linux-kernel - Re: [PATCH 2/2] sched/deadline: Throttle a constrained deadline task activated after the deadline

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Date:   Mon, 13 Feb 2017 12:12:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/2] sched/deadline: Throttle a constrained deadline task
 activated after the deadline

On Sat, Feb 11, 2017 at 08:15:26AM +0100, luca abeni wrote:
> Hi Daniel,
> 
> On Fri, 10 Feb 2017 20:48:11 +0100
> Daniel Bristot de Oliveira <bristot@...hat.com> wrote:
> 
> > During the activation, CBS checks if it can reuse the current task's
> > runtime and period. If the deadline of the task is in the past, CBS
> > cannot use the runtime, and so it replenishes the task. This rule
> > works fine for implicit deadline tasks (deadline == period), and the
> > CBS was designed for implicit deadline tasks. However, a task with
> > constrained deadline (deadine < period) might be awakened after the
> > deadline, but before the next period. In this case, replenishing the
> > task would allow it to run for runtime / deadline. As in this case
> > deadline < period, CBS enables a task to run for more than the
> > runtime/period. In a very load system, this can cause the domino
> > effect, making other tasks to miss their deadlines.
> 
> I think you are right: SCHED_DEADLINE implements the original CBS
> algorithm here, but uses relative deadlines different from periods in
> other places (while the original algorithm only considered relative
> deadlines equal to periods).
> An this mix is dangerous... I think your fix is correct, and cures a
> real problem.
> 

Made that a "Reviewed-by:" tag in your name again.