task_cgroup_from_root
struct cgroup *task_cgroup_from_root(struct task_struct *task,
cgrp = task_cgroup_from_root(tsk, root);
from_cgrp = task_cgroup_from_root(from, root);
src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
cgrp = task_cgroup_from_root(tsk, root);