sched: Initialize rq->age_stamp on processor start

If the sched_clock time starts at a large value, the kernel will spin
in sched_avg_update for a long time while rq->age_stamp catches up
with rq->clock.

The comment in kernel/sched/clock.c says that there is no strict promise
that it starts at zero.  So initialize rq->age_stamp when a cpu starts up
to avoid this.

I was seeing long delays on a simulator that didn't start the clock at
zero.  This might also be an issue on reboots on processors that don't
re-initialize the timer to zero on reset, and when using kexec.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1399574859-11714-1-git-send-email-minyard@acm.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -5089,10 +5089,20 @@ static struct notifier_block migration_notifier = {
 	.priority = CPU_PRI_MIGRATION,
 };
 
+static void __cpuinit set_cpu_rq_start_time(void)
+{
+	int cpu = smp_processor_id();
+	struct rq *rq = cpu_rq(cpu);
+	rq->age_stamp = sched_clock_cpu(cpu);
+}
+
 static int sched_cpu_active(struct notifier_block *nfb,
 				      unsigned long action, void *hcpu)
 {
 	switch (action & ~CPU_TASKS_FROZEN) {
+	case CPU_STARTING:
+		set_cpu_rq_start_time();
+		return NOTIFY_OK;
 	case CPU_DOWN_FAILED:
 		set_cpu_active((long)hcpu, true);
 		return NOTIFY_OK;
@@ -6970,6 +6980,7 @@ void __init sched_init(void)
 	if (cpu_isolated_map == NULL)
 		zalloc_cpumask_var(&cpu_isolated_map, GFP_NOWAIT);
 	idle_thread_set_boot_cpu();
+	set_cpu_rq_start_time();
 #endif
 	init_sched_fair_class();