linux-kernel - Re: [PATCH 2/7] rcu: limit PREEMPT

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Message-ID: <20241009180117.GS17263@noisy.programming.kicks-ass.net>
Date: Wed, 9 Oct 2024 20:01:17 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, paulmck@...nel.org,
	mingo@...nel.org, bigeasy@...utronix.de, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, frederic@...nel.org, efault@....de
Subject: Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations

On Wed, Oct 09, 2024 at 09:54:06AM -0700, Ankur Arora wrote:
> PREEMPT_LAZY can be enabled stand-alone or alongside PREEMPT_DYNAMIC
> which allows for dynamic switching of preemption models.
> 
> The choice of preemptible RCU or not, however, is fixed at compile
> time. Given the trade-offs made to have a preemptible RCU, some
> configurations which have limited preemption might prefer the
> stronger forward-progress guarantees of PREEMPT_RCU=n.
> 
> Accordingly, explicitly limit PREEMPT_RCU=y to PREEMPT_DYNAMIC,
> PREEMPT, PREEMPT_RT.
> 
> This means that (PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n), which selects
> PREEMPTION will run with PREEMPT_RCU=n. The combination (PREEMPT_LAZY=y,
> PREEMPT_DYNAMIC=y), will run with PREEMPT_RCU=y.

I am completely confused by this. Why do we want this?

> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  kernel/rcu/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 3e079de0f5b4..1bfe7016724f 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -18,7 +18,7 @@ config TREE_RCU
>  
>  config PREEMPT_RCU
>  	bool
> -	default y if PREEMPTION
> +	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
>  	select TREE_RCU
>  	help
>  	  This option selects the RCU implementation that is
> -- 
> 2.43.5
>