linux-kernel - Re: Doubt about push_dl_task() / find_lock_later

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Message-ID: <20170105082923.639adcf9@sweethome>
Date:   Thu, 5 Jan 2017 08:29:23 +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: Re: Doubt about push_dl_task() / find_lock_later_rq()

On Wed, 4 Jan 2017 15:49:35 +0100
luca abeni <luca.abeni@...tn.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