sched/fair: Do not announce throttled next buddy in dequeue_task_fair()

Hierarchy could be already throttled at this point. Throttled next
buddy could trigger a NULL pointer dereference in pick_next_task_fair().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ben Segall <bsegall@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/146608183552.21905.15924473394414832071.stgit@buzz
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 4 insertions, 5 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8c5d8c0c8827..bdcbeea90c95 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4537,15 +4537,14 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 
                 /* Don't dequeue parent if it has other entities besides us */
                 if (cfs_rq->load.weight) {
+                        /* Avoid re-evaluating load for this entity: */
+                        se = parent_entity(se);
                         /*
                          * Bias pick_next to pick a task from this cfs_rq, as
                          * p is sleeping when it is within its sched_slice.
                          */
-                        if (task_sleep && parent_entity(se))
-                                set_next_buddy(parent_entity(se));
-
-                        /* avoid re-evaluating load for this entity */
-                        se = parent_entity(se);
+                        if (task_sleep && se && !throttled_hierarchy(cfs_rq))
+                                set_next_buddy(se);
                         break;
                 }
                 flags |= DEQUEUE_SLEEP;