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

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Date:   Thu, 5 Jan 2017 12:48:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     luca abeni <luca.abeni@...tn.it>
Cc:     linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: Doubt about push_dl_task() / find_lock_later_rq()

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

Right, that's what I remember.

> 
> 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...