From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> Link: https://lkml.kernel.org/r/20220613182447.112191-2-bigeasy@xxxxxxxxxxxxx Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> --- 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
