tick-sched: Warn when next tick seems to be in the past

When the next tick is in the past, the delta between basemono and the next
tick gets negativ. But the next tick should never be in the past. The
negative effect of a wrong next tick might be a stop of the tick and timers
might expire late.

To prevent expensive debugging when changing underlying code, add a
WARN_ON_ONCE into this code path. To prevent complete misbehaviour, also
reset next_tick to basemono in this case.
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20231201092654.34614-4-anna-maria@linutronix.de

kernel/time/tick-sched.c

@@ -839,6 +839,10 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 		ts->next_timer = next_tick;
 	}
 
+	/* Make sure next_tick is never before basemono! */
+	if (WARN_ON_ONCE(basemono > next_tick))
+		next_tick = basemono;
+
 	/*
 	 * If the tick is due in the next period, keep it ticking or
 	 * force prod the timer.