Re: Possible enhancement to RCU CPU stall warning messages

[Clark Williams <clrkwllms@kernel.org>]

On Tue, Oct 13, 2026 at 05:41:41PM -0700, Paul E. McKenney wrote:
> Hello!
> 
> While chasing an "interesting" bug [1], I modified the RCU CPU
> stall-warning code so that when a task (as opposed to a CPU) is
> blocking the grace period due to being preempted, the stall-warning
> message includes the task that is currently running on the CPU that the
> preempted task wants to run on.  (In the case of the "interesting" bug,
> I kid you not, that CPU was idle.)
> 
> Would this stall-warning addition be useful for PREEMPT_RT users?
> 
> If it is not useful, I am going to keep it out of tree.  Something about
> it needing a way to dig into scheduler internals.  ;-)
> 
> But if you want it, please let me know.
> 
> 							Thanx, Paul
> 
> [1] https://lore.kernel.org/all/8fa7f795-518c-4507-b684-60b0c1cac2c5@paulmck-laptop/
> 

Paul,

While RCU isn't one of the common reasons for a latency spike, if it
*is*, then it's not one that's easily tracked down.  For that reason I'm
in favor of using your patch. 

That said, if it's going to be a pain to maintain, I can live without it. 

Clark