linux-kernel - Re: [PATCH v5] sched/deadline: fix earliest

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Date:	Wed, 2 Dec 2015 15:08:42 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Wanpeng Li <wanpeng.li@...mail.com>,
	Juri Lelli <juri.lelli@....com>,
	Wanpeng Li <kernellwp@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] sched/deadline: fix earliest_dl.next logic

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
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