sched: monitor clock underflows in /proc/sched_debug

We monitor clock overflows, let's also monitor clock underflows.
Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -465,7 +465,7 @@ struct rq {
 	u64 clock, prev_clock_raw;
 	s64 clock_max_delta;
 
-	unsigned int clock_warps, clock_overflows;
+	unsigned int clock_warps, clock_overflows, clock_underflows;
 	u64 idle_clock;
 	unsigned int clock_deep_idle_events;
 	u64 tick_timestamp;
@@ -3736,8 +3736,10 @@ void scheduler_tick(void)
 	/*
 	 * Let rq->clock advance by at least TICK_NSEC:
 	 */
-	if (unlikely(rq->clock < next_tick))
+	if (unlikely(rq->clock < next_tick)) {
 		rq->clock = next_tick;
+		rq->clock_underflows++;
+	}
 	rq->tick_timestamp = rq->clock;
 	update_cpu_load(rq);
 	curr->sched_class->task_tick(rq, curr, 0);

kernel/sched_debug.c

@@ -179,6 +179,7 @@ static void print_cpu(struct seq_file *m, int cpu)
 	PN(prev_clock_raw);
 	P(clock_warps);
 	P(clock_overflows);
+	P(clock_underflows);
 	P(clock_deep_idle_events);
 	PN(clock_max_delta);
 	P(cpu_load[0]);