Re: [PATCH v3] sched/deadline: fix earliest_dl.next logic

[Juri Lelli]

Hi,

On 24/11/15 09:58, Wanpeng Li wrote:
> Ping Peterz, :-)
> On 11/19/15 6:11 PM, 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.
> >
> >Signed-off-by: Wanpeng li <wanpeng.li@xxxxxxxxxxx>
> >---
> >v2 -> v3:
> >  * reset dl_rq->earliest_dl.next to 0 if !next_pushable
> >v1 -> v2:
> >  * fix potential NULL pointer dereference
> >
> >  kernel/sched/deadline.c | 63 ++++++++++---------------------------------------
> >  1 file changed, 12 insertions(+), 51 deletions(-)
> >
> >diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> >index 142df26..547d102 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);
> >@@ -181,11 +183,15 @@ 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)
> >  {
> >  	struct dl_rq *dl_rq = &rq->dl;
> >+	struct task_struct *next_pushable;
> >  	if (RB_EMPTY_NODE(&p->pushable_dl_tasks))
> >  		return;
> >@@ -199,6 +205,12 @@ 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);
> >+
> >+	next_pushable = pick_next_pushable_dl_task(rq);
> >+	if (next_pushable)
> >+		dl_rq->earliest_dl.next = next_pushable->dl.deadline;
> >+	else
> >+		dl_rq->earliest_dl.next = 0;

On a second thought, this might be useless as deadlines can wraparound.
However, there are other things that I need to check about pull_dl_
tasks(). Please, bear with me for a few other days :-).

Thanks,

- Juri

> >  }
> >  static inline int has_pushable_dl_tasks(struct rq *rq)
> >@@ -775,42 +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);
> >  	}
> >  }
> >@@ -832,7 +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);
> >  	}
> >  }
> >@@ -1267,28 +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:
> 
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