On Wed, 4 Jan 2017 15:49:35 +0100
luca abeni <luca.ab...@unitn.it> wrote:
> 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...
So, after re-reading the code I now see that my understanding here was
wrong: "dl.earliest_dl.curr" is really supposed to be the deadline of
the earliest deadline task on the runqueue... So, if I do not play
with affinities it should be the deadline of the task that is currently
executing on that CPU.
So, everything is fine.
I was confused by the fact that in some cases I saw
rq->dl.earliest_dl.curr != rq->curr->dl.deadline
I still do not understand how this can happen (I am not changing tasks
affinities), and I am investigating this.
Thanks,
Luca
> 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
