sched/fair: Avoid using decay_load_missed() with a negative value

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@arm.com>
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: perterz@infradead.org
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@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 564104227498867839747897b1170fb89d247c86..3b3dc39671f84a3ea72929ae63bf13340ece3ccb 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4509,9 +4509,17 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
 
                /* 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