linux-kernel - [PATCH 1/4] ARM: Disable jump-label on PREEMPT

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Message-Id: <20241210160556.2341497-2-arnd@kernel.org>
Date: Tue, 10 Dec 2024 17:05:53 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	linux-mm@...ck.org,
	linux-rt-devel@...ts.linux.dev,
	Ard Biesheuvel <ardb@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Jason Baron <jbaron@...mai.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Matthew Wilcox <willy@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King <linux@...linux.org.uk>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 1/4] ARM: Disable jump-label on PREEMPT_RT

From: Thomas Gleixner <tglx@...utronix.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@...utronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Ard Biesheuvel <ardb@...nel.org>
Link: https://lkml.kernel.org/r/20220613182447.112191-2-bigeasy@linutronix.de
Signed-off-by: Arnd Bergmann <arnd@...db.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