[PATCH v5] sched/deadline: fix earliest_dl.next logic

[PATCH v5] sched/deadline: fix earliest_dl.next logic

[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@unitn.it>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
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 | 69 ++++++++++---------------------------------------
 1 file changed, 13 insertions(+), 56 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 8b0a15e..087d090 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -176,13 +176,20 @@ 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);
 }
 
+static inline int has_pushable_dl_tasks(struct rq *rq)
+{
+	return !RB_EMPTY_ROOT(&rq->dl.pushable_dl_tasks_root);
+}
+
 static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
 {
 	struct dl_rq *dl_rq = &rq->dl;
@@ -199,11 +206,12 @@ 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);
-}
 
-static inline int has_pushable_dl_tasks(struct rq *rq)
-{
-	return !RB_EMPTY_ROOT(&rq->dl.pushable_dl_tasks_root);
+	if (has_pushable_dl_tasks(rq)) {
+		p = rb_entry(rq->dl.pushable_dl_tasks_leftmost,
+		     struct task_struct, pushable_dl_tasks);
+		dl_rq->earliest_dl.next = p->dl.deadline;
+	}
 }
 
 static int push_dl_task(struct rq *rq);
@@ -782,42 +790,14 @@ 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 +819,6 @@ 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 +1253,6 @@ 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

Re: [PATCH v5] sched/deadline: fix earliest_dl.next logic

[Juri Lelli]

Hi,

On 02/12/15 19:47, Wanpeng Li wrote:
> 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@unitn.it>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
> 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 | 69 ++++++++++---------------------------------------
>  1 file changed, 13 insertions(+), 56 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 8b0a15e..087d090 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -176,13 +176,20 @@ 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);
>  }
>  
> +static inline int has_pushable_dl_tasks(struct rq *rq)
> +{
> +	return !RB_EMPTY_ROOT(&rq->dl.pushable_dl_tasks_root);
> +}
> +
>  static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>  {
>  	struct dl_rq *dl_rq = &rq->dl;
> @@ -199,11 +206,12 @@ 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);
> -}
>  
> -static inline int has_pushable_dl_tasks(struct rq *rq)
> -{
> -	return !RB_EMPTY_ROOT(&rq->dl.pushable_dl_tasks_root);
> +	if (has_pushable_dl_tasks(rq)) {
> +		p = rb_entry(rq->dl.pushable_dl_tasks_leftmost,
> +		     struct task_struct, pushable_dl_tasks);

I just skimmed through this, but here you are pointing p to leftmost.
Couldn't this cause troubles afterwards?

We updated leftmost above, can't we simply use that path for this thing
below?

Thanks,

- Juri

> +		dl_rq->earliest_dl.next = p->dl.deadline;
> +	}
>  }
>  
>  static int push_dl_task(struct rq *rq);
> @@ -782,42 +790,14 @@ 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 +819,6 @@ 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 +1253,6 @@ 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
> 

Re: [PATCH v5] sched/deadline: fix earliest_dl.next logic

[Luca Abeni]

Hi,

On 12/02/2015 02:33 PM, Wanpeng Li wrote:
[...]
>> We updated leftmost above, can't we simply use that path for this thing
>> below?
>
> Do you mean something like below?
>
> @@ -195,6 +195,9 @@ 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 (has_pushable_dl_tasks(rq))
I do not know the rb trees code, but... Are you sre you can call has_pushable_tasks() here?
(I suspect pushable_dl_tasks_root is not updated yet, so maybe has_pushable_dl_tasks() risks
to return a wrong value?)

> +                       dl_rq->earliest_dl.next = rb_entry(rq->dl.pushable_dl_tasks_leftmost,
> +                               struct task_struct, pushable_dl_task)->dl.deadline;
I am not sure if this is what Juri meant, but maybe something like this?

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 087d090..26d3279 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -185,11 +185,6 @@ static void enqueue_pushable_dl_task(struct rq *rq, struct task_struct *p)
  	rb_insert_color(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
  }

-static inline int has_pushable_dl_tasks(struct rq *rq)
-{
-	return !RB_EMPTY_ROOT(&rq->dl.pushable_dl_tasks_root);
-}
-
  static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
  {
  	struct dl_rq *dl_rq = &rq->dl;
@@ -202,16 +197,18 @@ 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);
  	RB_CLEAR_NODE(&p->pushable_dl_tasks);
+}

-	if (has_pushable_dl_tasks(rq)) {
-		p = rb_entry(rq->dl.pushable_dl_tasks_leftmost,
-		     struct task_struct, pushable_dl_tasks);
-		dl_rq->earliest_dl.next = p->dl.deadline;
-	}
+static inline int has_pushable_dl_tasks(struct rq *rq)
+{
+	return !RB_EMPTY_ROOT(&rq->dl.pushable_dl_tasks_root);
  }

  static int push_dl_task(struct rq *rq);


I do not know if it is correct, but I ran some quick tests and seem to work
without problems.



				Luca

Re: [PATCH v5] sched/deadline: fix earliest_dl.next logic

[Wanpeng Li]

2015-12-02 22:08 GMT+08:00 Luca Abeni <luca.abeni@unitn.it>:
> Hi,
>
> On 12/02/2015 02:33 PM, Wanpeng Li wrote:
> [...]
>>>
>>> We updated leftmost above, can't we simply use that path for this thing
>>> below?
>>
>>
>> Do you mean something like below?
>>
>> @@ -195,6 +195,9 @@ 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 (has_pushable_dl_tasks(rq))
>
> I do not know the rb trees code, but... Are you sre you can call
> has_pushable_tasks() here?
> (I suspect pushable_dl_tasks_root is not updated yet, so maybe
> has_pushable_dl_tasks() risks
> to return a wrong value?)

Right.

>
>> +                       dl_rq->earliest_dl.next =
>> rb_entry(rq->dl.pushable_dl_tasks_leftmost,
>> +                               struct task_struct,
>> pushable_dl_task)->dl.deadline;
>
> I am not sure if this is what Juri meant, but maybe something like this?
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 087d090..26d3279 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -185,11 +185,6 @@ static void enqueue_pushable_dl_task(struct rq *rq,
> struct task_struct *p)
>         rb_insert_color(&p->pushable_dl_tasks,
> &dl_rq->pushable_dl_tasks_root);
>  }
>
> -static inline int has_pushable_dl_tasks(struct rq *rq)
> -{
> -       return !RB_EMPTY_ROOT(&rq->dl.pushable_dl_tasks_root);
> -}
> -
>  static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>  {
>         struct dl_rq *dl_rq = &rq->dl;
> @@ -202,16 +197,18 @@ 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;

Juri mentioned "updated leftmost", I'm not sure if it means that:

@@ -195,6 +195,9 @@ 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 (dl_rq->pushable_dl_tasks_leftmost)
+                       dl_rq->earliest_dl.next =
rb_entry(dl_rq->pushable_dl_tasks_leftmost,
+                               struct task_struct,
pushable_dl_tasks)->dl.deadline;
        }


Regards,
Wanpeng Li

Re: [PATCH v5] sched/deadline: fix earliest_dl.next logic

[Luca Abeni]

Hi,

On 12/03/2015 03:25 AM, Wanpeng Li wrote:
[...]
>> @@ -202,16 +197,18 @@ 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;
>
> Juri mentioned "updated leftmost", I'm not sure if it means that:
>
> @@ -195,6 +195,9 @@ 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 (dl_rq->pushable_dl_tasks_leftmost)
> +                       dl_rq->earliest_dl.next =
> rb_entry(dl_rq->pushable_dl_tasks_leftmost,
> +                               struct task_struct,
> pushable_dl_tasks)->dl.deadline;
>          }
This is basically the same thing I tested (I just used "next_node" instead
of "dl_rq->pushable_dl_tasks_leftmost" because the name is shorter), so I
think it should work.


			Luca

Re: [PATCH v5] sched/deadline: fix earliest_dl.next logic

[Wanpeng Li]

2015-12-03 16:37 GMT+08:00 Luca Abeni <luca.abeni@unitn.it>:
> Hi,
>
> On 12/03/2015 03:25 AM, Wanpeng Li wrote:
> [...]
>>>
>>> @@ -202,16 +197,18 @@ 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;
>>
>>
>> Juri mentioned "updated leftmost", I'm not sure if it means that:
>>
>> @@ -195,6 +195,9 @@ 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 (dl_rq->pushable_dl_tasks_leftmost)
>> +                       dl_rq->earliest_dl.next =
>> rb_entry(dl_rq->pushable_dl_tasks_leftmost,
>> +                               struct task_struct,
>> pushable_dl_tasks)->dl.deadline;
>>          }
>
> This is basically the same thing I tested (I just used "next_node" instead
> of "dl_rq->pushable_dl_tasks_leftmost" because the name is shorter), so I
> think it should work.

Yeah, the same, what I want to know is Juri's choice. Ping Juri, :-)

Regards,
Wanpeng Li

Re: [PATCH v5] sched/deadline: fix earliest_dl.next logic

[Juri Lelli]

Hi,

On 03/12/15 16:59, Wanpeng Li wrote:
> 2015-12-03 16:37 GMT+08:00 Luca Abeni <luca.abeni@unitn.it>:
> > Hi,
> >
> > On 12/03/2015 03:25 AM, Wanpeng Li wrote:
> > [...]
> >>>
> >>> @@ -202,16 +197,18 @@ 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;
> >>
> >>
> >> Juri mentioned "updated leftmost", I'm not sure if it means that:
> >>
> >> @@ -195,6 +195,9 @@ 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 (dl_rq->pushable_dl_tasks_leftmost)
> >> +                       dl_rq->earliest_dl.next =
> >> rb_entry(dl_rq->pushable_dl_tasks_leftmost,
> >> +                               struct task_struct,
> >> pushable_dl_tasks)->dl.deadline;
> >>          }
> >
> > This is basically the same thing I tested (I just used "next_node" instead
> > of "dl_rq->pushable_dl_tasks_leftmost" because the name is shorter), so I
> > think it should work.
> 
> Yeah, the same, what I want to know is Juri's choice. Ping Juri, :-)
> 

IMHO, Luca's solution looks shorter/better :-).

Thanks,

- Juri