refscale: Add measurement of clock readout

This commit adds a "clock" type to refscale, which checks the performance
of ktime_get_real_fast_ns().  Use the "clocksource=" kernel boot parameter
to select the underlying clock source.

[ paulmck: Work around compiler false positive per kernel test robot. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refscale.c

@@ -467,6 +467,40 @@ static struct ref_scale_ops acqrel_ops = {
 	.name		= "acqrel"
 };
 
+static volatile u64 stopopts;
+
+static void ref_clock_section(const int nloops)
+{
+	u64 x = 0;
+	int i;
+
+	preempt_disable();
+	for (i = nloops; i >= 0; i--)
+		x += ktime_get_real_fast_ns();
+	preempt_enable();
+	stopopts = x;
+}
+
+static void ref_clock_delay_section(const int nloops, const int udl, const int ndl)
+{
+	u64 x = 0;
+	int i;
+
+	preempt_disable();
+	for (i = nloops; i >= 0; i--) {
+		x += ktime_get_real_fast_ns();
+		un_delay(udl, ndl);
+	}
+	preempt_enable();
+	stopopts = x;
+}
+
+static struct ref_scale_ops clock_ops = {
+	.readsection	= ref_clock_section,
+	.delaysection	= ref_clock_delay_section,
+	.name		= "clock"
+};
+
 static void rcu_scale_one_reader(void)
 {
 	if (readdelay <= 0)
@@ -759,7 +793,7 @@ ref_scale_init(void)
 	int firsterr = 0;
 	static struct ref_scale_ops *scale_ops[] = {
 		&rcu_ops, &srcu_ops, &rcu_trace_ops, &rcu_tasks_ops, &refcnt_ops, &rwlock_ops,
-		&rwsem_ops, &lock_ops, &lock_irq_ops, &acqrel_ops,
+		&rwsem_ops, &lock_ops, &lock_irq_ops, &acqrel_ops, &clock_ops,
 	};
 
 	if (!torture_init_begin(scale_type, verbose))