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

BugLink: https://bugs.launchpad.net/bugs/1687512
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>
(cherry-picked from commit 754bd598be9bbc953bc709a9e8ed7f3188bfb9d7)
Signed-off-by: Daniel Axtens <daniel.axtens@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 377b987c3c08fb3309d5e129233ac6adfc0cc5b1..c1e78e9c8f05821390c7c99e7327f5102602e8a8 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4271,15 +4271,14 @@ 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;