[RFCv2 PATCH 17/23] sched: Likely idle state statistics placeholder

[Morten Rasmussen <morten.rasmussen@arm.com>]

The scheduler is currently completely unaware of idle-states. To make
informed decisions using the sched_group_energy idle_states list it
is necessary to know which idle-state a cpu (or group of cpus) is most
likely to be in when it is idle.

For example when migrating a task that wakes up periodically, the wakeup
energy expense depends on the idle-state the destination cpu is most
likely to be in when idle.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
---
 kernel/sched/fair.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9720f04..353e2d0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4254,6 +4254,19 @@ static inline bool energy_aware(void)
 	return sched_feat(ENERGY_AWARE);
 }
 
+/*
+ * Returns the index of the most likely idle-state that the sched_group is in
+ * when idle. The index can be used to identify the idle-state in the
+ * sched_group_energy idle_states list.
+ *
+ * This is currently just a placeholder. The information needs to come from
+ * cpuidle.
+ */
+static inline int likely_idle_state_idx(struct sched_group *sg)
+{
+	return 0;
+}
+
 static int wake_wide(struct task_struct *p)
 {
 	int factor = this_cpu_read(sd_llc_size);
-- 
1.7.9.5