rcu-tasks: Use schedule_hrtimeout_range() to wait for grace periods

The synchronous RCU-tasks grace-period-wait primitives invoke
schedule_timeout_idle() to give readers a chance to exit their
read-side critical sections.  Unfortunately, this fails during early
boot on PREEMPT_RT because PREEMPT_RT relies solely on ksoftirqd to run
timer handlers.  Because ksoftirqd cannot operate until its kthreads
are spawned, there is a brief period of time following scheduler
initialization where PREEMPT_RT cannot run the timer handlers that
schedule_timeout_idle() relies on, resulting in a hang.

To avoid this boot-time hang, this commit replaces schedule_timeout_idle()
with schedule_hrtimeout(), so that the timer expires in hardirq context.
This is ensures that the timer fires even on PREEMPT_RT throughout the
irqs-enabled portions of boot as well as during runtime.

The timer is set to expire between fract and fract + HZ / 2 jiffies in
order to align with any other timers that might expire during that time,
thus reducing the number of wakeups.

Note that RCU-tasks grace periods are infrequent, so the use of hrtimer
should be fine.  In contrast, in common-case code, user of hrtimer
could result in performance issues.

Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -647,13 +647,16 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 	fract = rtp->init_fract;
 
 	while (!list_empty(&holdouts)) {
+		ktime_t exp;
 		bool firstreport;
 		bool needreport;
 		int rtst;
 
 		// Slowly back off waiting for holdouts
 		set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
-		schedule_timeout_idle(fract);
+		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++;