sched, cgroup: Don't reject lower cpu.max on ancestors

While adding cgroup2 interface for the cpu controller, 0d593634
("sched: Implement interface for cgroup unified hierarchy") forgot to
update input validation and left it to reject cpu.max config if any
descendant has set a higher value.

cgroup2 officially supports delegation and a descendant must not be
able to restrict what its ancestors can configure.  For absolute
limits such as cpu.max and memory.max, this means that the config at
each level should only act as the upper limit at that level and
shouldn't interfere with what other cgroups can configure.

This patch updates config validation on cgroup2 so that the cpu
controller follows the same convention.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 0d593634 ("sched: Implement interface for cgroup unified hierarchy")
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org # v4.15+

kernel/sched/core.c

@@ -6678,14 +6678,19 @@ static int tg_cfs_schedulable_down(struct task_group *tg, void *data)
 		parent_quota = parent_b->hierarchical_quota;
 
 		/*
-		 * Ensure max(child_quota) <= parent_quota, inherit when no
+		 * Ensure max(child_quota) <= parent_quota.  On cgroup2,
+		 * always take the min.  On cgroup1, only inherit when no
 		 * limit is set:
 		 */
+		if (cgroup_subsys_on_dfl(cpu_cgrp_subsys)) {
+			quota = min(quota, parent_quota);
+		} else {
 			if (quota == RUNTIME_INF)
 				quota = parent_quota;
 			else if (parent_quota != RUNTIME_INF && quota > parent_quota)
 				return -EINVAL;
 		}
+	}
 	cfs_b->hierarchical_quota = quota;
 
 	return 0;