sched/core: Fix endless loop in pick_next_task()

1) Single cpu machine case.

When rq has only RT tasks, but no one of them can be picked
because of throttling, we enter in endless loop.

pick_next_task_{dl,rt} return NULL.

In pick_next_task_fair() we permanently go to retry

	if (rq->nr_running != rq->cfs.h_nr_running)
		return RETRY_TASK;

(rq->nr_running is not being decremented when rt_rq becomes
throttled).

No chances to unthrottle any rt_rq or to wake fair here,
because of rq is locked permanently and interrupts are
disabled.

2) In case of SMP this can cause a hang too. Although we unlock
   rq in idle_balance(), interrupts are still disabled.

The solution is to check for available tasks in DL and RT
classes instead of checking for sum.

Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1394098321.19290.11.camel@tkhai
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 0 insertions, 10 deletions

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index f3cee0a63b76..d8cdf1618551 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -470,11 +470,6 @@ static void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
 		dequeue_rt_entity(rt_se);
 }
 
-static inline int rt_rq_throttled(struct rt_rq *rt_rq)
-{
-	return rt_rq->rt_throttled && !rt_rq->rt_nr_boosted;
-}
-
 static int rt_se_boosted(struct sched_rt_entity *rt_se)
 {
 	struct rt_rq *rt_rq = group_rt_rq(rt_se);
@@ -545,11 +540,6 @@ static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
 {
 }
 
-static inline int rt_rq_throttled(struct rt_rq *rt_rq)
-{
-	return rt_rq->rt_throttled;
-}
-
 static inline const struct cpumask *sched_rt_period_mask(void)
 {
 	return cpu_online_mask;