linux-kernel - Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT

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Message-ID: <Zeq2FUkU4nVSjY7I@localhost.localdomain>
Date: Fri, 8 Mar 2024 07:54:13 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: Joel Fernandes <joel@...lfernandes.org>, paulmck@...nel.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
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Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO

On 07/03/24 19:49, Ankur Arora wrote:
> Joel Fernandes <joel@...lfernandes.org> writes:

..

> > Firstly, Maybe I misunderstood Ankur completely. Re-reading his comments above,
> > he seems to be suggesting preempting instantly for higher scheduling CLASSES
> > even for preempt=none mode, without having to wait till the next
> > scheduling-clock interrupt.
>
> Yes, that's what I was suggesting.
>
> > Not sure if that makes sense to me, I was asking not
> > to treat "higher class" any differently than "higher priority" for preempt=none.
>
> Ah. Understood.
>
> > And if SCHED_DEADLINE has a problem with that, then it already happens so with
> > CONFIG_PREEMPT_NONE=y kernels, so no need special treatment for higher class any
> > more than the treatment given to higher priority within same class. Ankur/Juri?
>
> No. I think that behaviour might be worse for PREEMPT_AUTO.
>
> PREEMPT_NONE=y (or PREEMPT_VOLUNTARY=y for that matter) don't
> quite have a policy around when preemption happens. Preemption
> might happen quickly, might happen slowly based on when the next
> preemption point is found.
>
> The PREEMPT_AUTO, preempt=none policy in this series will always
> cause preemption to be at user exit or the next tick. Seems like
> it would be worse for higher scheduling classes more often.
>
> But, I wonder what Juri thinks about this.

As I was saying in my last comment in the other discussion, I'm honestly
not sure, mostly because I'm currently fail to see what type of users
would choose preempt=none and have tasks scheduled with SCHED_DEADLINE
(please suggest example usecases, as I'm pretty sure I'm missing
something :). With that said, if the purpose of preempt=none is to have
a model which is super conservative wrt preemptions, having to wait one
tick to possibly schedule a DEADLINE task still seems kind of broken for
DEADLINE, but at least is predictably broken (guess one needs to account
for that somehow when coming up with parameters :).

Thanks,
Juri