Function sub_cgroup 

pub(crate) fn sub_cgroup(path: &str) -> Result<(u64, PathBuf)>

Resolve the attach cgroup: its id, which is its inode number, and the scheduler path its stats server lives under. The path mirrors the cgroup’s path below /sys/fs/cgroup under sub/, or is sub/ for a cgroup outside that mount or with a path too long for a socket. A path that is not a cgroup, or the cgroup root, is refused.