Re: [PATCH 6/6 v3] sched/eevdf: Speedup short slice task scheduling

[K Prateek Nayak]

Hello Peter,

On 6/26/2026 3:58 AM, Peter Zijlstra wrote:
> +static u64 ineligible_vruntime(struct cfs_rq *cfs_rq)
> +{
> +	struct sched_entity *curr = cfs_rq->curr;
> +	long weight = cfs_rq->sum_weight;
> +	s64 delta = 0;
> +
> +	if (curr && !curr->on_rq)
> +		curr = NULL;
> +
> +	/*
> +	 * This is called from set_next_task_fair(.first=true) /
> +	 * set_protect_slice() so curr had better be set and on_rq.
> +	 */
> +	WARN_ON_ONCE(!curr);

set_protect_slice() is indeed called from set_next_entity(.first=true)
but it is done after __dequeue_entity() and before "cfs_rq->curr" is
set (both sched/flat and sched/core have the same pattern).

You should be hitting this splat very easily unless you have moved
set_protect_slice() after the setting of cfs_rq->curr in your tree.

> +
> +	if (weight) {
> +		s64 runtime = cfs_rq->sum_w_vruntime;
> +
> +		/*
> +		 * Do not add @curr to obtain the effective '- w_j' terms.
> +		 */
> +
> +		/* sign flips effective floor / ceiling */
> +		if (runtime < 0)
> +			runtime -= (weight - 1);
> +
> +		delta = div64_long(runtime, weight);
> +	}
> +
> +	return cfs_rq->zero_vruntime + delta + 1;
> +}
> +
>  static inline u64 cfs_rq_max_slice(struct cfs_rq *cfs_rq);
>  
>  /*
> @@ -1058,8 +1119,14 @@ static inline void set_protect_slice(str
>  		slice = cfs_rq_min_slice(cfs_rq);
>  
>  	slice = min(slice, se->slice);
> -	if (slice != se->slice)
> -		vprot = min_vruntime(vprot, se->vruntime + calc_delta_fair(slice, se));
> +
> +	/* If there are shorter slices than se's one */
> +	if (slice != se->slice) {

I guess that condition was protecting a simple boot test but If I run:

  for i in 10000000 20000000 30000000 40000000 50000000;
  do
      sudo taskset -c 0 chrt -v -o -T $i 0 loop&     # Simple while(1) loop
  done


I see the splat:

  ------------[ cut here ]------------
  !curr
  WARNING: kernel/sched/fair.c:858 at set_next_task_fair+0x1d1/0x870, CPU#0: kworker/0:5/3750
  ...  

> +		if (sched_feat(PREEMPT_SHORT))
> +			vprot = min_vruntime(vprot, ineligible_vruntime(cfs_rq));
> +		else
> +			vprot = min_vruntime(vprot, se->vruntime + calc_delta_fair(slice, se));
> +	}
>  
>  	se->vprot = vprot;
>  }

-- 
Thanks and Regards,
Prateek