Review power saving modes kernel configurations

[carlossvg]

@shuhaowu reported some CPU frequency changes. A possible explanation could be something related with power management. We should review all the related configurations. From the top of my head these are some of the things that we should take into account:

• Apparently, power issues could cause a kernel log message which might affect the latency measurements - https://pimylifeup.com/raspberry-pi-low-voltage-warning/ - https://forums.raspberrypi.com/viewtopic.php?t=224493
• dwc_otg has to be disabled - https://community.emlid.com/t/rt-kernel-unusual-dwc-otg-high-8k-s-interrupt-rate-using-quite-some-cpu/1002 - https://www.osadl.org/Single-View.111+M5c03315dc57.0.html - https://www.osadl.org/monitoring/patches/r7s3/usb-dwc_otg-fix-system-lockup-when-interrupts-are-threaded.patch.html
• CPU idle
  • https://wiki.linuxfoundation.org/realtime/documentation/howto/debugging/no-cpu-idle
  • A Checklist for Writing Linux Real-Time Applications: Kernel configuration
  • Enea Linux Real-Time Guide (2.8.2 CPU Power States and 3.2.2 Optimize Power Save) https://linux.enea.com/4.0/documentation/book-enea-linux-realtime-guide.pdf

[carlossvg]

cc @LanderU @razr @shuhaowu

[razr]

are CPU core caches separated in the pi4 image?