Re: [PATCH v2] sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, May 15, 2026 at 02:56:08PM -0400, Steven Rostedt wrote:
> I just want to make sure that my analysis is correct. Since only one CPU
> can initiate the RT_PUSH_IPI. That for this to be a problem, other CPUs
> need to be constantly running RT tasks for short periods of time so that
> when the RT task schedules off the CPU, the CPU then initiates the "pull".
> And it's not that it happens all at once. It's more serialized where the RT
> tasks are scheduling off at different times to constantly feed the
> RT_PUSH_IPI logic without seeing that it's already in place.
> 
> It would require this to happen enough times to keep the overloaded CPU
> from finishing the softirq until a time when the softirq is scheduled
> again. And it would maintain this abuse for long enough to trigger the
> watchdog.
> 
> Does that fit the scenario of your environment?

Yeah, I think that's coming from the FIFO threaded irq handling for mpi3mr.
We tried two mitigations - one dropping the irq threads to SCHED_OTHER and
NO_RT_PUSH_IPI. Both worked. While the former is not conclusive in itself,
it is in line with the theory at least.

Thanks.

-- 
tejun