> > > I notice you are using -rt kernel. Do you actually need realtime
> > > features?
> >
> > Yes, I actually need the realtime feature. I have one task which
> > needs to run periodically in realtime (triggered every 10ms by an
> > external IRQ).
>
> How fast response do you need to the external IRQ?

We do not have "Hard-Realtime" requirements, but we should always handle the IRQ inside of 1ms.

> > > Can you try to reproduce the problem on 4.19.193?
> >
> > This is only a problem with the realtime patch. The patch
> > introduces migrate_enable which is part of the callstack:
> >
> > Jun 1 09:11:46 sicam kernel: [46802.944165] BUG: using smp_processor_id() in preemptible [00000000] code: TCPTSK/1809
> > Jun 1 09:11:46 sicam kernel: [46802.944210] caller is
> > migrate_enable+0x40/0x488
>
> Fun :-(. As Jan mentioned, testing if it can be reproduced with
> 4.19.193-rt81 would be useful.

Ok, I will give 4.19.193-rt81 a try.

Before we decided to switch to the 4.19.x kernel we tried in March this year the 5.10.25 kernel with -rt35.
There we observed a very bad realtime behaviour. Our external interrupt had a latency of more then 4ms when our complete application was running. The IRQ task has realtime prio 51 and is therefore over all other IRQ tasks in the system (they have by default 50).
With kernel 4.19.x we do not see a latency over 1ms in the same configuration.

Best regards,
Rainer