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FromVaidyanathan Srinivasan <>
Subject[RFC PATCH v5 3/7] sched: nominate preferred wakeup cpu
DateThu, 11 Dec 2008 23:12:57 +0530
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 | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 6bea99b..0918677 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -493,6 +493,14 @@ 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
};

/*
@@ -3407,6 +3415,10 @@ 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


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