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

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Message-ID: <20241008132416.9cVldCGG@linutronix.de>
Date: Tue, 8 Oct 2024 15:24:16 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
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 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
> 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).
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?

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).

> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Sebastian