linux-kernel - Re: [PATCH 3/5] sched: Enable PREEMPT_DYNAMIC for PREEMPT

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Message-ID: <20241008144000.GE14587@noisy.programming.kicks-ass.net>
Date: Tue, 8 Oct 2024 16:40:00 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, ankur.a.arora@...cle.com,
	efault@....de, Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 3/5] sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT

On Tue, Oct 08, 2024 at 03:24:16PM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-10-07 09:46:12 [+0200], Peter Zijlstra wrote:
> > In order to enable PREEMPT_DYNAMIC for PREEMPT_RT, remove PREEMPT_RT
> > from the 'Preemption Model' choice. Strictly speaking PREEMPT_RT is
> > not a change in how preemption works, but rather it makes a ton more
> > code preemptible.
> > 
> > Notably, take away NONE and VOLATILE options for PREEMPT_RT, they make

As I think Mike already noted, typing is hard and this should of course
have been Voluntary :-)

> > no sense (but are techincally possible).
> 
> So this is what we do. Okay. This means we can enable the DYNAMIC mode
> on PREEMPT_RT enabled kernels and switch between "full" and the "lazy"
> mode(s).

Right.

> On PREEMPT_RT enabled kernels with PREEMPT_DYNAMIC the UTS_VERSION
> string is set to PREEMPT_RT and PREEMPT_DYNAMIC is not exposed. Is this
> on purpose or just happened?

I noticed it, didn't care and promptly forgot about it again :-)

> Clark was asking for a file to expose whether or not PREEMPT_RT is
> enabled and I was pointing him to UTS_VERSION but then suggested that it
> might be possible if we expose the current setting of the preemption
> model and use this. 
> But with this it won't work.
> I am not sure if PREEMPT_DYNAMIC is needed to be exposed and if
> everybody is happy parsing UTS_VERSION (we used to have a
> /sys/kernel/realtime file in the RT queue).

Yeah, IDK. This is all just pick a colour :-)