[PATCH 4.5 018/238] sched/fair: Avoid using decay_load_missed() with a negative value

[Greg Kroah-Hartman]

4.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Byungchul Park <byungchul.park@xxxxxxx>

commit 7400d3bbaa229eb8e7631d28fb34afd7cd2c96ff upstream.

decay_load_missed() cannot handle nagative values, so we need to prevent
using the function with a negative value.

Reported-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Paul E . McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: perterz@xxxxxxxxxxxxx
Fixes: 59543275488d ("sched/fair: Prepare __update_cpu_load() to handle active tickless")
Link: http://lkml.kernel.org/r/20160115070749.GA1914@X58A-UD3R
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/sched/fair.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4459,9 +4459,17 @@ static void __update_cpu_load(struct rq
 
 		/* scale is effectively 1 << i now, and >> i divides by scale */
 
-		old_load = this_rq->cpu_load[i] - tickless_load;
+		old_load = this_rq->cpu_load[i];
 		old_load = decay_load_missed(old_load, pending_updates - 1, i);
-		old_load += tickless_load;
+		if (tickless_load) {
+			old_load -= decay_load_missed(tickless_load, pending_updates - 1, i);
+			/*
+			 * old_load can never be a negative value because a
+			 * decayed tickless_load cannot be greater than the
+			 * original tickless_load.
+			 */
+			old_load += tickless_load;
+		}
 		new_load = this_load;
 		/*
 		 * Round up the averaging division if load is increasing. This