sched: don't allow rt_runtime_us to be zero for groups having rt tasks

This patch checks if we can set the rt_runtime_us to 0. If there is a
realtime task in the group, we don't want to set the rt_runtime_us as 0
or bad things will happen. (that task wont get any CPU time despite
being TASK_RUNNNG)
Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -7750,6 +7750,17 @@ static int __rt_schedulable(struct task_group *tg, u64 period, u64 runtime)
 	return total + to_ratio(period, runtime) < global_ratio;
 }
 
+/* Must be called with tasklist_lock held */
+static inline int tg_has_rt_tasks(struct task_group *tg)
+{
+	struct task_struct *g, *p;
+	do_each_thread(g, p) {
+		if (rt_task(p) && rt_rq_of_se(&p->rt)->tg == tg)
+			return 1;
+	} while_each_thread(g, p);
+	return 0;
+}
+
 int sched_group_set_rt_runtime(struct task_group *tg, long rt_runtime_us)
 {
 	u64 rt_runtime, rt_period;
@@ -7761,12 +7772,18 @@ int sched_group_set_rt_runtime(struct task_group *tg, long rt_runtime_us)
 		rt_runtime = RUNTIME_INF;
 
 	mutex_lock(&rt_constraints_mutex);
+	read_lock(&tasklist_lock);
+	if (rt_runtime_us == 0 && tg_has_rt_tasks(tg)) {
+		err = -EBUSY;
+		goto unlock;
+	}
 	if (!__rt_schedulable(tg, rt_period, rt_runtime)) {
 		err = -EINVAL;
 		goto unlock;
 	}
 	tg->rt_runtime = rt_runtime;
  unlock:
+	read_unlock(&tasklist_lock);
 	mutex_unlock(&rt_constraints_mutex);
 
 	return err;