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SubjectRe: [PATCH] sched: Tweak default dynamic preempt mode selection
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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


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