[RFC PATCH v4 3/7] 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.  Hence this information is stored in root_domain
struct which is one copy per partitioned sched domain.
The root_domain can be accessed from each cpu's runqueue
and there is one copy per partitioned sched domain.

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

 kernel/sched.c |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 79b71f3..d28cd98 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -493,6 +493,17 @@ struct root_domain {
 #ifdef CONFIG_SMP
 	struct cpupri cpupri;
 #endif
+#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
+
+	/*
+	 * 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;
+
+#endif
+
 };
 
 /*
@@ -3090,6 +3101,7 @@ static int move_one_task(struct rq *this_rq, int this_cpu, struct rq *busiest,
 	return 0;
 }
 
+
 /*
  * find_busiest_group finds and returns the busiest CPU group within the
  * domain. It calculates and returns the amount of weighted load which
@@ -3406,6 +3418,9 @@ out_balanced:
 
 	if (this == group_leader && group_leader != group_min) {
 		*imbalance = min_load_per_task;
+		if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP)
+			cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu =
+					first_cpu(group_leader->cpumask);
 		return group_min;
 	}
 #endif