sched: Ensure cpu_power periodic update

With a lot of small tasks, the softirq sched is nearly never called
when no_hz is enabled. In this case load_balance() is mainly called
with the newly_idle mode which doesn't update the cpu_power.

Add a next_update field which ensure a maximum update period when
there is short activity.

Having stale cpu_power information can skew the load-balancing
decisions, this is cured by the guaranteed update.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1323717668-2143-1-git-send-email-vincent.guittot@linaro.org

1 files changed, 16 insertions, 8 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7c6414fc669d..8e77a6bd597b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -215,6 +215,8 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
 
 const struct sched_class fair_sched_class;
 
+static unsigned long __read_mostly max_load_balance_interval = HZ/10;
+
 /**************************************************************
  * CFS operations on generic schedulable entities:
  */
@@ -3776,6 +3778,11 @@ void update_group_power(struct sched_domain *sd, int cpu)
         struct sched_domain *child = sd->child;
         struct sched_group *group, *sdg = sd->groups;
         unsigned long power;
+        unsigned long interval;
+
+        interval = msecs_to_jiffies(sd->balance_interval);
+        interval = clamp(interval, 1UL, max_load_balance_interval);
+        sdg->sgp->next_update = jiffies + interval;
 
         if (!child) {
                 update_cpu_power(sd, cpu);
@@ -3883,12 +3890,15 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
          * domains. In the newly idle case, we will allow all the cpu's
          * to do the newly idle load balance.
          */
-        if (idle != CPU_NEWLY_IDLE && local_group) {
-                if (balance_cpu != this_cpu) {
-                        *balance = 0;
-                        return;
-                }
-                update_group_power(sd, this_cpu);
+        if (local_group) {
+                if (idle != CPU_NEWLY_IDLE) {
+                        if (balance_cpu != this_cpu) {
+                                *balance = 0;
+                                return;
+                        }
+                        update_group_power(sd, this_cpu);
+                } else if (time_after_eq(jiffies, group->sgp->next_update))
+                        update_group_power(sd, this_cpu);
         }
 
         /* Adjust by relative CPU power of the group */
@@ -4945,8 +4955,6 @@ static int __cpuinit sched_ilb_notifier(struct notifier_block *nfb,
 
 static DEFINE_SPINLOCK(balancing);
 
-static unsigned long __read_mostly max_load_balance_interval = HZ/10;
-
 /*
  * Scale the max load_balance interval with the number of CPUs in the system.
  * This trades load-balance latency on larger machines for less cross talk.