Messages in this thread | | | Date | Mon, 24 Apr 2017 11:53:13 +0100 | From | Juri Lelli <> | Subject | Re: [PATCH] sched/deadline: fix switching to -deadline |
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On 24/04/17 12:36, Luca Abeni wrote: > On Mon, 24 Apr 2017 11:16:24 +0100 > Juri Lelli <juri.lelli@arm.com> wrote: > > > On 21/04/17 21:08, Luca Abeni wrote:
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> > > > > > Well, double thinking about it, this is an interesting problem... > > > What do we want to do with do_set_cpus_allowed()? (I mean: what is > > > the expected behaviour?) > > > > > > With this patch, if a task is moved to a different runqueue when its > > > deadline is in the past (because we are doing gEDF, or because of > > > timer granularity issues) its scheduling deadline is reinitialized > > > to current time + relative deadline... I think this makes perfect > > > sense, doesn't it? > > > > > > > Mmm, I don't think we will (with this patch) actually reinitialize the > > deadline when a "normal" gEDF migration happen (push/pull), as > > (de)activate_task() have no flag set. Which brings the question, > > should we actually take care of this corner case (as what you say > > makes sense to me too)? > > I might be misunderstanding the problem, here... Are you talking about > do_set_cpus_allowed()? Or about push/pull migrations happening because > of the gEDF algorithm? >
My concern was about do_set_cpus_allowed(), but then you mentioned "because we are doing gEDF" and that made me think of what happens when we do push/pull. :)
> If you are referring to do_set_cpus_allowed, this is my understanding: > 1) If do_set_cpus_allowed() is called on a queued task, then > dequeue_task() with DEQUEUE_SAVE is called, followed by > enqueue_task() with ENQUEUE_RESTORE... So, if the deadline is in the > past it is correctly reinitialized > 2) If do_set_cpus_allowed() is called on a non-queued task, this means > the task is blocked, no? So, when it will wake up enqueue_dl_entity() > will invoke update_dl_entity() that will check if the deadline is in > the past. >
OK. I think it makes sense, and your patch should cure the problem. Maybe add a comment to note this down.
> If you are referring to push/pull migrations due to gEDF, then > enqueue_dl_entity() will be invoked with "flags" = 0, so the deadline > will not be changed (and this is correct: we do not want to > initialize / change tasks' deadlines during gEDF migrations). >
Ok, but I was wondering about the (admittedly) corner case in which we migrate (via push/pull) a task on a rq, the rq_clock of which is after the task's deadline (because clocks on src_rq and dst_rq are not in sync). Anyway, maybe it's so corner case that we don't really want to deal with it right now? I guess bigger things to fix first. :)
> In my previous email, with "a task is moved to a different runqueue" I > wanted to say that the taks is forced to moved to a different runqueue > because its affinity is changed; I did not want to talk about "regular > migrations" due to the push/pull (gEDF) mechanism. >
Thanks for claryfing. As said, I just got distracted by what you mentioned as examples between parenthesis.
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