[PATCH v4 2/6] sched: set initial value of runnable avg for new forked task

[Alex Shi <alex.shi@intel.com>]

We need initialize the se.avg.{decay_count, load_avg_contrib} for a
new forked task.
Otherwise random values of above variables cause mess when do new task
enqueue:
    enqueue_task_fair
        enqueue_entity
            enqueue_entity_load_avg

and make forking balancing imbalance since incorrect load_avg_contrib.

set avg.decay_count = 0, and avg.load_avg_contrib = se->load.weight to
resolve such issues.

Signed-off-by: Alex Shi <alex.shi@intel.com>
---
 kernel/sched/core.c | 6 ++++++
 kernel/sched/fair.c | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 86e87f1..e0c003a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1564,6 +1564,7 @@ static void __sched_fork(struct task_struct *p)
 #ifdef CONFIG_SMP
 	p->se.avg.runnable_avg_period = 0;
 	p->se.avg.runnable_avg_sum = 0;
+	p->se.avg.decay_count = 0;
 #endif
 #ifdef CONFIG_SCHEDSTATS
 	memset(&p->se.statistics, 0, sizeof(p->se.statistics));
@@ -1651,6 +1652,11 @@ void sched_fork(struct task_struct *p)
 		p->sched_reset_on_fork = 0;
 	}
 
+	/* New forked task assumed with full utilization */
+#if defined(CONFIG_SMP)
+	p->se.avg.load_avg_contrib = p->se.load.weight;
+#endif
+
 	if (!rt_prio(p->prio))
 		p->sched_class = &fair_sched_class;
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9bcb7f3..85ff367 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1509,6 +1509,10 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
 	 * We track migrations using entity decay_count <= 0, on a wake-up
 	 * migration we use a negative decay count to track the remote decays
 	 * accumulated while sleeping.
+	 *
+	 * When enqueue a new forked task, the se->avg.decay_count == 0, so
+	 * we bypass update_entity_load_avg(), use avg.load_avg_contrib initial
+	 * value: se->load.weight.
 	 */
 	if (unlikely(se->avg.decay_count <= 0)) {
 		se->avg.last_runnable_update = rq_of(cfs_rq)->clock_task;
-- 
1.7.12