sched/uclamp: Fix a bug in propagating uclamp value in new cgroups

When a new cgroup is created, the effective uclamp value wasn't updated
with a call to cpu_util_update_eff() that looks at the hierarchy and
update to the most restrictive values.

Fix it by ensuring to call cpu_util_update_eff() when a new cgroup
becomes online.

Without this change, the newly created cgroup uses the default
root_task_group uclamp values, which is 1024 for both uclamp_{min, max},
which will cause the rq to to be clamped to max, hence cause the
system to run at max frequency.

The problem was observed on Ubuntu server and was reproduced on Debian
and Buildroot rootfs.

By default, Ubuntu and Debian create a cpu controller cgroup hierarchy
and add all tasks to it - which creates enough noise to keep the rq
uclamp value at max most of the time. Imitating this behavior makes the
problem visible in Buildroot too which otherwise looks fine since it's a
minimal userspace.

Fixes: 0b60ba2 ("sched/uclamp: Propagate parent clamps")
Reported-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Doug Smythies <dsmythies@telus.net>
Link: https://lore.kernel.org/lkml/000701d5b965$361b6c60$a2524520$@net/

kernel/sched/core.c

@@ -7099,6 +7099,12 @@ static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
 
 	if (parent)
 		sched_online_group(tg, parent);
+
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+	/* Propagate the effective uclamp value for the new group */
+	cpu_util_update_eff(css);
+#endif
+
 	return 0;
 }