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

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Date: Wed, 6 Mar 2024 15:42:10 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
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Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO

Hi Ankur,

On 3/5/2024 3:11 AM, Ankur Arora wrote:
> 
> Joel Fernandes <joel@...lfernandes.org> writes:
> 
[..]
>> IMO, just kill 'voluntary' if PREEMPT_AUTO is enabled. There is no
>> 'voluntary' business because
>> 1. The behavior vs =none is to allow higher scheduling class to preempt, it
>> is not about the old voluntary.
> 
> What do you think about folding the higher scheduling class preemption logic
> into preempt=none? As Juri pointed out, prioritization of at least the leftmost
> deadline task needs to be done for correctness.
> 
> (That'll get rid of the current preempt=voluntary model, at least until
> there's a separate use for it.)

Yes I am all in support for that. Its less confusing for the user as well, and
scheduling higher priority class at the next tick for preempt=none sounds good
to me. That is still an improvement for folks using SCHED_DEADLINE for whatever
reason, with a vanilla CONFIG_PREEMPT_NONE=y kernel. :-P. If we want a new mode
that is more aggressive, it could be added in the future.

>> 2. you are also planning to remove cond_resched()s via this series and leave
>> it to the scheduler right?
> 
> Yeah, under PREEMPT_AUTO, cond_resched() will /almost/ be not there. Gets
> defined to:
> 
> static inline int _cond_resched(void)
> {
>         klp_sched_try_switch();
>         return 0;
> }
> 
> Right now, we need cond_resched() to make timely forward progress while
> doing live-patching.

Cool, got it!

>> Or call it preempt=higher, or something? No one is going to understand the
>> meaning of voluntary the way it is implied here IMHO.
> 
> I don't think there's enough to make it worth adding a new model. For
> now I'm tending towards moving the correctness parts to preempt=none and
> making preempt=voluntary identical to preempt=none.

Got it, sounds good.

> Thanks for the review.

Sure! Thanks for this work. Looking forward to the next series,

 - Joel