Re: [PATCH] sched/deadline: fix earliest_dl.next is not the pushable candidate

[Juri Lelli <juri.lelli@arm.com>]

Hi,

On 10/22/15, Wanpeng Li wrote:
> earliest_dl.next is used to cache the next earliest ready task 
> which also is pushable in order to be a candidate of pushable 
> tasks during pull algorithm. If the earliest_dl.next deadline 
> of the sr_rq is earlier than the earliest_dl.curr deadline of 
> current rq, the task from the sr_rq can be pulled. However, 
> current implementation logic of earliest_dl.next just guarantee 
> it is the next earliest ready task instead of the next pushable 
> earliest task, which will result in hold both rqs' lock and find 
> nothing to preempt our current since the affinity in pull algorithm. 
> In addition, current logic doesn't update the next candidate for 
> pushing in pick_next_task_dl() even if the running task is never 
> eligible.
> 
> This patch fix it by updating earliest_dl.next when pushable dl task is 
> enqueued/dequeued the same way as rt class.
> 
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
>  kernel/sched/deadline.c | 58 ++++++-------------------------------------------
>  1 file changed, 7 insertions(+), 51 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 142df26..6175c94 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -87,6 +87,8 @@ void init_dl_rq(struct dl_rq *dl_rq)
>  
>  #ifdef CONFIG_SMP
>  
> +static struct task_struct *pick_next_pushable_dl_task(struct rq *rq);
> +
>  static inline int dl_overloaded(struct rq *rq)
>  {
>  	return atomic_read(&rq->rd->dlo_count);
> @@ -181,6 +183,9 @@ static void enqueue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>  
>  	rb_link_node(&p->pushable_dl_tasks, parent, link);
>  	rb_insert_color(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
> +
> +	if (dl_time_before(p->dl.deadline, dl_rq->earliest_dl.next))
> +		dl_rq->earliest_dl.next = p->dl.deadline;
>  }
>  
>  static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
> @@ -199,6 +204,8 @@ static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>  
>  	rb_erase(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
>  	RB_CLEAR_NODE(&p->pushable_dl_tasks);
> +
> +	dl_rq->earliest_dl.next = pick_next_pushable_dl_task(rq)->dl.deadline;

NULL pointer dereference if rq doesn't have pushable tasks.

Thanks,

- Juri

>  }
>  
>  static inline int has_pushable_dl_tasks(struct rq *rq)
> @@ -775,42 +782,14 @@ static void update_curr_dl(struct rq *rq)
>  
>  #ifdef CONFIG_SMP
>  
> -static struct task_struct *pick_next_earliest_dl_task(struct rq *rq, int cpu);
> -
> -static inline u64 next_deadline(struct rq *rq)
> -{
> -	struct task_struct *next = pick_next_earliest_dl_task(rq, rq->cpu);
> -
> -	if (next && dl_prio(next->prio))
> -		return next->dl.deadline;
> -	else
> -		return 0;
> -}
> -
>  static void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
>  {
>  	struct rq *rq = rq_of_dl_rq(dl_rq);
>  
>  	if (dl_rq->earliest_dl.curr == 0 ||
>  	    dl_time_before(deadline, dl_rq->earliest_dl.curr)) {
> -		/*
> -		 * If the dl_rq had no -deadline tasks, or if the new task
> -		 * has shorter deadline than the current one on dl_rq, we
> -		 * know that the previous earliest becomes our next earliest,
> -		 * as the new task becomes the earliest itself.
> -		 */
> -		dl_rq->earliest_dl.next = dl_rq->earliest_dl.curr;
>  		dl_rq->earliest_dl.curr = deadline;
>  		cpudl_set(&rq->rd->cpudl, rq->cpu, deadline, 1);
> -	} else if (dl_rq->earliest_dl.next == 0 ||
> -		   dl_time_before(deadline, dl_rq->earliest_dl.next)) {
> -		/*
> -		 * On the other hand, if the new -deadline task has a
> -		 * a later deadline than the earliest one on dl_rq, but
> -		 * it is earlier than the next (if any), we must
> -		 * recompute the next-earliest.
> -		 */
> -		dl_rq->earliest_dl.next = next_deadline(rq);
>  	}
>  }
>  
> @@ -832,7 +811,6 @@ static void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
>  
>  		entry = rb_entry(leftmost, struct sched_dl_entity, rb_node);
>  		dl_rq->earliest_dl.curr = entry->deadline;
> -		dl_rq->earliest_dl.next = next_deadline(rq);
>  		cpudl_set(&rq->rd->cpudl, rq->cpu, entry->deadline, 1);
>  	}
>  }
> @@ -1267,28 +1245,6 @@ static int pick_dl_task(struct rq *rq, struct task_struct *p, int cpu)
>  	return 0;
>  }
>  
> -/* Returns the second earliest -deadline task, NULL otherwise */
> -static struct task_struct *pick_next_earliest_dl_task(struct rq *rq, int cpu)
> -{
> -	struct rb_node *next_node = rq->dl.rb_leftmost;
> -	struct sched_dl_entity *dl_se;
> -	struct task_struct *p = NULL;
> -
> -next_node:
> -	next_node = rb_next(next_node);
> -	if (next_node) {
> -		dl_se = rb_entry(next_node, struct sched_dl_entity, rb_node);
> -		p = dl_task_of(dl_se);
> -
> -		if (pick_dl_task(rq, p, cpu))
> -			return p;
> -
> -		goto next_node;
> -	}
> -
> -	return NULL;
> -}
> -
>  /*
>   * Return the earliest pushable rq's task, which is suitable to be executed
>   * on the CPU, NULL otherwise:
> -- 
> 1.9.1
> 
>