torture: Increase stutter-end intensity

Currently, all stuttered kthreads block a jiffy at a time, which can
result in them starting at different times.  (Note: This is not an
energy-efficiency problem unless you run torture tests in production,
in which case you have other problems!)  This commit increases the
intensity of the restart event by causing kthreads to spin through the
last jiffy, restarting when they see the variable change.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

kernel/torture.c

@@ -533,7 +533,11 @@ void stutter_wait(const char *title)
 	while (ACCESS_ONCE(stutter_pause_test) ||
 	       (torture_runnable && !ACCESS_ONCE(*torture_runnable))) {
 		if (stutter_pause_test)
-			schedule_timeout_interruptible(1);
+			if (ACCESS_ONCE(stutter_pause_test) == 1)
+				schedule_timeout_interruptible(1);
+			else
+				while (ACCESS_ONCE(stutter_pause_test))
+					cond_resched();
 		else
 			schedule_timeout_interruptible(round_jiffies_relative(HZ));
 		torture_shutdown_absorb(title);
@@ -550,7 +554,11 @@ static int torture_stutter(void *arg)
 	VERBOSE_TOROUT_STRING("torture_stutter task started");
 	do {
 		if (!torture_must_stop()) {
-			schedule_timeout_interruptible(stutter);
+			if (stutter > 1) {
+				schedule_timeout_interruptible(stutter - 1);
+				ACCESS_ONCE(stutter_pause_test) = 2;
+			}
+			schedule_timeout_interruptible(1);
 			ACCESS_ONCE(stutter_pause_test) = 1;
 		}
 		if (!torture_must_stop())