linux-kernel - Re: [RFC PATCH V3 6/6] sched/fair: Implement starvation monitor

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Date:   Fri, 16 Jun 2023 15:27:15 +0200
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        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>,
        Joel Fernandes <joel@...lfernandes.org>,
        Vineeth Pillai <vineeth@...byteword.org>,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [RFC PATCH V3 6/6] sched/fair: Implement starvation monitor

On 6/16/23 13:51, Peter Zijlstra wrote:
> On Thu, Jun 08, 2023 at 05:58:18PM +0200, Daniel Bristot de Oliveira wrote:
>> From: Juri Lelli <juri.lelli@...hat.com>
>>
>> Starting deadline server for lower priority classes right away when
>> first task is enqueued might break guarantees, as tasks belonging to
>> intermediate priority classes could be uselessly preempted. E.g., a well
>> behaving (non hog) FIFO task can be preempted by NORMAL tasks even if
>> there are still CPU cycles available for NORMAL tasks to run, as they'll
>> be running inside the fair deadline server for some period of time.
>>
>> To prevent this issue, implement a starvation monitor mechanism that
> 
> Why should this be prevented?  FIFO can be preempted by a higher
> priority FIFO/RR, or in this case by DEADLINE, and always by stop_task.

Yeah, but in the context of "real-time throttling" the user is not asking to run things
as SCHED_DEADLINE, they are just running FIFO, and expecting it not to have SCHED_OTHER
jumping in front of them.

If we do not "deffer" the starting of the server for a point in which the starvation
is eminent, we can create some anomalies... for example:

A Interrupt wakes up a FIFO and a CFS task, which will will run first? The CFS!

A FIFO schedules a deferred work on a kworker... the kworker will preempt the
FIFO.

It is counter intuitive for the majority of people out there... It will
be counter-intuitive for most of PREEMPT_RT users :-/. From one day to the other
their tests using cyclictest will... break...

> Just accept this.
> 
> Anything that's build around FIFO-99 not getting preempted is just plain
> broken. Don't try and pander to broken