rcu-tasks: Restore use of timers for non-RT kernels

The use of hrtimers for RCU-tasks grace-period delays works well in
general, but can result in excessive grace-period delays for some
corner-case workloads.  This commit therefore reverts to the use of
timers for non-RT kernels to mitigate those grace-period delays.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -654,9 +654,13 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 
 		// Slowly back off waiting for holdouts
 		set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
-		exp = jiffies_to_nsecs(fract);
-		__set_current_state(TASK_IDLE);
-		schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD);
+		if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			schedule_timeout_idle(fract);
+		} else {
+			exp = jiffies_to_nsecs(fract);
+			__set_current_state(TASK_IDLE);
+			schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD);
+		}
 
 		if (fract < HZ)
 			fract++;