linux-kernel - Re: [PATCH V7 0/9] SCHED_DEADLINE server infrastructure

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Message-ID: <ZnWCBF9QkQmO01nw@jlelli-thinkpadt14gen4.remote.csb>
Date: Fri, 21 Jun 2024 15:37:08 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Daniel Bristot de Oliveira <bristot@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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

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>

> 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?

Thanks!
Juri