hrtimer: Kick lowres dynticks targets on timer enqueue

In lowres mode, hrtimers are serviced by the tick instead of a clock
event. It works well as long as the tick stays periodic but we must also
make sure that the hrtimers are serviced in dynticks mode targets,
pretty much like timer list timers do.

Note that all dynticks modes are concerned: get_nohz_timer_target()
tries not to return remote idle CPUs but there is nothing to prevent
the elected target from entering dynticks idle mode until we lock its
base. It's also prefectly legal to enqueue hrtimers on full dynticks CPU.

So there are two requirements to correctly handle dynticks:

1) On target's tick stop time, we must not delay the next tick further
the next hrtimer.

2) On hrtimer queue time. If the tick of the target is stopped, we must
wake up that CPU such that it sees the new hrtimer and recalculate
the next tick accordingly.

The point 1 is well handled currently through get_nohz_timer_interrupt() and
cmp_next_hrtimer_event().

But the point 2 isn't handled at all.

Fixing this is easy though as we have the necessary API ready for that.
All we need is to call wake_up_nohz_cpu() on a target when a newly
enqueued hrtimer requires tick rescheduling, like timer list timer do.

Signed-off-by: Viresh Kumar <[e-mail]>
Signed-off-by: Frederic Weisbecker <[e-mail]>
Link: http://lkml.kernel.org/r/3d7e[e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

kernel/time/hrtimer.c

···
 
 	leftmost = enqueue_hrtimer(timer, new_base);
 
-	/*
-	 * Only allow reprogramming if the new base is on this CPU.
-	 * (it might still be on another CPU if the timer was pending)
-	 *
-	 * XXX send_remote_softirq() ?
-	 */
-	if (leftmost && new_base->cpu_base == &__get_cpu_var(hrtimer_bases)
-		&& hrtimer_enqueue_reprogram(timer, new_base)) {
+	if (!leftmost) {
+		unlock_hrtimer_base(timer, &flags);
+		return ret;
+	}
+
+	if (!hrtimer_is_hres_active(timer)) {
+		/*
+		 * Kick to reschedule the next tick to handle the new timer
+		 * on dynticks target.
+		 */
+		wake_up_nohz_cpu(new_base->cpu_base->cpu);
+	} else if (new_base->cpu_base == &__get_cpu_var(hrtimer_bases) &&
+			hrtimer_enqueue_reprogram(timer, new_base)) {
+		/*
+		 * Only allow reprogramming if the new base is on this CPU.
+		 * (it might still be on another CPU if the timer was pending)
+		 *
+		 * XXX send_remote_softirq() ?
+		 */
 		if (wakeup) {
 			/*
 			 * We need to drop cpu_base->lock to avoid a