linux-kernel - Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server

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Date:   Wed, 8 Nov 2023 14:58:12 +0100
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vineeth Pillai <vineeth@...byteword.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server

On 11/8/23 13:44, Peter Zijlstra wrote:
> Should we rather not cap the runtime, something like so?
> 
> Because the above also causes period drift, which we do not want.

like in the example I showed before:

- 3/10 reservation (30%).
- w=waiting
- r=running
- s=sleeping
- T=throttled
- fair server dispatched at 0, starvation from RT.


|wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww|rrrrrrrrrrrrrrrrrrrrrrrrrrrrrr|TTTTTTTTTT[...]TTTTTTTTTTT|rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr|TTTTTTT
|___________________________period 1_________________________________|_________period 2________________________[...]___________|___period 3____________________|[.... internal-period
0---------1---------2---------3---------4---------5---------6--------7--------8---------9----------10.......11.[...]16.........17........18........19........20|[.... < Real-time
---------------------------------------------------------------------+---------------------------------------------------------|
                                                                     |                                                         +new period

>From "real-world/wall clock" the internal period shift produces the
"zerolax" timeline. It runs 3 units of time before the 10's.

If one has a mix of DL and FIFO task, and want to enforce
a given response time to the fair server, they can reduce the
fair server period to achieve that.

-- Daniel