sched: Fix nohz_idle_balance()

On tickless systems, one CPU runs load balance for all idle CPUs.

The cpu_load of this CPU is updated before starting the load balance
of each other idle CPUs. We should instead update the cpu_load of
the balance_cpu.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Venkatesh Pallipadi <venki@google.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1347509486-8688-1-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -4794,14 +4794,15 @@ static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle)
 		if (need_resched())
 			break;
 
-		raw_spin_lock_irq(&this_rq->lock);
-		update_rq_clock(this_rq);
-		update_idle_cpu_load(this_rq);
-		raw_spin_unlock_irq(&this_rq->lock);
+		rq = cpu_rq(balance_cpu);
+
+		raw_spin_lock_irq(&rq->lock);
+		update_rq_clock(rq);
+		update_idle_cpu_load(rq);
+		raw_spin_unlock_irq(&rq->lock);
 
 		rebalance_domains(balance_cpu, CPU_IDLE);
 
-		rq = cpu_rq(balance_cpu);
 		if (time_after(this_rq->next_balance, rq->next_balance))
 			this_rq->next_balance = rq->next_balance;
 	}