Document disabling the realtime kernel

[farshidtz]

$ sudo pro enable realtime-kernel
One moment, checking your subscription first
No variant specified. To specify a variant, use the variant option.
Auto-selecting generic variant. Proceed? (y/N) y
The Real-time kernel is an Ubuntu kernel with PREEMPT_RT patches integrated.

This will change your kernel. To revert to your original kernel, you will need
to make the change manually.

Do you want to continue? [ default = Yes ]: (Y/n) Y
Configuring APT access to Real-time kernel
Updating Real-time kernel package lists
Updating standard Ubuntu package lists
Installing Real-time kernel packages
Real-time kernel enabled
A reboot is required to complete install.
$ sudo reboot

$ uname -r
6.8.1-1014-realtime

$ dpkg --list | grep linux-image
ii  linux-image-6.8.0-49-generic            6.8.0-49.49                             amd64        Signed kernel image generic
ii  linux-image-6.8.1-1014-realtime         6.8.1-1014.15                           amd64        Signed kernel image realtime
ii  linux-image-realtime                    6.8.1-1014.15                           amd64        Linux kernel image for real-time systems.
ii  linux-image-virtual                     6.8.0-49.49                             amd64        Virtual Linux kernel image


$ sudo pro disable realtime-kernel 
This will remove the boot order preference for the Real-time kernel and
disable updates to the Real-time kernel.

This will NOT fully remove the kernel from your system.

After this operation is complete you must:
  - Ensure a different kernel is installed and configured to boot
  - Reboot into that kernel
  - Fully remove the realtime kernel packages from your system
      - This might look something like `apt remove linux*realtime`,
        but you must ensure this is correct before running it.

Are you sure? (y/N) y
Removing APT access to Real-time kernel

$ sudo cat /boot/grub/grub.cfg | grep "menuentry "
menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-631f71ce-256c-4124-ab53-b0408b646d43' {
	menuentry 'Ubuntu, with Linux 6.8.1-1014-realtime' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.8.1-1014-realtime-advanced-631f71ce-256c-4124-ab53-b0408b646d43' {
	menuentry 'Ubuntu, with Linux 6.8.1-1014-realtime (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.8.1-1014-realtime-recovery-631f71ce-256c-4124-ab53-b0408b646d43' {
	menuentry 'Ubuntu, with Linux 6.8.0-49-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.8.0-49-generic-advanced-631f71ce-256c-4124-ab53-b0408b646d43' {
	menuentry 'Ubuntu, with Linux 6.8.0-49-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.8.0-49-generic-recovery-631f71ce-256c-4124-ab53-b0408b646d43' {
		menuentry 'UEFI Firmware Settings' $menuentry_id_option 'uefi-firmware' {

$ sudo apt remove linux*realtime

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Note, selecting 'linux-tools-6.8.1-1014-realtime' for glob 'linux*realtime'
Note, selecting 'linux-modules-extra-6.8.1-1014-realtime' for glob 'linux*realtime'
Note, selecting 'linux-headers-realtime' for glob 'linux*realtime'
Note, selecting 'linux-headers-6.8.1-1014-realtime' for glob 'linux*realtime'
Note, selecting 'linux-image-realtime' for glob 'linux*realtime'
Note, selecting 'linux-realtime' for glob 'linux*realtime'
Note, selecting 'linux-image-6.8.1-1014-realtime' for glob 'linux*realtime'
Note, selecting 'linux-image-unsigned-6.8.1-1014-realtime' for glob 'linux*realtime'
Note, selecting 'linux-modules-6.8.1-1014-realtime' for glob 'linux*realtime'
Package 'linux-image-unsigned-6.8.1-1014-realtime' is not installed, so not removed
The following packages were automatically installed and are no longer required:
  amd64-microcode linux-realtime-headers-6.8.1-1014 linux-realtime-tools-6.8.1-1014 wireless-regdb
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  linux-headers-6.8.1-1014-realtime linux-headers-realtime linux-image-6.8.1-1014-realtime linux-image-realtime linux-modules-6.8.1-1014-realtime
  linux-modules-extra-6.8.1-1014-realtime linux-realtime linux-tools-6.8.1-1014-realtime
0 upgraded, 0 newly installed, 8 to remove and 43 not upgraded.
After this operation, 193 MB disk space will be freed.
Do you want to continue? [Y/n] Y
(Reading database ... 118374 files and directories currently installed.)
Removing linux-realtime (6.8.1-1014.15) ...
Removing linux-headers-realtime (6.8.1-1014.15) ...
Removing linux-headers-6.8.1-1014-realtime (6.8.1-1014.15) ...
Removing linux-image-realtime (6.8.1-1014.15) ...
Removing linux-image-6.8.1-1014-realtime (6.8.1-1014.15) ...
W: Removing the running kernel
I: /boot/vmlinuz is now a symlink to vmlinuz-6.8.0-49-generic
I: /boot/initrd.img is now a symlink to initrd.img-6.8.0-49-generic
/etc/kernel/postrm.d/initramfs-tools:
update-initramfs: Deleting /boot/initrd.img-6.8.1-1014-realtime
/etc/kernel/postrm.d/zz-update-grub:
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/50-cloudimg-settings.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-6.8.0-49-generic
Found initrd image: /boot/initrd.img-6.8.0-49-generic
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DISABLE_OS_PROBER documentation entry.
Adding boot menu entry for UEFI Firmware Settings ...
done
Removing linux-modules-extra-6.8.1-1014-realtime (6.8.1-1014.15) ...
Removing linux-modules-6.8.1-1014-realtime (6.8.1-1014.15) ...
Removing linux-tools-6.8.1-1014-realtime (6.8.1-1014.15) ...

$ sudo reboot

$ uname -r
6.8.0-49-generic

$ sudo cat /boot/grub/grub.cfg | grep "menuentry "
menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-631f71ce-256c-4124-ab53-b0408b646d43' {
	menuentry 'Ubuntu, with Linux 6.8.0-49-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.8.0-49-generic-advanced-631f71ce-256c-4124-ab53-b0408b646d43' {
	menuentry 'Ubuntu, with Linux 6.8.0-49-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-6.8.0-49-generic-recovery-631f71ce-256c-4124-ab53-b0408b646d43' {
		menuentry 'UEFI Firmware Settings' $menuentry_id_option 'uefi-firmware' {

This only works because linux-image-*-generic still exists. Ubuntu/apt seem to keep two kernel versions by default.

It should not work after an upgrade to linux-image-*-realtime, leading to auto removal of the generic one. In that case, the generic kernel should be installed manually before doing sudo apt remove linux*realtime.

See also:

• https://askubuntu.com/questions/1459582/how-do-i-fully-remove-the-realtime-kernel