clockevents: Prevent timer interrupt starvation

Calvin reported an odd NMI watchdog lockup which claims that the CPU locked
up in user space. He provided a reproducer, which sets up a timerfd based
timer and then rearms it in a loop with an absolute expiry time of 1ns.

As the expiry time is in the past, the timer ends up as the first expiring
timer in the per CPU hrtimer base and the clockevent device is programmed
with the minimum delta value. If the machine is fast enough, this ends up
in a endless loop of programming the delta value to the minimum value
defined by the clock event device, before the timer interrupt can fire,
which starves the interrupt and consequently triggers the lockup detector
because the hrtimer callback of the lockup mechanism is never invoked.

As a first step to prevent this, avoid reprogramming the clock event device
when:
     - a forced minimum delta event is pending
     - the new expiry delta is less then or equal to the minimum delta

Thanks to Calvin for providing the reproducer and to Borislav for testing
and providing data from his Zen5 machine.

The problem is not limited to Zen5, but depending on the underlying
clock event device (e.g. TSC deadline timer on Intel) and the CPU speed
not necessarily observable.

This change serves only as the last resort and further changes will be made
to prevent this scenario earlier in the call chain as far as possible.

[ tglx: Updated to restore the old behaviour vs. !force and delta <= 0 and
  	fixed up the tick-broadcast handlers as pointed out by Borislav ]

Fixes: d316c57ff6 ("[PATCH] clockevents: add core functionality")
Reported-by: Calvin Owens <calvin@wbinvd.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Calvin Owens <calvin@wbinvd.org>
Tested-by: Borislav Petkov <bp@alien8.de>
Link: https://lore.kernel.org/lkml/acMe-QZUel-bBYUh@mozart.vkv.me/
Link: https://patch.msgid.link/20260407083247.562657657@kernel.org

kernel/time/clockevents.c

@@ -172,6 +172,7 @@ void clockevents_shutdown(struct clock_event_device *dev)
 {
 	clockevents_switch_state(dev, CLOCK_EVT_STATE_SHUTDOWN);
 	dev->next_event = KTIME_MAX;
+	dev->next_event_forced = 0;
 }
 
 /**
@@ -305,7 +306,6 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
 {
 	unsigned long long clc;
 	int64_t delta;
-	int rc;
 
 	if (WARN_ON_ONCE(expires < 0))
 		return -ETIME;
@@ -324,16 +324,27 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
 		return dev->set_next_ktime(expires, dev);
 
 	delta = ktime_to_ns(ktime_sub(expires, ktime_get()));
-	if (delta <= 0)
-		return force ? clockevents_program_min_delta(dev) : -ETIME;
 
-	delta = min(delta, (int64_t) dev->max_delta_ns);
-	delta = max(delta, (int64_t) dev->min_delta_ns);
+	/* Required for tick_periodic() during early boot */
+	if (delta <= 0 && !force)
+		return -ETIME;
 
-	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
-	rc = dev->set_next_event((unsigned long) clc, dev);
+	if (delta > (int64_t)dev->min_delta_ns) {
+		delta = min(delta, (int64_t) dev->max_delta_ns);
+		clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
+		if (!dev->set_next_event((unsigned long) clc, dev))
+			return 0;
+	}
 
-	return (rc && force) ? clockevents_program_min_delta(dev) : rc;
+	if (dev->next_event_forced)
+		return 0;
+
+	if (dev->set_next_event(dev->min_delta_ticks, dev)) {
+		if (!force || clockevents_program_min_delta(dev))
+			return -ETIME;
+	}
+	dev->next_event_forced = 1;
+	return 0;
 }
 
 /*

kernel/time/hrtimer.c

@@ -1888,6 +1888,7 @@ void hrtimer_interrupt(struct clock_event_device *dev)
 	BUG_ON(!cpu_base->hres_active);
 	cpu_base->nr_events++;
 	dev->next_event = KTIME_MAX;
+	dev->next_event_forced = 0;
 
 	raw_spin_lock_irqsave(&cpu_base->lock, flags);
 	entry_time = now = hrtimer_update_base(cpu_base);

kernel/time/tick-broadcast.c

@@ -76,8 +76,10 @@ const struct clock_event_device *tick_get_wakeup_device(int cpu)
  */
 static void tick_broadcast_start_periodic(struct clock_event_device *bc)
 {
-	if (bc)
+	if (bc) {
+		bc->next_event_forced = 0;
 		tick_setup_periodic(bc, 1);
+	}
 }
 
 /*
@@ -403,6 +405,7 @@ static void tick_handle_periodic_broadcast(struct clock_event_device *dev)
 	bool bc_local;
 
 	raw_spin_lock(&tick_broadcast_lock);
+	tick_broadcast_device.evtdev->next_event_forced = 0;
 
 	/* Handle spurious interrupts gracefully */
 	if (clockevent_state_shutdown(tick_broadcast_device.evtdev)) {
@@ -696,6 +699,7 @@ static void tick_handle_oneshot_broadcast(struct clock_event_device *dev)
 
 	raw_spin_lock(&tick_broadcast_lock);
 	dev->next_event = KTIME_MAX;
+	tick_broadcast_device.evtdev->next_event_forced = 0;
 	next_event = KTIME_MAX;
 	cpumask_clear(tmpmask);
 	now = ktime_get();
@@ -1063,6 +1067,7 @@ static void tick_broadcast_setup_oneshot(struct clock_event_device *bc,
 
 
 	bc->event_handler = tick_handle_oneshot_broadcast;
+	bc->next_event_forced = 0;
 	bc->next_event = KTIME_MAX;
 
 	/*
@@ -1175,6 +1180,7 @@ void hotplug_cpu__broadcast_tick_pull(int deadcpu)
 		}
 
 		/* This moves the broadcast assignment to this CPU: */
+		bc->next_event_forced = 0;
 		clockevents_program_event(bc, bc->next_event, 1);
 	}
 	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);

kernel/time/tick-common.c

@@ -110,6 +110,7 @@ void tick_handle_periodic(struct clock_event_device *dev)
 	int cpu = smp_processor_id();
 	ktime_t next = dev->next_event;
 
+	dev->next_event_forced = 0;
 	tick_periodic(cpu);
 
 	/*

kernel/time/tick-sched.c

@@ -1513,6 +1513,7 @@ static void tick_nohz_lowres_handler(struct clock_event_device *dev)
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
 
 	dev->next_event = KTIME_MAX;
+	dev->next_event_forced = 0;
 
 	if (likely(tick_nohz_handler(&ts->sched_timer) == HRTIMER_RESTART))
 		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);