Re: Re: [linus:master] [sched/eevdf] 2227a957e1: BUG:kernel_NULL_pointer_dereference,address

[Abel Wu]

On 2/19/24 8:49 PM, Chen Yu Wrote:
> While looking at pick_eevdf(), I have a thought.
> Currently the sched entity is sorted by their deadline. During task
> pickup, the pick_eevdf() scans for an candidate sched entity with the
> smallest deadline. Meanwhile this candidate sched entity must also be
> eligible.
>
> The scan is O(lgn) on average, and O(1) at best case. How about making the
> average scan even faster by sorting the sched entity not only by deadline,
> but also the eligibility? The idea is that, the eligible sched entity with
> smaller deadline is sorted at the front the tree. Otherwise, if the entity
> is not eligible, even if it has a smaller deadline, it should be sorted
> at the end of the tree.

Eligibility is dynamic due to the nature of weighted average vruntime.
IIUC if doing so like above, update_curr() should take the responsibility
to re-sort the tree which seems to be O(logN).

> After the change, pick_eevdf() get the leftmost sched entity at O(1) on
> average. Besides, it is guaranteed to return non-NULL sched entity in
> pick_eevdf(), which prevents suspicious NULL pointer exception in pick_eevdf().

It is guaranteed when doing pick that the rbtree is non-NULL, and given
that rq lock is held, I don't think the bug is inside pick_eevdf().

> For example, suppose there are two sched entities to be queued, se_a and se_b.
> Consider their eligibility and deadline, there are 6 combination:
>
> 1. se_a is eligible, se_b is eligible, se_a.deadline < se_b.deadline
> 2. se_a is eligible, se_b is eligible, se_a.deadline >= se_b.deadline
> 3. se_a is eligible, se_b is not eligible
> 4. se_a is not eligible, se_b is eligible
> 5. se_a is not eligible, se_b is not eligible, se_a.deadline < se_b.deadline
> 6. se_a is not eligible, se_b is not eligible, se_a.deadline >= se_b.deadline
>
> In scenario 1, 3, 5, sched_entity se_a should be sorted before se_b,
> so pick_eevdf() would pick se_a first.
>
> When enqueuing a new sched entity, it is regarded as eligible if its
> vlag is positive. In theory later in pick_eevdf(), the eligibility
> of this sched entity should be re-checked via entity_eligible(). But
> consider if the sched entity is eliglble when enqueued, it is very
> likely the same sched entity remains eligible when pick_eevdf(), because
> the V keeps moving forward but the vruntime of this sched entity remain
> unchanged - the vlag could get larger.
>
> Something like this untested:
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 533547e3c90a..831043cc1432 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -551,11 +551,19 @@ static inline u64 min_vruntime(u64 min_vruntime, u64 vruntime)
>   static inline bool entity_before(const struct sched_entity *a,
>   				 const struct sched_entity *b)
>   {
> -	/*
> -	 * Tiebreak on vruntime seems unnecessary since it can
> -	 * hardly happen.
> -	 */
> -	return (s64)(a->deadline - b->deadline) < 0;
> +	bool eli_a, eli_b;
> +
> +	eli_a = (a->vlag >= 0) ? true : false;
> +	eli_b = (b->vlag >= 0) ? true : false;
> +
> +	if ((eli_a && eli_b) || (!eli_a && !eli_b))
> +		/*
> +		 * Tiebreak on vruntime seems unnecessary since it can
> +		 * hardly happen.
> +		 */
> +		return (s64)(a->deadline - b->deadline) < 0;
> +
> +	return eli_a ? 1 : 0;
>   }
>   
>   static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
> @@ -877,10 +885,8 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
>    */
>   static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>   {
> -	struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
>   	struct sched_entity *se = __pick_first_entity(cfs_rq);
>   	struct sched_entity *curr = cfs_rq->curr;
> -	struct sched_entity *best = NULL;
>   
>   	/*
>   	 * We can safely skip eligibility check if there is only one entity
> @@ -899,45 +905,8 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
>   	if (sched_feat(RUN_TO_PARITY) && curr && curr->vlag == curr->deadline)
>   		return curr;
>   
> -	/* Pick the leftmost entity if it's eligible */
> -	if (se && entity_eligible(cfs_rq, se)) {
> -		best = se;
> -		goto found;
> -	}
> -
> -	/* Heap search for the EEVD entity */
> -	while (node) {
> -		struct rb_node *left = node->rb_left;
> -
> -		/*
> -		 * Eligible entities in left subtree are always better
> -		 * choices, since they have earlier deadlines.
> -		 */
> -		if (left && vruntime_eligible(cfs_rq,
> -					__node_2_se(left)->min_vruntime)) {
> -			node = left;
> -			continue;
> -		}
> -
> -		se = __node_2_se(node);
> -
> -		/*
> -		 * The left subtree either is empty or has no eligible
> -		 * entity, so check the current node since it is the one
> -		 * with earliest deadline that might be eligible.
> -		 */
> -		if (entity_eligible(cfs_rq, se)) {
> -			best = se;
> -			break;
> -		}
> -
> -		node = node->rb_right;
> -	}
> -found:
> -	if (!best || (curr && entity_before(curr, best)))
> -		best = curr;
> -
> -	return best;
> +	/* Pick the leftmost entity */
> +	return se;
>   }
>   
>   #ifdef CONFIG_SCHED_DEBUG