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Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when calculating overflow

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On 02/15/2017 01:59 PM, Juri Lelli wrote:
> Actually, another thing that we noticed, talking on IRC with Peter, is
> that we seem to be replenishing differently on different occasions:

When a task is awakened (not by the replenishment timer), it is not
possible to know if the absolute deadline var stores the absolute
deadline of activation which took place in the instant
(current time) - dl_period.

Therefore, assuming the next deadline is one dl_deadline away from now
is correct.

IOW: that is a sporadic activation - the task is activated after at
least minimum inter-arrival time between activation/replenishment:

>  - on wakeup (if overflowing) we do
> 
>    dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
>    dl_se->runtime = pi_se->dl_runtime;


In the replenishment timer, it is known that the absolute deadline
instant of the previous activation is in the deadline var. So
putting the absolute deadline one dl_period away is correct [1].

Another point is that this case avoids creating time drift due
to latencies. For instance, in the case of a 1 ms delay of the timer
(interrupts disabled?), the wakeup replenishment would push the
absolute a relative deadline + 1 ms away from the previous deadline.

IOW: the replenishment timer makes the periodic case - a fixed time
offset from the previous activation/replenishment.

>  - when the replenishment timer fires (un-thottle and with runtime < 0)
> 
>    dl_se->deadline += pi_se->dl_period;
>    dl_se->runtime += pi_se->dl_runtime;

So I think it is correct. Am I missing something?

[1] For the sake of completeness:

- dl_se->deadline = Absolute deadline
- dl_se->dl_deadline = Relative deadline

the next absolute deadline is at:

dl_se->deadline = dl_next_period(dl_se) + dl_se->dl_deadline;

as dl_next_period(dl_se) is:

dl_se->deadline - dl_se->dl_deadline + dl_se->dl_period;

the next deadline is at:

dl_se->deadline = dl_se->deadline - dl_se->dl_deadline +
dl_se->dl_period + dl_se->dl_deadline

Which can be simplified to:

dl_se->deadline = dl_se->deadline += pi_se->dl_period;

because we have (- dl_se->dl_deadline) + dl_se->dl_deadline.

-- Daniel

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[PATCH V2 0/2] sched/deadline: Fixes for constrained deadline tasks Daniel Bristot de Oliveira <bristot@redhat.com> - 2017-02-13 20:10 +0100
  [PATCH V2 2/2] sched/deadline: Throttle a constrained deadline task activated after the deadline Daniel Bristot de Oliveira <bristot@redhat.com> - 2017-02-13 20:10 +0100
    Re: [PATCH V2 2/2] sched/deadline: Throttle a constrained deadline  task activated after the deadline Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it> - 2017-02-14 18:00 +0100
      Re: [PATCH V2 2/2] sched/deadline: Throttle a constrained deadline  task activated after the deadline Daniel Bristot de Oliveira <bristot@redhat.com> - 2017-02-14 18:40 +0100
    Re: [PATCH V2 2/2] sched/deadline: Throttle a constrained deadline  task activated after the deadline Steven Rostedt <rostedt@goodmis.org> - 2017-02-14 20:40 +0100
  [PATCH V2 1/2] sched/deadline: Replenishment timer should fire in the next period Daniel Bristot de Oliveira <bristot@redhat.com> - 2017-02-13 20:10 +0100
  [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow "Steven Rostedt (VMware)" <rostedt@goodmis.org> - 2017-02-14 20:30 +0100
    Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow luca abeni <luca.abeni@santannapisa.it> - 2017-02-14 23:50 +0100
      Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Steven Rostedt <rostedt@goodmis.org> - 2017-02-15 01:20 +0100
        Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Luca Abeni <luca.abeni@santannapisa.it> - 2017-02-15 08:50 +0100
          Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Juri Lelli <juri.lelli@arm.com> - 2017-02-15 11:30 +0100
            Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Peter Zijlstra <peterz@infradead.org> - 2017-02-15 12:40 +0100
            Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Luca Abeni <luca.abeni@santannapisa.it> - 2017-02-15 13:40 +0100
              Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Juri Lelli <juri.lelli@arm.com> - 2017-02-15 14:00 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Luca Abeni <luca.abeni@santannapisa.it> - 2017-02-15 14:20 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Juri Lelli <juri.lelli@arm.com> - 2017-02-15 15:20 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Daniel Bristot de Oliveira <bristot@redhat.com> - 2017-02-15 14:40 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Daniel Bristot de Oliveira <bristot@redhat.com> - 2017-02-15 14:50 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Steven Rostedt <rostedt@goodmis.org> - 2017-02-15 15:10 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Juri Lelli <juri.lelli@arm.com> - 2017-02-15 15:20 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it> - 2017-02-16 17:40 +0100
                Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when  calculating overflow Steven Rostedt <rostedt@goodmis.org> - 2017-02-16 17:50 +0100

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