kernfs_root_to_node
rdtgroup_default.kn = kernfs_root_to_node(rdt_root);
sysfs_root_kn = kernfs_root_to_node(sysfs_root);
struct kernfs_node *kernfs_root_to_node(struct kernfs_root *root);
struct cgroup *root_cgrp = kernfs_root_to_node(kf_root)->priv;
root_cgrp->kn = kernfs_root_to_node(root->kf_root);