T7870: Kernel add option CONFIG_NO_HZ_FULL

[sever-sever]

Change summary

Add options CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU for:

• system option kernel cpu nohz-full
• system option kernel cpu rcu-no-cbs

Fixes:

Sep 26 14:47:11 r14 kernel: Kernel command line: BOOT_IMAGE=/boot/2025.09.26-1054-integration/vmlinuz boot=live rootdelay=5 noautologin net.ifnames=0 biosdevname=0 vyos-union=/boot/2025.09.26-1054-integration hpet=disable mce=off nosoftlockup isolcpus=2-3 nohz_full=2-3 rcu_nocbs=2-3 nmi_watchdog=0 numa_balancing=disable default_hugepagesz=2M hugepagesz=2M hugepages=4096 console=tty0
Sep 26 14:47:11 r14 kernel: Housekeeping: nohz unsupported. Build with CONFIG_NO_HZ_FULL
Sep 26 14:47:11 r14 kernel: Unknown kernel command line parameters "noautologin BOOT_IMAGE=/boot/2025.09.26-1054-integration/vmlinuz boot=live biosdevname=0 vyos-union=/boot/2025.09.26-1054-integration nohz_full=2-3 rcu_nocbs=2-3", will be passed to user space.
Sep 26 14:47:11 r14 kernel: random: crng init done

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T7870

Related PR(s)

Reproduce

set system option kernel cpu isolate-cpus '2-3'
set system option kernel cpu nohz-full '2-3'
set system option kernel cpu rcu-no-cbs '2-3'

Reboot the system, and check logs

Sep 26 14:47:11 r14 kernel: Kernel command line: BOOT_IMAGE=/boot/2025.09.26-1054-integration/vmlinuz boot=live rootdelay=5 noautologin net.ifnames=0 biosdevname=0 vyos-union=/boot/2025.09.26-1054-integration hpet=disable mce=off nosoftlockup isolcpus=2-3 nohz_full=2-3 rcu_nocbs=2-3 nmi_watchdog=0 numa_balancing=disable default_hugepagesz=2M hugepagesz=2M hugepages=4096 console=tty0
Sep 26 14:47:11 r14 kernel: Housekeeping: nohz unsupported. Build with CONFIG_NO_HZ_FULL
Sep 26 14:47:11 r14 kernel: Unknown kernel command line parameters "noautologin BOOT_IMAGE=/boot/2025.09.26-1054-integration/vmlinuz boot=live biosdevname=0 vyos-union=/boot/2025.09.26-1054-integration nohz_full=2-3 rcu_nocbs=2-3", will be passed to user space.
Sep 26 14:47:11 r14 kernel: random: crng init done

Before fix:

vyos@r14:~$ zcat /proc/config.gz | grep CONFIG_NO_HZ_FULL
# CONFIG_NO_HZ_FULL is not set
vyos@r14:~$ 

vyos@r14:~$ zcat /proc/config.gz | grep CONFIG_RCU_NOCB_CPU
vyos@r14:~$

After fix:

Sep 29 11:42:27 r14 kernel: NO_HZ: Full dynticks CPUs: 2-3.
Sep 29 11:42:27 r14 kernel: rcu:         Offload RCU callbacks from CPUs: 2-3.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[sarthurdev]

Adds needed kernel config for existing CLI

[dmbaturin]

It's likely a good idea to set NO_HZ_FULL explicitly even regardless of that CLI bug.