linux-kernel - Re: [PATCH] sched/deadline: comment update for constrained deadline tasks explanation

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Message-ID: <f6dc63d8-6afc-4a0b-2fcf-9aaf9a8d8fc6@redhat.com>
Date:   Fri, 24 Nov 2017 16:56:14 +0100
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Alessio Balsini <alessio.balsini@....com>,
        linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [PATCH] sched/deadline: comment update for constrained deadline
 tasks explanation

On 11/24/2017 12:12 PM, Alessio Balsini wrote:
> Remove the adjective "relative" associated to the period of a periodic
> task, since the period is an absolute value.
> 
> Rephrased the comment with the aim of making it even more clear.
> 
> Signed-off-by: Alessio Balsini <alessio.balsini@....com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: Luca Abeni <luca.abeni@...tannapisa.it>
> Cc: Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
> Cc: linux-kernel@...r.kernel.org
> ---
>  kernel/sched/deadline.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index f349f7e..902b2e0 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -770,9 +770,9 @@ update_dl_revised_wakeup(struct sched_dl_entity *dl_se, struct rq *rq)
>  }
>  
>  /*
> - * Regarding the deadline, a task with implicit deadline has a relative
> - * deadline == relative period. A task with constrained deadline has a
> - * relative deadline <= relative period.
> + * Regarding periodic real-time tasks, a constrained deadline task has the

Not only periodic, but also sporadic. Moreover, for sporadic it is not
_period_, but _minimum inter-arrival time_...

IMHO, it is not worth to add such level of details, not even on articles
people do, it just turn the explanation more confuse. See this, in the
page 5:

http://www-users.cs.york.ac.uk/~robdavis/papers/MPSurveyv5.0.pdf

There is no mention for the kind of period.

> + * relative deadline <= period. An implicit deadline task is a particular case

It is important to keep the "relative" to avoid confusion with the
variable "period" which is the "absolute period." I put relative to
helps with the ambiguity of the term "period."

-- Daniel