Message-ID: <b14879eb-cbc9-4e53-b703-ab7b451b7687@kernel.org>
Date: Fri, 21 Jun 2024 15:43:31 +0200
From: Daniel Bristot de Oliveira <bristot@...nel.org>
To: Juri Lelli <juri.lelli@...hat.com>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>
Cc: 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>, Phil Auld <pauld@...hat.com>,
 Suleiman Souhlal <suleiman@...gle.com>,
 Youssef Esmat <youssefesmat@...gle.com>
Subject: Re: [PATCH V7 0/9] SCHED_DEADLINE server infrastructure

On 6/21/24 15:37, Juri Lelli wrote:
> Hi Daniel,
> 
> On 27/05/24 14:06, Daniel Bristot de Oliveira wrote:
>> This is v7 of Peter's SCHED_DEADLINE server infrastructure
>> implementation [1].
> 
> I finally managed to give this a go and can report that it works great
> for what I've seen. :)
> 
> So, please consider this reply a
> 
> Tested-by: Juri Lelli <juri.lelli@...hat.com>

Thanks!

>> SCHED_DEADLINE servers can help fixing starvation issues of low priority
>> tasks (e.g., SCHED_OTHER) when higher priority tasks monopolize CPU
>> cycles. Today we have RT Throttling; DEADLINE servers should be able to
>> replace and improve that.
> 
> ...
> 
>> The problem with DL server only implementation is that FIFO tasks might
>> suffer preemption from NORMAL even when spare CPU cycles are available.
>> In fact, fair deadline server is enqueued right away when NORMAL tasks
>> wake up and they are first scheduled by the server, thus potentially
>> preempting a well behaving FIFO task. This is of course not ideal.
>>
>> We had discussions about it, and one of the possibilities would be
>> using a different scheduling algorithm for this. But IMHO that is
>> an overkill.
>>
>> Juri and I discussed this and though about delaying the server
>> activation for the (period - runtime), thus enabling the server
>> only if the fair scheduler is about to starve. We called it
>> the defer server.
>>
>> The defer the server start to the (absolute deadline - runtime)
>> point in time. This is achieved by starting the dl server throttled,
>> with a next replenishing time set to activate the server at
>> (absolute deadline - runtime).
>>
>> The server is enqueued with the runtime replenished. As the fair
>> scheduler runs without boost, its runtime is consumed. If the
>> fair server has its runtime before the runtime - deadline time,
>> the a new period is set, and the timer armed for the new
>> deadline.
> 
> I also wanted to pay particular attention to this part implementing the
> deferred server, but failed to find enough focus time for now. I will
> keep trying. One thing that I wondered though is if this change (and the
> move towards this replacing current RT throttling) would call for a Doc
> update. What do you think?

Yeah, am I planning a v8 for the next week. It has no code changes, just a rebase
and the addition of documentation.

I am not mentioning the RT throttling in the documentation. Instead, I am treating
this as a new feature on its own, which is inline with the comments over the code.

I will add an rv monitor to it, extending the documentation, but I will do it
on another series... once we get this done.

Thoughts?

Peter/Ingo, which branch should I rebase it?

-- Daniel

> Thanks!
> Juri
> 

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