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

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Message-ID: <20170215141651.GF1368@e106622-lin>
Date:   Wed, 15 Feb 2017 14:16:51 +0000
From:   Juri Lelli <juri.lelli@....com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Luca Abeni <luca.abeni@...tannapisa.it>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Mike Galbraith <efault@....de>,
        Romulo Silva de Oliveira <romulo.deoliveira@...c.br>
Subject: Re: [PATCH 3/2] sched/deadline: Use deadline instead of period when
 calculating overflow

On 15/02/17 14:33, Daniel Bristot de Oliveira wrote:
> 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?
> 

Nope, you are right, no problems here.

Thanks,

- Juri