Re: [PATCH v2 3/6] hrtimer,sched: Add fuzzy hrtimer mode for HRTICK

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, Jan 21 2026 at 17:20, Peter Zijlstra wrote:
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -928,7 +928,8 @@ void hrtick_start(struct rq *rq, u64 del
>  static void hrtick_rq_init(struct rq *rq)
>  {
>  	INIT_CSD(&rq->hrtick_csd, __hrtick_start, rq);
> -	hrtimer_setup(&rq->hrtick_timer, hrtick, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
> +	hrtimer_setup(&rq->hrtick_timer, hrtick, CLOCK_MONOTONIC,
> +		      HRTIMER_MODE_REL_HARD | HRTIMER_MODE_FUZZY);

SHouldn't this be HRTIMER_MODE_REL_PINNED_HARD? I know it's set when
starting the timer, but I had to double check it.

>  }
>  #else /* !CONFIG_SCHED_HRTICK: */
>  static inline void hrtick_clear(struct rq *rq)
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -1122,7 +1122,7 @@ static void __remove_hrtimer(struct hrti
>  	 * an superfluous call to hrtimer_force_reprogram() on the
>  	 * remote cpu later on if the same timer gets enqueued again.
>  	 */
> -	if (reprogram && timer == cpu_base->next_timer)
> +	if (!timer->is_fuzzy && reprogram && timer == cpu_base->next_timer)
>  		hrtimer_force_reprogram(cpu_base, 1);
>  }
>  
> @@ -1269,6 +1269,19 @@ static int __hrtimer_start_range_ns(stru
>  	if (new_base->cpu_base->in_hrtirq)
>  		return 0;
>  
> +	if (timer->is_fuzzy) {
> +		/*
> +		 * XXX fuzzy implies pinned!  not sure how to deal with
> +		 * retrigger_next_event() for the !local case.

I'd rather say:

           Fuzzy requires pinned as the lazy reprogramming only works
           for CPU local timers.

> +		 */
> +		WARN_ON_ONCE(!(mode & HRTIMER_MODE_PINNED));

Other than that:

Reviewed-by: Thomas Gleixner <tglx@kernel.org>


Thanks,

        tglx