Sorry, I tested further…it is not solved, it even worse than I thought:
I have had originally isolated as a test core 3 with boot.cmd and recompile. Got core 3 isolated.
Now, to be clean again:
I took all the isolcpus statements out of boot.cmd and recompiled. I took everything out of dietpiEnv.txt as well.
Unexpected Result:
We still have core 3 isolated now, no matter what i do (reboot, poweroff, take it from the PSU):
root@DietPi:~# cat /sys/devices/system/cpu/isolated
3
root@DietPi:~# grep extraargs /boot/dietpiEnv.txt /boot/boot.scr
/boot/dietpiEnv.txt:extraargs=net.ifnames=0
grep: /boot/boot.scr: binary file matches
So, I guess some with the compile from boot.cmd—>boot.scr goes wrong.
Even if I delete /rename boot.scr and create a complete new boot.scr, I still have core 3 isolated.