sched: more sanity checks on the bandwidth settings

While playing around with it, I noticed we missed some sanity checks.
Also add some comments while we're there.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -8866,11 +8866,29 @@ static int tg_schedulable(struct task_group *tg, void *data)
 		runtime = d->rt_runtime;
 	}
 
+	/*
+	 * Cannot have more runtime than the period.
+	 */
+	if (runtime > period && runtime != RUNTIME_INF)
+		return -EINVAL;
+
+	/*
+	 * Ensure we don't starve existing RT tasks.
+	 */
 	if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg))
 		return -EBUSY;
 
 	total = to_ratio(period, runtime);
 
+	/*
+	 * Nobody can have more than the global setting allows.
+	 */
+	if (total > to_ratio(global_rt_period(), global_rt_runtime()))
+		return -EINVAL;
+
+	/*
+	 * The sum of our children's runtime should not exceed our own.
+	 */
 	list_for_each_entry_rcu(child, &tg->children, siblings) {
 		period = ktime_to_ns(child->rt_bandwidth.rt_period);
 		runtime = child->rt_bandwidth.rt_runtime;
@@ -8978,19 +8996,24 @@ long sched_group_rt_period(struct task_group *tg)
 
 static int sched_rt_global_constraints(void)
 {
-	struct task_group *tg = &root_task_group;
-	u64 rt_runtime, rt_period;
+	u64 runtime, period;
 	int ret = 0;
 
 	if (sysctl_sched_rt_period <= 0)
 		return -EINVAL;
 
-	rt_period = ktime_to_ns(tg->rt_bandwidth.rt_period);
-	rt_runtime = tg->rt_bandwidth.rt_runtime;
+	runtime = global_rt_runtime();
+	period = global_rt_period();
+
+	/*
+	 * Sanity check on the sysctl variables.
+	 */
+	if (runtime > period && runtime != RUNTIME_INF)
+		return -EINVAL;
 
 	mutex_lock(&rt_constraints_mutex);
 	read_lock(&tasklist_lock);
-	ret = __rt_schedulable(tg, rt_period, rt_runtime);
+	ret = __rt_schedulable(NULL, 0, 0);
 	read_unlock(&tasklist_lock);
 	mutex_unlock(&rt_constraints_mutex);