[PATCH 2/2] sched:fix: Signed overflow prevention for vacancycalculation

[Lukasz Majewski]

Nasty bug with vacancy calculation has been fixed.
In the if statement the FULL_UTIL is a large constant, max_cfs_util()
returns unsigned and putil is also defined as unsigned. The outcome
is that this condition is always true.

As observed, this was the reason for frequent jumps of processes between
CPUs.

Signed-off-by: Lukasz Majewski <l.majewski@xxxxxxxxxxx>
Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3a4917c..56a9d16 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3468,10 +3468,10 @@ find_leader_cpu(struct sched_group *group, struct task_struct *p, int this_cpu,
 	unsigned putil;
 	putil = div_u64(((u64)p->se.avg.runnable_avg_sum << SCHED_POWER_SHIFT),
 				p->se.avg.runnable_avg_period + 1);
+	vacancy = FULL_UTIL - max_cfs_util(this_cpu, wakeup) - (putil << 2);
 
 	/* bias toward local cpu */
-	if (cpumask_test_cpu(this_cpu, tsk_cpus_allowed(p)) &&
-		FULL_UTIL - max_cfs_util(this_cpu, wakeup) - (putil << 2) > 0)
+	if (cpumask_test_cpu(this_cpu, tsk_cpus_allowed(p)) && vacancy > 0)
 		return this_cpu;
 
 	/* Traverse only the allowed CPUs */
-- 
1.7.12


-- 
Thanks
    Alex
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