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.

If there are no additional concerns about this version, I will
forward it to Russell's patch system
---
 arch/arm/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index fb4e1da3bb98..ed850cc0ed3c 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -80,7 +80,7 @@ config ARM
 	select HAS_IOPORT if PCI || PCMCIA || ISA || ARCH_FOOTBRIDGE || ARCH_RPC
 	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 || !SMP)
 	select HAVE_ARCH_KFENCE if MMU && !XIP_KERNEL
 	select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
 	select HAVE_ARCH_KASAN if MMU && !XIP_KERNEL
-- 
2.39.5

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

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