How to modify kernel boot parameters

The real-time performance-tuning features compiled into the kernel can be controlled by various means. The approach used here is a common one: passing command line parameters via a bootloader.

The Linux kernel supports a wide range of parameters, but only a few of them are relevant to real-time tuning. Kernel boot parameters is good a reference.

Depending on the underlying boot mechanism, there are multiple ways to pass command line parameters to the kernel:

Modify kernel boot parameters via GRUB

You can temporarily or permanently configure parameters which GRUB will pass to Real-time Ubuntu.

When first testing a real-time tuning feature, configure it temporarily. Temporary bootloader parameters are passed to the kernel on a single startup. To re-use them you’ll need to re-enter them — they don’t persist across reboots.

When you’re satisfied that a parameter achieves what you want, you can make it permanent by updating GRUB’s configuration then rebooting.

Configure GRUB temporarily

Reboot, and interrupt GRUB’s startup sequence at its bootloader menu.

Highlight the entry — usually Ubuntu — that would boot Real-time Ubuntu.

GRUB menu

Press e to edit its boot parameters.

Append parameters to the line that starts with linux:

GRUB edit

  • Leave a space before each new parameter

  • Don’t add space round = and other punctuations for each key-value

  • Don’t add line breaks

For example, after adding nohz=on and nohz_full=1-3:

linux        /boot/vmlinuz-5.15.0-1056-realtime root=UUID=06cfb4f1-21f9-4568-b656-926224e20fb3 ro quiet splash $vt_handoff nohz=on nohz_full=1-3

Press Ctrl+X to exit the editor and continue booting with the new parameters.

Configure GRUB permanently

Once your device has booted, use a text editor to open /etc/default/grub.

Add parameters to GRUB_CMDLINE_LINUX:

  • Enter parameters inside the double-quotes

  • Leave a space before each new parameter

  • Don’t add space round = and other punctuations for each key-value

  • Don’t add line breaks

For example, after adding nohz=on and nohz_full=1-3:

GRUB_CMDLINE_LINUX="nohz=on nohz_full=1-3"

Save and close the editor.

Update GRUB with its new configuration:

sudo update-grub

Reboot.

Modify kernel boot parameters on Raspberry Pi

On a Raspberry Pi running Ubuntu Server / Desktop, the kernel command line arguments are found under /boot/firmware/cmdline.txt.

For example, on Ubuntu 24.04 with the real-time kernel:

$ cat /boot/firmware/cmdline.txt
console=serial0,115200 multipath=off dwc_otg.lpm_enable=0 console=tty1 root=LABEL=writable rootfstype=ext4 rootwait fixrtc

Use a text editor to edit this file /boot/firmware/cmdline.txt.

Append parameters to the line:

  • Leave a space after each new parameter

  • Don’t add space round = and other punctuations for each key-value

  • Don’t add line breaks

For example, after adding nohz=on and nohz_full=1-3 to the above example:

$ cat /boot/firmware/cmdline.txt
console=serial0,115200 multipath=off dwc_otg.lpm_enable=0 console=tty1 root=LABEL=writable rootfstype=ext4 rootwait fixrtc nohz=on nohz_full=1-3

Save and close the editor.

Reboot.

Verify the parameters passed to the kernel at boot time:

$ cat /proc/cmdline
reboot=w coherent_pool=1M 8250.nr_uarts=1 pci=pcie_bus_safe snd_bcm2835.enable_compat_alsa=0 snd_bcm2835.enable_hdmi=1  smsc95xx.macaddr=D8:3A:DD:E4:0B:D3 vc_mem.mem_base=0x3fc00000 vc_mem.mem_size=0x40000000  console=ttyAMA10,115200 multipath=off dwc_otg.lpm_enable=0 console=tty1 root=LABEL=writable rootfstype=ext4 rootwait fixrtc nohz=on nohz_full=1-3

Modify boot parameters on Ubuntu Core

Note

This section assumes access to features currently available exclusively to dedicated Snap Store users.

Ubuntu Core doesn’t have the /etc/default/grub file mapped for the kernel parameters, nor the update-grub utility. On Ubuntu Core, the kernel parameters can be modified statically or dynamically. Static configurations are added during the image build. On the other hand, dynamic configurations are set using snap options on a running system.

This section focuses on the dynamic method for configuring kernel boot parameters, suitable for iteratively tuning the operating system.

Hint

Refer to How to create a Real-time Ubuntu Core image if you want to create a Real-time Ubuntu Core image.

Open a terminal and access your Real-time Ubuntu Core instance. The instance should be on a physical device rather than a virtual machine. While a virtual machine can be used to follow the steps in this guide, it will not provide the benefits of a real-time kernel.

There are two snap options for setting the kernel parameters:

When using the reference gadget snaps (pc-gadget and pi-gadget), you need to use the system.kernel.dangerous-cmdline-append option, as the allow list is not present in them.

When creating a custom gadget snap for a real-time Ubuntu Core system, it is advisable to add an allow list with the relevant parameters.

The kernel parameters in the snap configurations exactly match the kernel parameters that would be passed to the kernel on the grub command line, GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub file. Parameters are passed as a single string in the key=value format, with each parameter separated by a space.

For example, the command below sets nohz=on, nohz_full=2-N, and irqaffinity=0-1:

snap set system system.kernel.dangerous-cmdline-append="nohz=on nohz_full=2-N irqaffinity=0-1"

After setting the kernel parameters, it’s necessary to reboot the system to apply the changes. Then you can check if the parameters were applied:

$ cat /proc/cmdline
snapd_recovery_mode=run console=ttyS0,115200n8 console=tty1 panic=-1 nohz=on nohz_full=2-N irqaffinity=0-1

To undo all the modifications made to the kernel parameters, use the snap unset command:

snap unset system system.kernel.dangerous-cmdline-append

And reboot.

Once you are satisfied with the configurations, consider configuring them statically so that they are built into the image. Read more on How to create a Real-time Ubuntu Core image.