Re: [PATCH 4/5] ARM: Disable jump-label on PREEMPT_RT.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-10-30 10:39:59 [+0800], Xie Yuanbin wrote:
> On Wed, 29 Oct 2025 16:59:17 +0100, Thomas Gleixner wrote:
> > 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.
> >
> > Disable jump-label support on a PREEMPT_RT system.
> 
> This sounds like a performance issue rather than a feature issue.
> Banning users from using the jump label because of a performance problem
> seems like throwing the baby out with the bathwater.
> Perhaps we could add a config like CONFIG_ARM_JUMP_LABEL_RT_FORCE that
> allows users to select, and inform users in the description about
> potential latency.

It is not a performance issue, it is a latency issue. If the scheduling
latency increases by 300us because of this then the PREEMPT_RT system
becomes worthless.

> Xie Yuanbin

Sebastian