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

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Message-ID: <63380f0a-329c-43df-8e6c-4818de5eb371@paulmck-laptop>
Date: Thu, 7 Mar 2024 11:01:02 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
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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	konrad.wilk@...cle.com
Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO

On Wed, Mar 06, 2024 at 03:42:10PM -0500, Joel Fernandes wrote:
> 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.

This would be something that happens only after removing cond_resched()
might_sleep() functionality from might_sleep(), correct?

							Thanx, Paul

> >> 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
>