[tip: timers/urgent] clockevents: Prevent timer interrupt starvation

tip-bot2 for Thomas Gleixner
2026-04-07T20:23:26Z

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID: b81207470caf597a518cd3706844810a7ed7cbe4
Gitweb: https://git.kernel.org/tip/b81207470caf597a518cd3706844810a7ed7cbe4
Author: Thomas Gleixner <tglx@kernel.org>
AuthorDate: Tue, 07 Apr 2026 10:54:17 +02:00
Committer: Thomas Gleixner <tglx@kernel.org>
CommitterDate: Tue, 07 Apr 2026 20:07:19 +02:00

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.

Fixes: d316c57ff6bf ("[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>
Link: https://lore.kernel.org/lkml/acMe-QZUel-bBYUh@mozart.vkv.me/
Link: https://patch.msgid.link/20260407083247.562657657@kernel.org
---
include/linux/clockchips.h | 2 ++
kernel/time/clockevents.c | 23 +++++++++++++++--------
kernel/time/hrtimer.c | 1 +
kernel/time/tick-common.c | 1 +
kernel/time/tick-sched.c | 1 +
5 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index b0df28d..50cdc9d 100644
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -80,6 +80,7 @@ enum clock_event_state {
* @shift: nanoseconds to cycles divisor (power of two)
* @state_use_accessors:current state of the device, assigned by the core code
* @features: features
+ * @next_event_forced: True if the last programming was a forced event
* @retries: number of forced programming retries
* @set_state_periodic: switch state to periodic
* @set_state_oneshot: switch state to oneshot
@@ -108,6 +109,7 @@ struct clock_event_device {
u32 shift;
enum clock_event_state state_use_accessors;
unsigned int features;
+ unsigned int next_event_forced;
unsigned long retries;

int (*set_state_periodic)(struct clock_event_device *);
diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index eaae1ce..7278fb2 100644
--- a/kernel/time/clockevents.c
+++ b/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 +324,23 @@ 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);
+ 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;
+ }

- clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
- rc = dev->set_next_event((unsigned long) clc, dev);
+ if (dev->next_event_forced)
+ return 0;

- return (rc && force) ? clockevents_program_min_delta(dev) : rc;
+ 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;
}

/*
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 860af7a..1e37142 100644
--- a/kernel/time/hrtimer.c
+++ b/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);
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index d305d85..6a9198a 100644
--- a/kernel/time/tick-common.c
+++ b/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);

/*
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 36449f0..d1f27df 100644
--- a/kernel/time/tick-sched.c
+++ b/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);