linux-kernel - Doubt about push_dl_task() / find_lock_later

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Message-ID: <20170104154935.7f08d550@sweethome>
Date:   Wed, 4 Jan 2017 15:49:35 +0100
From:   luca abeni <luca.abeni@...tn.it>
To:     linux-kernel@...r.kernel.org
Cc:     Juri Lelli <juri.lelli@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Doubt about push_dl_task() / find_lock_later_rq()

Hi all,

trying to debug a reclaiming issue discovered by Daniel, I find myself
confused by the push logic... Maybe I am misunderstanding something
very obvious, so I ask here:

- push_dl_task() selects a task to be pushed, and then searches for a
  runqueue to push the task to by calling find_lock_later_rq()
- if I understand well, find_lock_later_rq() checks all the candidate
  runqueues for pushing, and then compares the deadline of the task
  with "dl.earliest_dl.curr" of the candidate runqueue, to check if
  pushing the task there makes sense or not
- now, my understanding is that in order to implement gEDF task T must
  be pushed on CPU C if the deadline of T is smaller than the earliest
  deadline of tasks on C... That is to say, the deadline of T must be
  smaller than the deadline of the task that is currently executing on
  C... No?
- But as far as I understand "dl.earliest_dl.curr" is the earliest
  deadline of _pushable_ tasks that are on the remote runqueue... That
  is to say, "earliest_dl.curr" does not consider the deadline of the
  task currently executing on the remote runqueue
- So, it seems to me that tasks are sometimes pushed to other runqueues
  even if they have a deadline that is not smaller than the deadline of
  the task executing on the "target" runqueue (so, a task is pushed but
  not immediately scheduled for execution). Is this correct? What is
  the logic behind this behaviour?
I would be tempted to say that the correct check is not
	dl_time_before(task->dl.deadline, later_rq->dl.earliest_dl.curr)
  (as it is now in find_lock_later_rq()), but
	dl_time_before(task->dl.deadline, later_rq->curr->dl.deadline)
This, in my view, would migrate a task only when it is going to preempt
the current of the remote runqueue. What am I missing?


			Thanks,
				Luca