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

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Date: Tue, 05 Mar 2024 00:11:28 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, peterz@...radead.org,
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        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO


Joel Fernandes <joel@...lfernandes.org> writes:

> Hi Anukr,
>
> On Mon, Feb 12, 2024 at 09:55:50PM -0800, Ankur Arora wrote:
>> The default preemption policy for voluntary preemption under
>> PREEMPT_AUTO is to schedule eagerly for tasks of higher scheduling
>> class, and lazily for well-behaved, non-idle tasks.
>>
>> This is the same policy as preempt=none, with an eager handling of
>> higher priority scheduling classes.
>
> AFAICS, the meaning of the word 'voluntary' has changed versus the old
> CONFIG_PREEMPT_VOLUNTARY, with this patch.
>
> So the word voluntary does not completely make sense in this context. What is
> VOLUNTARY about choosing a higher scheduling class?
>
> For instance, even in the same scheduling class, there is a notion of higher
> priority, not just between classes. Example, higher RT priority within RT, or
> earlier deadline within EEVDF (formerly CFS).

Agreed. The higher scheduling class line is pretty fuzzy and after the discussion
with Juri, almost non existent: https://lore.kernel.org/lkml/ZeBPXNFkipU9yytp@localhost.localdomain/.

> 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.)

> 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.

> 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.

Thanks for the review.

--
ankur