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

[Steven Rostedt]

On Thu, 30 Oct 2025 08:55:58 +0100
Sebastian Andrzej Siewior <bigeasy at 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 ;-)

Instead, we could print a message to the console stating that a huge
latency has just occurred due to user activation. Similar to what we used
to post when RT Throttling happened (I don't see that message anymore since
dl_server was added :-/).

-- Steve