Hi all

I finally found the root cause of my problem! What made me very suspicious was the fact, that the spikes occur pretty much exactly after 950ms each time. 
It turns out that throttling was activated having sched_rt_runtime_us set to 950000. Switching it to -1 and everything runs just fine!

Just out of curiosity: in case the throttling kicks in, are all the rt task just in a suspended state?

Again thanx to Sebastian for helping me out here!

BR
Andy