[RFC PATCH v2 5/7] sched: nominate preferred wakeup cpu

[Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>]

When the system utilisation is low and more cpus are idle,
then the process waking up from sleep should prefer to
wakeup an idle cpu from semi-idle cpu package (multi core
package) rather than a completely idle cpu package which
would waste power.

Use the sched_mc balance logic in find_busiest_group() to
nominate a preferred wakeup cpu.

This info can be sored in appropriate sched_domain, but
updating this info in all copies of sched_domain is not
practical.  For now lets try with a global variable.

Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
---

 kernel/sched.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 569fc8d..4ae79f5 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3380,6 +3380,9 @@ out_balanced:
 
 	if (this == group_leader && group_leader != group_min) {
 		*imbalance = min_load_per_task;
+		if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP)
+			sched_mc_preferred_wakeup_cpu =
+					first_cpu(group_leader->cpumask);
 		return group_min;
 	}
 #endif
@@ -6911,6 +6914,13 @@ static void sched_domain_node_span(int node, cpumask_t *span)
 int sched_smt_power_savings = 0, sched_mc_power_savings = 0;
 
 /*
+ * Preferred wake up cpu nominated by sched_mc balance that will be used when
+ * most cpus are idle in the system indicating overall very low system
+ * utilisation. Triggered at POWERSAVINGS_BALANCE_WAKEUP (2).
+ */
+unsigned int sched_mc_preferred_wakeup_cpu;
+
+/*
  * SMT sched-domains:
  */
 #ifdef CONFIG_SCHED_SMT