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

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Message-ID: <CACRpkdYP6yWHuWGwHZCe7SQLg-BFtLb-KKHxqrXbeM2hhrsgfA@mail.gmail.com>
Date: Wed, 11 Dec 2024 14:04:03 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, 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>, 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: Re: [PATCH 1/4] ARM: Disable jump-label on PREEMPT_RT

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

> 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>

Makes sense.

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

Yours,
Linus Walleij