Re: [PATCH v2] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[Peter Zijlstra]

On Wed, May 27, 2026 at 03:21:13PM +0800, Li kunyu wrote:
> The logic of the function here should be to convert the virtual runtime
> into actual time by combining it with the weight of the scheduling
> entity.
> However, it cannot be converted by simply converting actual time into
> virtual running time. A reverse conversion is required to obtain the
> correct time.
> 
> Signed-off-by: Li kunyu <likunyu10@xxxxxxx>
> ---
> v2: Use the div64_ul function to perform division operation
> 
>  kernel/sched/fair.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69361c63353a..53f3373c1735 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7033,7 +7033,9 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
>  			resched_curr(rq);
>  		return;
>  	}
> -	delta = (se->load.weight * vdelta) / NICE_0_LOAD;
> +
> +	if (unlikely(se->load.weight != NICE_0_LOAD))
> +		delta = div64_ul(NICE_0_LOAD * vdelta, se->load.weight);
>  
>  	/*
>  	 * Correct for instantaneous load of other classes.

This seems wrong. Note that calc_delta_fair() does: 

  vdelta := (delta * NICE_0_LOAD) / se->load.weight

which gets us time -> vtime. Therefore the vtime -> time transform is
the inverse of that:

  delta := (vdelta * se->load.weight) / NICE_0_LOAD;

which is exactly what the current code does.

Specifically:

                  NICE_0_LOAD      se->load.weight
       (delta * ---------------) * --------------- = delta
                se->load.weight      NICE_0_LOAD