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

Sebastian Andrzej Siewior
2024-12-11T13:26:16Z

On 2024-12-10 17:05:53 [+0100], Arnd Bergmann 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>
> ---
> I slightly changed the patch from the version currently in linux-rt.git
> to leave jump labels enabled on single-CPU kernels that are still
> fairly common on 32-bit arm.

So HOTPLUG_CPU depends on SMP and without SMP there no HOTPLUG_CPU so
the patch function will be invoked directly. So we patch and flush.
Well, okay.

> If there are no additional concerns about this version, I will
> forward it to Russell's patch system

I'm fine with it, thank you.

Sebastian