linux-kernel - Re: [PATCH V2 2/2] sched/deadline: Throttle a constrained deadline task activated after the deadline

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Message-ID: <501277c7-d7b2-e1c2-4f10-eda9b6624956@santannapisa.it>
Date:   Tue, 14 Feb 2017 16:54:13 +0100
From:   Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mike Galbraith <efault@....de>,
        Romulo Silva de Oliveira <romulo.deoliveira@...c.br>
Subject: Re: [PATCH V2 2/2] sched/deadline: Throttle a constrained deadline
 task activated after the deadline

On 13/02/2017 20:05, Daniel Bristot de Oliveira wrote:
> To avoid this problem, in the activation of a constrained deadline
> task after the deadline but before the next period, throttle the
> task and set the replenishing timer to the begin of the next period,
> unless it is boosted.

my only comment is that, by throttling on (dl < wakeuptime < period), we force the app to sync its activation time with the kernel, and the cbs doesn't self-sync anymore with the app own periodicity, which is what normally happens with dl=period. With dl=period, we loose the cbs self-sync and we force the app to sync with the kernel periodic timer only if we use explicitly yield(), but now this becomes also implicit just if we set dl<period.

> 	attr.sched_policy   = SCHED_DEADLINE;
> 	attr.sched_runtime  = 2 * 1000 * 1000;		/* 2 ms */
> 	attr.sched_deadline = 2 * 1000 * 1000;		/* 2 ms */
> 	attr.sched_period   = 2 * 1000 * 1000 * 1000;	/* 2 s */
...
> On my box, this reproducer uses almost 50% of the CPU time, which is
> obviously wrong for a task with 2/2000 reservation.

just a note here: in this example of runtime=deadline=2ms, shall we rely
on a utilization-based test, then we should assume the task is taking 100%.
More precise tests for EDF with deadline<period would properly count the
1998ms/2000ms free space, instead.

my2c,

	T.
-- 
Tommaso Cucinotta, Computer Engineering PhD
Associate Professor at the Real-Time Systems Laboratory (ReTiS)
Scuola Superiore Sant'Anna, Pisa, Italy
http://retis.sssup.it/people/tommaso