linux-kernel - Re: [PATCH RFC] sched/deadline: Use the revised wakeup rule for suspending constrained dl tasks

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Message-ID: <9eb70e53-0d8a-94e8-7da4-b577aaafd56e@redhat.com>
Date:   Thu, 11 May 2017 19:07:01 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Xunlei Pang <xpang@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Romulo Silva de Oliveira <romulo.deoliveira@...c.br>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] sched/deadline: Use the revised wakeup rule for
 suspending constrained dl tasks

On 05/04/2017 04:23 PM, Peter Zijlstra wrote:
> On Thu, May 04, 2017 at 04:17:21PM +0200, Peter Zijlstra wrote:
>> On Mon, Apr 24, 2017 at 05:18:35PM +0200, Daniel Bristot de Oliveira wrote:
>>> +static void
>>> +update_dl_revised_wakeup(struct sched_dl_entity *dl_se, struct rq *rq)
>>> +{
>>> +	u64 density = div64_u64(dl_se->dl_runtime << 20, dl_se->dl_deadline);
>>> +	u64 laxity = dl_se->deadline - rq_clock(rq);
>>> +
>>> +	BUG_ON(laxity < 0);
>> Compiler will make that go away, by virtue of laxity being unsigned.
> Also, so we want that to BUG (or even WARN) in a soft RT setting.
> Remember, GEDF does not guarantee we make our deadlines.

The point is that, a task with laxity < 0 should be throttled before
this point by:

df8eac8cafce ("sched/deadline: Throttle a constrained deadline task
activated after the deadline")

Arrive here with (laxity < 0) meas that we skipped that check.

>> We use two difference CBS rules:
> _different_, clearly I cannot type anymore either ;-)

ops... :-)

-- Daniel