cgroup: don't print subsystems for the default hierarchy

It doesn't make sense to print subsystems on mount option or
/proc/PID/cgroup for the default hierarchy.

* cgroup.controllers file at the root of the default hierarchy lists
  the currently attached controllers.

* The default hierarchy is catch-all for unmounted subsystems.

* The default hierarchy doesn't accept any mount options.

Suppress subsystem printing on mount options and /proc/PID/cgroup for
the default hierarchy.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: cgroups@vger.kernel.org

kernel/cgroup.c

@@ -1333,6 +1333,7 @@ static int cgroup_show_options(struct seq_file *seq,
 	struct cgroup_subsys *ss;
 	int ssid;
 
+	if (root != &cgrp_dfl_root)
 		for_each_subsys(ss, ssid)
 			if (root->subsys_mask & (1 << ssid))
 				seq_printf(seq, ",%s", ss->name);
@@ -5137,9 +5138,11 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
 			continue;
 
 		seq_printf(m, "%d:", root->hierarchy_id);
+		if (root != &cgrp_dfl_root)
 			for_each_subsys(ss, ssid)
 				if (root->subsys_mask & (1 << ssid))
-				seq_printf(m, "%s%s", count++ ? "," : "", ss->name);
+					seq_printf(m, "%s%s", count++ ? "," : "",
+						   ss->name);
 		if (strlen(root->name))
 			seq_printf(m, "%sname=%s", count ? "," : "",
 				   root->name);