sched: skip updating rq's next_balance under null SD

Was playing with sched_smt_power_savings/sched_mc_power_savings and
found out that while the scheduler domains are reconstructed when sysfs
settings change, rebalance_domains() can get triggered with null domain
on other cpus, which is setting next_balance to jiffies + 60*HZ.
Resulting in no idle/busy balancing for 60 seconds.

Fix this.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 12 insertions, 2 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index d96030db8ff7..a4b22d93e00d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3043,6 +3043,7 @@ static inline void rebalance_domains(int cpu, enum cpu_idle_type idle)
         struct sched_domain *sd;
         /* Earliest time when we have to do rebalance again */
         unsigned long next_balance = jiffies + 60*HZ;
+        int update_next_balance = 0;
 
         for_each_domain(cpu, sd) {
                 if (!(sd->flags & SD_LOAD_BALANCE))
@@ -3079,8 +3080,10 @@ static inline void rebalance_domains(int cpu, enum cpu_idle_type idle)
                 if (sd->flags & SD_SERIALIZE)
                         spin_unlock(&balancing);
 out:
-                if (time_after(next_balance, sd->last_balance + interval))
+                if (time_after(next_balance, sd->last_balance + interval)) {
                         next_balance = sd->last_balance + interval;
+                        update_next_balance = 1;
+                }
 
                 /*
                  * Stop the load balance at this level. There is another
@@ -3090,7 +3093,14 @@ out:
                 if (!balance)
                         break;
         }
-        rq->next_balance = next_balance;
+
+        /*
+         * next_balance will be updated only when there is a need.
+         * When the cpu is attached to null domain for ex, it will not be
+         * updated.
+         */
+        if (likely(update_next_balance))
+                rq->next_balance = next_balance;
 }
 
 /*