode, and I'd assume other distros do the same. Can we reasonably support both?

I think, it's not a big deal. We just need to configure root paths for different cgroup controllers to be the same. But I may be wrong :) I think it should be straightforward to create a NixOS VMs with hybrid and unified hierarchy so that we can test (even on github) that both versions work. I'll try to prepare this later.

[wentasah]

Now, when #55 is closed, we can start developing support for unified cgroup hierarchy. The example below shows how to test it and what currently fails (even the simplest command).

[host:~]# cd demos-sched/test
[host:~/demos-sched/test]# nix-shell -p nixos-shell --command nixos-shell
...
<<< Welcome to NixOS 21.11.git.1e6fe5276cdM (x86_64) - ttyS0 >>>
Log in as "root" with an empty password.
nixos login: root
[root@nixos:~/demos-sched/test]# demos-sched -C '{windows: [], partitions: []}'
>>> 14:55:13.197 [critical] Assertion failed: `!freezer_path.empty()`
>>> 14:55:13.199 [critical]     at ../src/cgroup_setup.cpp:76 (bool cgroup_setup::create_toplevel_cgroups(Cgroup&, Cgroup&, Cgroup&, const string&))
>>> 14:55:13.201 [critical] Unwinding stack and exiting...
>>> 14:55:13.202 [error] Exception: Assertion failed (see above)

[wentasah]

Github actions's ubuntu-latest switch to Ubuntu 22.04, which uses unified cgroup hierarchy by default. Somebody should look at implementing the necessary support :-).