Documentation: SEAPATH ignores GRUB_CMDLINE_LINUX_DEFAULT — kernel parameters must use GRUB_CMDLINE_LINUX

[ni8towl]

Summary

Many guides for RT Linux configuration (including ABB's SSC600 Engineering Manual and various kernel tuning references) instruct engineers to add kernel parameters via GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub. On SEAPATH, this variable is silently ignored — parameters must be added to GRUB_CMDLINE_LINUX instead.

This means instructions like "add isolcpus=4-7 to GRUB_CMDLINE_LINUX_DEFAULT" will appear to succeed (update-grub runs without error) but the parameters will not appear in the generated boot entry and will have no effect after reboot.

Relationship to Open Issues

This finding may be relevant to several open issues where kernel parameters are not taking effect as expected:

• Harmonize kernel parameters add #730 ("Harmonize kernel parameters add") — the effort to unify how kernel params are added between Debian and Yocto should document that GRUB_CMDLINE_LINUX_DEFAULT is not used on SEAPATH Debian
• IRQ handling still assigned to isolated CPUs #865 ("IRQ handling still assigned to isolated CPUs") — if irqaffinity was added to GRUB_CMDLINE_LINUX_DEFAULT instead of GRUB_CMDLINE_LINUX, it would silently not take effect, which matches the reported symptom
• irqaffinity kernel parameter is wrong when isolcpus variable contains the "N" value  #868 ("irqaffinity kernel parameter is wrong when isolcpus variable contains the N value") — similar: if the parameter was applied to the wrong GRUB variable, it would appear to be configured but have no effect

Engineers troubleshooting these issues should verify which GRUB variable their parameters are in.

How to Reproduce

1. On a SEAPATH Debian node, edit /etc/default/grub
2. Add isolcpus=4-7 to GRUB_CMDLINE_LINUX_DEFAULT
3. Run sudo update-grub — completes without error
4. Inspect the generated boot entry:

   sudo grep "linux.*vmlinuz" /boot/grub/grub.cfg | head -1

5. Observe: isolcpus=4-7 is absent from the generated kernel command line
6. Reboot and check cat /proc/cmdline — parameter is not present

How to Verify the Correct Variable

# Before rebooting — check the generated grub.cfg
sudo grep "linux.*vmlinuz" /boot/grub/grub.cfg | head -1
# If parameters are missing — you edited the wrong variable

# After reboot — confirm parameters took effect
cat /proc/cmdline

Proposed Documentation Change

Add a note to the SEAPATH documentation and any kernel parameter configuration guides stating explicitly that SEAPATH ignores GRUB_CMDLINE_LINUX_DEFAULT and that all kernel parameters must be added to GRUB_CMDLINE_LINUX. A verification step (grepping the generated grub.cfg before rebooting) should be included as standard practice.

Environment

• SEAPATH Debian standalone, kernel 6.1.0-41-rt-amd64
• GRUB with SEAPATH-specific configuration (custom /etc/grub.d/ scripts)
• Discovered during CPU isolation configuration for ABB vSSC600 deployment

Reported by @ni8towl