I have been able to resolve this issue on one PC by building deb files of the 6.1 kernel from pristine sources from kernel.org with the PREEMPT_RT patch applied.
The only change I made using xconfig was to enable the fully preemptible kernel and set the following keys to permit building of the debs.
scripts/config --disable SYSTEM_REVOCATION_KEYS
scripts/config --disable DEBUG_INFO
scripts/config --enable DEBUG_INFO_NONE
scripts/config --set-str SYSTEM_TRUSTED_KEYS ""
Once this kernel and the R8168-dkms driver for my hardware was installed, the issue appears to be resolved.
I also built Linuxcnc from source on Arch Linux where the issue was not evident.
These two data points confirm in my mind the Debian implementation of the Realtek network device drivers is buggy, certainly for the RT image and probably for the default image.
The only change I made using xconfig was to enable the fully preemptible kernel and set the following keys to permit building of the debs.
scripts/config --disable SYSTEM_REVOCATION_KEYS
scripts/config --disable DEBUG_INFO
scripts/config --enable DEBUG_INFO_NONE
scripts/config --set-str SYSTEM_TRUSTED_KEYS ""
Once this kernel and the R8168-dkms driver for my hardware was installed, the issue appears to be resolved.
I also built Linuxcnc from source on Arch Linux where the issue was not evident.
These two data points confirm in my mind the Debian implementation of the Realtek network device drivers is buggy, certainly for the RT image and probably for the default image.
I believe this will become a major support issue once Bookworm becomes the stable release.
It would be great if the kernel team could review this and find a resolution before then.