Re: [PATCH 1/4] ARM: Disable jump-label on PREEMPT_RT

[Linus Walleij <linus.walleij@linaro.org>]

On Tue, Dec 10, 2024 at 5:06 PM Arnd Bergmann <arnd@kernel.org> wrote:

> From: Thomas Gleixner <tglx@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.
>
> On 64-bit Arm, it is possible to use jump labels without the
> stop_machine() call, which architecturally provides a way to atomically
> change one 32-bit instruction word while keeping maintaining consistency,
> but this is not generally the case on 32-bit, in particular in thumb2
> mode.
>
> Disable jump-label support on a PREEMPT_RT system when SMP is enabled.
>
> [bigeasy: Patch description.]
> [arnd: add !SMP case, extend changelog]
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Link: https://lkml.kernel.org/r/20220613182447.112191-2-bigeasy@linutronix.de
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Makes sense.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Yours,
Linus Walleij