Re: [patch 03/12] hrtimer: Use hrtimer_start_expires_user() for hrtimer sleepers

[Peter Zijlstra]

On Tue, Apr 07, 2026 at 10:54:27AM +0200, Thomas Gleixner wrote:
> Most hrtimer sleepers are user controlled and user space can hand arbitrary
> expiry values in as long as they are valid timespecs. If the expiry value
> is in the past then this requires a full loop through reprogramming the
> clock event device, taking the hrtimer interrupt, waking the task and
> reprogram again.
> 
> Use hrtimer_start_expires_user() which avoids the full round trip by
> checking the timer for expiry on enqueue.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
>  kernel/time/hrtimer.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -2152,7 +2152,11 @@ void hrtimer_sleeper_start_expires(struc
>  	if (IS_ENABLED(CONFIG_PREEMPT_RT) && sl->timer.is_hard)
>  		mode |= HRTIMER_MODE_HARD;
>  
> -	hrtimer_start_expires(&sl->timer, mode);
> +	/* If already expired, clear the task pointer and set current state to running */
> +	if (!hrtimer_start_expires_user(&sl->timer, mode)) {
> +		sl->task = NULL;
> +		__set_current_state(TASK_RUNNING);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(hrtimer_sleeper_start_expires);
>  
>