tick: Shut down low-res tick from dying CPU

The timekeeping duty is handed over from the outgoing CPU within stop
machine. This works well if CONFIG_NO_HZ_COMMON=n or the tick is in
high-res mode. However in low-res dynticks mode, the tick isn't
cancelled until the clockevent is shut down, which can happen later. The
tick may therefore fire again once IRQs are re-enabled on stop machine
and until IRQs are disabled for good upon the last call to idle.

That's so many opportunities for a timekeeper to go idle and the
outgoing CPU to take over that duty. This is why
tick_nohz_idle_stop_tick() is called one last time on idle if the CPU
is seen offline: so that the timekeeping duty is handed over again in
case the CPU has re-taken the duty.

This means there are two timekeeping handovers on CPU down hotplug with
different undocumented constraints and purposes:

1) A handover on stop machine for !dynticks || highres. All online CPUs
   are guaranteed to be non-idle and the timekeeping duty can be safely
   handed-over. The hrtimer tick is cancelled so it is guaranteed that in
   dynticks mode the outgoing CPU won't take again the duty.

2) A handover on last idle call for dynticks && lowres.  Setting the
   duty to TICK_DO_TIMER_NONE makes sure that a CPU will take over the
   timekeeping.

Prepare for consolidating the handover to a single place (the first one)
with shutting down the low-res tick as well from
tick_cancel_sched_timer() as well. This will simplify the handover and
unify the tick cancellation between high-res and low-res.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240225225508.11587-15-frederic@kernel.org

kernel/time/tick-common.c

@@ -410,7 +410,8 @@ int tick_cpu_dying(unsigned int dying_cpu)
 	if (tick_do_timer_cpu == dying_cpu)
 		tick_do_timer_cpu = cpumask_first(cpu_online_mask);
 
-	tick_cancel_sched_timer(dying_cpu);
+	/* Make sure the CPU won't try to retake the timekeeping duty */
+	tick_sched_timer_dying(dying_cpu);
 
 	/* Remove CPU from timer broadcasting */
 	tick_offline_cpu(dying_cpu);

kernel/time/tick-sched.c

@@ -308,6 +308,14 @@ static enum hrtimer_restart tick_nohz_handler(struct hrtimer *timer)
 	return HRTIMER_RESTART;
 }
 
+static void tick_sched_timer_cancel(struct tick_sched *ts)
+{
+	if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
+		hrtimer_cancel(&ts->sched_timer);
+	else if (tick_sched_flag_test(ts, TS_FLAG_NOHZ))
+		tick_program_event(KTIME_MAX, 1);
+}
+
 #ifdef CONFIG_NO_HZ_FULL
 cpumask_var_t tick_nohz_full_mask;
 EXPORT_SYMBOL_GPL(tick_nohz_full_mask);
@@ -1040,10 +1048,7 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
 	 * the tick timer.
 	 */
 	if (unlikely(expires == KTIME_MAX)) {
-		if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
-			hrtimer_cancel(&ts->sched_timer);
-		else
-			tick_program_event(KTIME_MAX, 1);
+		tick_sched_timer_cancel(ts);
 		return;
 	}
 
@@ -1598,14 +1603,27 @@ void tick_setup_sched_timer(bool hrtimer)
 	tick_nohz_activate(ts);
 }
 
-void tick_cancel_sched_timer(int cpu)
+/*
+ * Shut down the tick and make sure the CPU won't try to retake the timekeeping
+ * duty before disabling IRQs in idle for the last time.
+ */
+void tick_sched_timer_dying(int cpu)
 {
+	struct tick_device *td = &per_cpu(tick_cpu_device, cpu);
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+	struct clock_event_device *dev = td->evtdev;
 	ktime_t idle_sleeptime, iowait_sleeptime;
 	unsigned long idle_calls, idle_sleeps;
 
-	if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
-		hrtimer_cancel(&ts->sched_timer);
+	/* This must happen before hrtimers are migrated! */
+	tick_sched_timer_cancel(ts);
+
+	/*
+	 * If the clockevents doesn't support CLOCK_EVT_STATE_ONESHOT_STOPPED,
+	 * make sure not to call low-res tick handler.
+	 */
+	if (tick_sched_flag_test(ts, TS_FLAG_NOHZ))
+		dev->event_handler = clockevents_handle_noop;
 
 	idle_sleeptime = ts->idle_sleeptime;
 	iowait_sleeptime = ts->iowait_sleeptime;

kernel/time/tick-sched.h

@@ -106,9 +106,9 @@ extern struct tick_sched *tick_get_tick_sched(int cpu);
 
 extern void tick_setup_sched_timer(bool hrtimer);
 #if defined CONFIG_NO_HZ_COMMON || defined CONFIG_HIGH_RES_TIMERS
-extern void tick_cancel_sched_timer(int cpu);
+extern void tick_sched_timer_dying(int cpu);
 #else
-static inline void tick_cancel_sched_timer(int cpu) { }
+static inline void tick_sched_timer_dying(int cpu) { }
 #endif
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST