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

Quoted 6 lines · On Wed, 4 Jan 2017 15:49:35 +0100 luca abeni <[email protected]> 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:

Quoted 13 lines

- 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.

On Thu, Jan 05, 2017 at 08:29:23AM +0100, luca abeni wrote:

Quoted 5 lines

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.

I'm having trouble spotting code that does that...