Re: [PATCH] clockevents: Prevent timer interrupt starvation

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 02, 2026 at 07:07:49PM +0200, Thomas Gleixner wrote:

> --- a/kernel/time/clockevents.c
> +++ b/kernel/time/clockevents.c
> @@ -172,6 +172,7 @@ void clockevents_shutdown(struct clock_e
>  {
>  	clockevents_switch_state(dev, CLOCK_EVT_STATE_SHUTDOWN);
>  	dev->next_event = KTIME_MAX;
> +	dev->next_event_forced = 0;
>  }
>  
>  /**
> @@ -224,13 +225,7 @@ static int clockevents_increase_min_delt
>  	return 0;
>  }
>  
> -/**
> - * clockevents_program_min_delta - Set clock event device to the minimum delay.
> - * @dev:	device to program
> - *
> - * Returns 0 on success, -ETIME when the retry loop failed.
> - */
> -static int clockevents_program_min_delta(struct clock_event_device *dev)
> +static int __clockevents_program_min_delta(struct clock_event_device *dev)
>  {
>  	unsigned long long clc;
>  	int64_t delta;
> @@ -263,13 +258,7 @@ static int clockevents_program_min_delta
>  
>  #else  /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */
>  
> -/**
> - * clockevents_program_min_delta - Set clock event device to the minimum delay.
> - * @dev:	device to program
> - *
> - * Returns 0 on success, -ETIME when the retry loop failed.
> - */
> -static int clockevents_program_min_delta(struct clock_event_device *dev)
> +static int __clockevents_program_min_delta(struct clock_event_device *dev)
>  {
>  	unsigned long long clc;
>  	int64_t delta = 0;
> @@ -293,6 +282,21 @@ static int clockevents_program_min_delta
>  #endif /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */
>  
>  /**
> + * clockevents_program_min_delta - Set clock event device to the minimum delay.
> + * @dev:	device to program
> + *
> + * Returns 0 on success, -ETIME when the retry loop failed.
> + */
> +static int clockevents_program_min_delta(struct clock_event_device *dev)
> +{
> +	if (dev->next_event_forced)
> +		return 0;
> +
> +	dev->next_event_forced = 1;
> +	return __clockevents_program_min_delta(dev);
> +}
> +
> +/**
>   * clockevents_program_event - Reprogram the clock event device.
>   * @dev:	device to program
>   * @expires:	absolute expiry time (monotonic clock)
> @@ -324,6 +328,11 @@ int clockevents_program_event(struct clo
>  		return dev->set_next_ktime(expires, dev);
>  
>  	delta = ktime_to_ns(ktime_sub(expires, ktime_get()));
> +
> +	/* Don't reprogram when a forced event is pending */
> +	if (dev->next_event_forced && delta <= (int64_t)dev->min_delta_ns)
> +		return 0;
> +
>  	if (delta <= 0)
>  		return force ? clockevents_program_min_delta(dev) : -ETIME;
>  

This last hunk seems duplicate of the clockevents_program_min_delta()
change, that also will bail when next_event_forced is set.

Also, I note, thing (except shutdown) will ever clear next_event_forced;
and I'm thinking that any interrupt or normal reprogram should be doing
that.

Now, there doesn't seem to be a generic intercept for
dev->event_handler, so clearing on interrupt is hard, but would
something like the completely untested below work?

---
diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index b0df28ddd394..a79f8fa10104 100644
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -108,6 +108,7 @@ 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 eaae1ce9f060..8f6621361e46 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;
 }
 
 /**
@@ -324,16 +325,31 @@ 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;
+	if (delta <= 0) {
+		rc = -ETIME;
+		goto error;
+	}
 
 	delta = min(delta, (int64_t) dev->max_delta_ns);
 	delta = max(delta, (int64_t) dev->min_delta_ns);
 
 	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
 	rc = dev->set_next_event((unsigned long) clc, dev);
+	if (rc)
+		goto error;
 
-	return (rc && force) ? clockevents_program_min_delta(dev) : rc;
+	dev->next_event_forced = 0;
+	return 0;
+
+error:
+	if (force) {
+		if (dev->next_event_forced)
+			return 0;
+
+		dev->next_event_forced = 1;
+		return clockevents_program_min_delta(dev);
+	}
+	return rc;
 }
 
 /*