timers: Assert no next dyntick timer look-up while CPU is offline

The next timer (re-)evaluation, with the purpose of entering/updating
the dyntick mode, can happen from 3 sites and none of them are relevant
while the CPU is offline:

1) The idle loop:
	a) From the quick check helping the cpuidle governor to heuristically
	   predict the best C-state.
	b) While stopping the tick.

   But if the CPU is offline, the tick has been cancelled and there is
   consequently no need to further stop the tick.

2) Remote expiry: when a CPU remotely expires global timers on behalf of
   another CPU, the latter target's next timer is re-evaluated
   afterwards. However remote expîry doesn't happen on offline CPUs.

3) IRQ exit: on nohz_full mode, the tick is (re-)evaluated on IRQ exit.
   But full dynticks is disabled on offline CPUs.

Therefore it is safe to assume that no next dyntick timer lookup can
be performed on offline CPUs.

Assert this expectation to report any surprise.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240225225508.11587-17-frederic@kernel.org

kernel/time/timer.c

@@ -2233,10 +2233,10 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
 	bool idle_is_possible;
 
 	/*
-	 * Pretend that there is no timer pending if the cpu is offline.
-	 * Possible pending timers will be migrated later to an active cpu.
+	 * When the CPU is offline, the tick is cancelled and nothing is supposed
+	 * to try to stop it.
 	 */
-	if (cpu_is_offline(smp_processor_id())) {
+	if (WARN_ON_ONCE(cpu_is_offline(smp_processor_id()))) {
 		if (idle)
 			*idle = true;
 		return tevt.local;