Re: [patch 01/12] clockevents: Prevent timer interrupt starvation

[Thomas Gleixner]

Calvin!

On Tue, Apr 07 2026 at 10:54, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxx>
>
> 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.

It'd be great if you could re-test this one independently of the other
changes, so we can get that on the way ASAP.

Thanks,

        tglx