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>

1 files changed, 17 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index b8ee864c7481..52b98675acb2 100644
--- a/kernel/sched.c
+++ b/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;