torture: Make torture_stutter() use hrtimer

The torture_stutter() function uses schedule_timeout_interruptible()
to time the stutter duration, but this can miss race conditions due to
its being time-synchronized with everything else that is based on the
timer wheels.  This commit therefore converts torture_stutter() to use
the high-resolution timers via schedule_hrtimeout(), and also to fuzz
the stutter interval.  While in the area, this commit also limits the
spin-loop portion of the stutter_wait() function's wait loop to two
jiffies, down from about one second.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/torture.c

@@ -641,20 +641,27 @@ EXPORT_SYMBOL_GPL(stutter_wait);
  */
 static int torture_stutter(void *arg)
 {
+	ktime_t delay;
+	DEFINE_TORTURE_RANDOM(rand);
 	int wtime;
 
 	VERBOSE_TOROUT_STRING("torture_stutter task started");
 	do {
 		if (!torture_must_stop() && stutter > 1) {
 			wtime = stutter;
-			if (stutter > HZ + 1) {
+			if (stutter > 2) {
 				WRITE_ONCE(stutter_pause_test, 1);
-				wtime = stutter - HZ - 1;
-				schedule_timeout_interruptible(wtime);
-				wtime = HZ + 1;
+				wtime = stutter - 3;
+				delay = ktime_divns(NSEC_PER_SEC * wtime, HZ);
+				delay += (torture_random(&rand) >> 3) % NSEC_PER_MSEC;
+				set_current_state(TASK_INTERRUPTIBLE);
+				schedule_hrtimeout(&delay, HRTIMER_MODE_REL);
+				wtime = 2;
 			}
 			WRITE_ONCE(stutter_pause_test, 2);
-			schedule_timeout_interruptible(wtime);
+			delay = ktime_divns(NSEC_PER_SEC * wtime, HZ);
+			set_current_state(TASK_INTERRUPTIBLE);
+			schedule_hrtimeout(&delay, HRTIMER_MODE_REL);
 		}
 		WRITE_ONCE(stutter_pause_test, 0);
 		if (!torture_must_stop())