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

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

On 2025-10-30 05:46:26 [-0400], Steven Rostedt wrote:
> On Thu, 30 Oct 2025 08:55:58 +0100
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> 
> > You can't really control when jump labels are enabled/ disabled. For
> > instance this happens if you enable/ disable trace points.
> > Arm64 and X86 replaced the stop_machine() logic with something else that
> > does not affect latency in that magnitude. If Arm could something
> > similar then fine but until then I don't see a way around it.
> > There are other things that are disabled on PREEMPT_RT due to latency
> > reasons such as transparent huge pages or expedited RCU.
> 
> But transparent huge pages and expedited RCU are not user controlled events.
> 
> Enabling function tracing was known to cause huge latency for a very long
> time and it came from the PREEMPT_RT patch set. Heck, printk use to as well ;-)

trace events were an example and this is used to enable tracing in
production or development to gather information if something is not
working right _or_ on the edge to investigate. A latency spike at this
point would be a timing violation forcing the system into a safe state
an so not allowing further investigations.

I hope I made it clear that using jump-labels at this point is not
doable at this point and a warning does not improve things. Improving
the implementation on ARM as it has been done on ARM64 or X86 is how I
would compare the situation to printk. Not adding a warning at boot
time.

> -- Steve

Sebastian