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

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Message-ID: <ZnWFQqQ_Yix9tS8D@jlelli-thinkpadt14gen4.remote.csb>
Date: Fri, 21 Jun 2024 15:50:58 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Daniel Bristot de Oliveira <bristot@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...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>,
	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 21/06/24 15:43, Daniel Bristot de Oliveira wrote:
> 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?

Works for me! Guess we can deal with the RT throttling references in the
future when that gets eventually pruned.