Re: [PATCH v6] sched/deadline: fix earliest_dl.next logic

[Wanpeng Li]

2015-12-03 17:42 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>:

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

>From my hotmail instead of gmail. :-)

Regards,
Wanpeng Li

> earliest_dl.next should cache deadline of the earliest ready task that
> is also enqueued in the pushable rbtree, as pull algorithm uses this
> information to find candidates for migration: if the earliest_dl.next
> deadline of source rq is earlier than the earliest_dl.curr deadline of
> destination rq, the task from the source rq can be pulled.
>
> However, current implementation only guarantees that earliest_dl.next is
> the deadline of the next ready task instead of the next pushable task;
> which will result in potentially holding both rqs' lock and find nothing
> to migrate because of affinity constraints. 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 fixes both problems by updating earliest_dl.next when
> pushable dl task is enqueued/dequeued, similar to what we already do for
> RT.
>
> Tested-by: Luca Abeni <luca.abeni@xxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v5 -> v6:
>  * take advantage of next_node
> v4 -> v5:
>  * remove useless pick_next_earliest_dl_task declare
> v3 -> v4:
>  * move earliest_dl.next caculation under if (leftmost)
>  * don't reset dl_rq->earliest_dl.next
>  * just checking and eventually using the updated leftmost in
>    dequeue_pushable_dl_task()
> v2 -> v3:
>  * reset dl_rq->earliest_dl.next to 0 if !next_pushable
> v1 -> v2:
>  * fix potential NULL pointer dereference
>
>  kernel/sched/deadline.c | 58 +++++--------------------------------------------
>  1 file changed, 6 insertions(+), 52 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 8b0a15e..a35e24a 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -176,8 +176,10 @@ static void enqueue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>                 }
>         }
>
> -       if (leftmost)
> +       if (leftmost) {
>                 dl_rq->pushable_dl_tasks_leftmost = &p->pushable_dl_tasks;
> +               dl_rq->earliest_dl.next = p->dl.deadline;
> +       }
>
>         rb_link_node(&p->pushable_dl_tasks, parent, link);
>         rb_insert_color(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
> @@ -195,6 +197,9 @@ static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>
>                 next_node = rb_next(&p->pushable_dl_tasks);
>                 dl_rq->pushable_dl_tasks_leftmost = next_node;
> +               if (next_node)
> +                       dl_rq->earliest_dl.next = rb_entry(next_node,
> +                               struct task_struct, pushable_dl_tasks)->dl.deadline;
>         }
>
>         rb_erase(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
> @@ -782,42 +787,14 @@ 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);
>         }
>  }
>
> @@ -839,7 +816,6 @@ 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);
>         }
>  }
> @@ -1274,28 +1250,6 @@ 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
>



-- 
Regards,
Wanpeng Li
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