[Devel] [PATCH RHEL7 COMMIT] ms/sched/fair: Do not announce throttled next buddy in dequeue_task_fair()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-327.22.2.vz7.16.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-327.22.2.el7
------>
commit 825c3143ddabf5ebaa833ff6fb2dc05a94c3f1e9
Author: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
Date:   Wed Jul 27 17:05:50 2016 +0400

    ms/sched/fair: Do not announce throttled next buddy in dequeue_task_fair()
    
    ms commit: 754bd598be9bbc953bc709a9e8ed7f3188bfb9d7.
    
    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 at yandex-team.ru>
    
    Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
    Reviewed-by: Ben Segall <bsegall at google.com>
    Cc: Linus Torvalds <torvalds at linux-foundation.org>
    Cc: Peter Zijlstra <peterz at infradead.org>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Link: http://lkml.kernel.org/r/146608183552.21905.15924473394414832071.stgit@buzz
    Signed-off-by: Ingo Molnar <mingo at kernel.org>
    
    https://jira.sw.ru/browse/PSBM-50099
    
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 kernel/sched/fair.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b6f70a3..a87bacf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4215,15 +4215,14 @@ 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;