linux-kernel - Re: [PATCH] sched: Tweak default dynamic preempt mode selection

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Date:   Sat, 06 Nov 2021 05:40:30 +0100
From:   Mike Galbraith <efault@....de>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH] sched: Tweak default dynamic preempt mode selection

On Fri, 2021-11-05 at 10:40 +0000, Valentin Schneider wrote:
> Commit c597bfddc9e9 ("sched: Provide Kconfig support for default dynamic
> preempt mode") changed the selectable config names for the preemption
> model. This means a config file must now select
>
>   CONFIG_PREEMPT_BEHAVIOUR=y
>
> rather than
>
>   CONFIG_PREEMPT=y
>
> to get a preemptible kernel. This means all arch config files need to be
> updated - right now arm64 defconfig selects CONFIG_PREEMPT=y but ends up
> with CONFIG_PREEMPT_NONE_BEHAVIOUR=y.
>
> Instead, have CONFIG_*PREEMPT be the selectable configs again, and make
> them select their _BEHAVIOUR equivalent if CONFIG_PREEMPT_DYNAMIC is set.


Is there any way to get to PREEMPT_RT in the first selection again as
well?  I had created a behavior entry for RT (below) and inverted the
dependency to make it appear in the initial selection again, but that's
clearly not gonna fly.

Starting with a 5.15 config, to select RT you currently must first
select a model you don't want, then reject PREEMPT_DYNAMIC and you'll
be offered the full menu of models immediately. With your patch added,
that became worse.  After rejecting PREEMPT_DYNAMIC, I had to go
through new 5.15+ options before finally being offered the full menu.

	-Mike

--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -56,10 +56,10 @@ config PREEMPT_BEHAVIOUR
 	  embedded system with latency requirements in the
milliseconds
 	  range.

-config PREEMPT_RT
+config PREEMPT_RT_BEHAVIOR
 	bool "Fully Preemptible Kernel (Real-Time)"
-	depends on EXPERT && ARCH_SUPPORTS_RT && !PREEMPT_DYNAMIC
-	select PREEMPTION
+	depends on EXPERT && ARCH_SUPPORTS_RT
+	select PREEMPT_RT
 	help
 	  This option turns the kernel into a real-time kernel by
replacing
 	  various locking primitives (spinlocks, rwlocks, etc.) with
@@ -86,6 +86,10 @@ config PREEMPT
 	select PREEMPTION
 	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK

+config PREEMPT_RT
+	bool
+	select PREEMPTION
+
 config PREEMPT_COUNT
        bool

@@ -101,7 +105,7 @@ config PREEMPT_LAZY

 config PREEMPT_DYNAMIC
 	bool "Preemption behaviour defined on boot"
-	depends on HAVE_PREEMPT_DYNAMIC
+	depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT
 	select PREEMPT
 	default y
 	help