[PATCH 4/5] ARM: Disable jump-label on PREEMPT_RT.

[Arnd Bergmann]

On Wed, Oct 29, 2025, at 16:59, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx at linutronix.de>
>
> jump-labels are used to efficiently switch between two possible code
> paths. To achieve this, stop_machine() is used to keep the CPU in a
> known state while the opcode is modified. The usage of stop_machine()
> here leads to large latency spikes which can be observed on PREEMPT_RT.
>
> Jump labels may change the target during runtime and are not restricted
> to debug or "configuration/ setup" part of a PREEMPT_RT system where
> high latencies could be defined as acceptable.
>
> Disable jump-label support on a PREEMPT_RT system.
>
> [bigeasy: Patch description.]
>
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
> ---
>  arch/arm/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 864a14a434b08..99c9b1c320af8 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -80,7 +80,7 @@ config ARM
>  	select HAS_IOPORT
>  	select HAVE_ARCH_AUDITSYSCALL if AEABI && !OABI_COMPAT
>  	select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
> -	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
> +	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU 
> && !PREEMPT_RT

I posted a slightly different version last year, which optimizes
the uniprocessor version and has a little more information
in the changelog:

https://lore.kernel.org/all/20241210160556.2341497-2-arnd@kernel.org/

     Arnd