refperf: Tune reader measurement interval

This commit moves a printk() out of the measurement interval, converts
a atomic_dec()/atomic_read() pair to atomic_dec_and_test(), and adds
a smp_mb__before_atomic() to avoid potential wake/wait hangs.  These
changes have the added benefit of reducing the number of loops required
for amortizing loop overhead for CONFIG_PREEMPT=n RCU measurements from
1,000,000 to 10,000.  This reduction in turn shortens the test, reducing
the probability of interference.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refperf.c

@@ -252,15 +252,16 @@ ref_perf_reader(void *arg)
 	// Make sure that the CPU is affinitized appropriately during testing.
 	WARN_ON_ONCE(smp_processor_id() != me);
 
+	smp_mb__before_atomic();
 	atomic_dec(&rt->start);
 
+	VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d started", me, exp_idx);
+
 	// To prevent noise, keep interrupts disabled. This also has the
 	// effect of preventing entries into slow path for rcu_read_unlock().
 	local_irq_save(flags);
 	start = ktime_get_mono_fast_ns();
 
-	VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d started", me, exp_idx);
-
 	cur_ops->readsection(loops);
 
 	duration = ktime_get_mono_fast_ns() - start;
@@ -268,14 +269,12 @@ ref_perf_reader(void *arg)
 
 	rt->last_duration_ns = WARN_ON_ONCE(duration < 0) ? 0 : duration;
 
-	atomic_dec(&nreaders_exp);
+	if (atomic_dec_and_test(&nreaders_exp))
+		wake_up(&main_wq);
 
 	VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d ended, (readers remaining=%d)",
 			me, exp_idx, atomic_read(&nreaders_exp));
 
-	if (!atomic_read(&nreaders_exp))
-		wake_up(&main_wq);
-
 	if (!torture_must_stop())
 		goto repeat;
 end: