All linux kernel options

Kernel parameters

There are three ways to pass options to the kernel and thus control its behaviour:

1. When building the kernel—in the kernel’s config file. See Kernel#Compilation for details.
2. When starting the kernel—using command line parameters (usually through a boot loader).
3. At runtime—through the files in /proc/sys/ (see sysctl) and /sys/ .

Between the three methods, the configurable options differ in availability, their name and the method in which they are specified. This page only explains the second method (kernel command line parameters) and shows a list of the most used kernel parameters in Arch Linux.

Most parameters are associated with subsystems and work only if the kernel is configured with those subsystems built in. They also depend on the presence of the hardware they are associated with.

Kernel command line parameters either have the format parameter or parameter=value .

Contents

Configuration

Kernel parameters can be set either temporarily by editing the boot entry in the boot loader’s boot selection menu, or permanently by modifying the boot loader’s configuration file.

The following examples add the quiet and splash parameters to Syslinux, systemd-boot, GRUB, GRUB Legacy, LILO, and rEFInd.

Syslinux

• Press Tab when the menu shows up and add them at the end of the string:

Press Enter to boot with these parameters.

• To make the change persistent after reboot, edit /boot/syslinux/syslinux.cfg and add them to the APPEND line:

For more information on configuring Syslinux, see the Syslinux article.

systemd-boot

• Press e when the menu appears and add the parameters to the end of the string:

Press Enter to boot with these parameters.

• To make the change persistent after reboot, edit /boot/loader/entries/arch.conf (assuming you set up your EFI system partition) and add them to the options line:

For more information on configuring systemd-boot, see the systemd-boot article.

• Press e when the menu shows up and add them on the linux line:

Press Ctrl+x to boot with these parameters.

• To make the change persistent after reboot, you could manually edit /boot/grub/grub.cfg with the exact line from above, or if using grub-mkconfig:

Edit /etc/default/grub and append your kernel options between the quotes in the GRUB_CMDLINE_LINUX_DEFAULT line: And then automatically re-generate the grub.cfg file with:

For more information on configuring GRUB, see the GRUB article.

GRUB Legacy

• Press e when the menu shows up and add them on the kernel line:

Press b to boot with these parameters.

• To make the change persistent after reboot, edit /boot/grub/menu.lst and add them to the kernel line, exactly like above.

For more information on configuring GRUB Legacy, see the GRUB Legacy article.

For more information on configuring LILO, see the LILO article.

rEFInd

• Press + , F2 , or Insert on the desired menu entry and press it again on the submenu entry. Add kernel parameters at the end of the string:

Press Enter to boot with these parameters.

• To make the change persistent after reboot, edit /boot/refind_linux.conf and append them between the quotes in all required lines, for example

• If you have disabled auto-detection of OSes in rEFInd and are defining OS stanzas instead in esp/EFI/refind/refind.conf to load your OSes, you can edit it like:

For more information on configuring rEFInd, see the rEFInd article.

EFISTUB

dracut

dracut is capable of embedding the kernel parameters in the initramfs, thus allowing to omit them from the boot loader configuration. See dracut#Kernel command line options.

Hijacking cmdline

Even without access to your bootloader it is possible to change your kernel parameters to enable debugging (if you have root access). This can be accomplished by overwriting /proc/cmdline which stores the kernel parameters. However /proc/cmdline is not writable even as root, so this hack is accomplished by using a bind mount to mask the path.

First create a file containing the desired kernel parameters

Then use a bind mount to overwrite the parameters

The -n option skips adding the mount to /etc/mtab , so it will work even if root is mounted read-only. You can cat /proc/cmdline to confirm that your change was successful.

Parameter list

This list is not comprehensive. For a complete list of all options, please see the kernel documentation.

parameter	Description
init	Run specified binary instead of /sbin/init as init process. The systemd-sysvcompat package symlinks /sbin/init to /usr/lib/systemd/systemd to use systemd. Set it to /bin/sh to boot to the shell.
initrd	Specify the location of the initial ramdisk. For UEFI boot managers and EFISTUB, the path must be specified using backslashes ( \ ) as path separators.
debug	Enable kernel debugging (events log level).
lsm	Set the initialisation order of the Linux security modules, used to enable AppArmor, SELinux or TOMOYO.
maxcpus	Maximum number of processors that an SMP kernel will bring up during bootup.
mem	Force usage of a specific amount of memory to be used.
netdev	Network devices parameters.
nomodeset	Disable Kernel mode setting.
panic	Time before automatic reboot on kernel panic.
resume	Specify a swap device to use when waking from hibernation.
ro	Mount root device read-only on boot (default 1 ).
root	Root filesystem. See init/do_mounts.c for kernel’s supported device name formats. Note that an initramfs with udev supports more name formats.
rootflags	Root filesystem mount options. Useful for setting options that cannot be applied by remounting (i.e. by systemd-remount-fs.service(8) ). For example, the discard option of an XFS root volume.
rw	Mount root device read-write on boot.
systemd.unit	Boot to a specified target.
video	Override framebuffer video defaults.

1 mkinitcpio uses ro as default value when neither rw or ro is set by the boot loader. Boot loaders may set the value to use, for example GRUB uses rw by default (see FS#36275 as a reference).

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The kernel’s command-line parameters¶

The following is a consolidated list of the kernel parameters as implemented by the __setup(), early_param(), core_param() and module_param() macros and sorted into English Dictionary order (defined as ignoring all punctuation and sorting digits before letters in a case insensitive manner), and with descriptions where known.

The kernel parses parameters from the kernel command line up to “ — “; if it doesn’t recognize a parameter and it doesn’t contain a ‘.’, the parameter gets passed to init: parameters with ‘=’ go into init’s environment, others are passed as command line arguments to init. Everything after “ — ” is passed as an argument to init.

Module parameters can be specified in two ways: via the kernel command line with a module name prefix, or via modprobe, e.g.:

Parameters for modules which are built into the kernel need to be specified on the kernel command line. modprobe looks through the kernel command line (/proc/cmdline) and collects module parameters when it loads a module, so the kernel command line can be used for loadable modules too.

Hyphens (dashes) and underscores are equivalent in parameter names, so:

can also be entered as:

Double-quotes can be used to protect spaces in values, e.g.:

cpu lists:В¶

Some kernel parameters take a list of CPUs as a value, e.g. isolcpus, nohz_full, irqaffinity, rcu_nocbs. The format of this list is:

— (must be a positive range in ascending order)

Note that for the special case of a range one can split the range into equal sized groups and for each group use some amount from the beginning of that group:

For example one can add to the command line following parameter:

where the final item represents CPUs 100,101,125,126,150,151,…

The value “N” can be used to represent the numerically last CPU on the system, i.e “foo_cpus=16-N” would be equivalent to “16-31” on a 32 core system.

Keep in mind that “N” is dynamic, so if system changes cause the bitmap width to change, such as less cores in the CPU list, then N and any ranges using N will also change. Use the same on a small 4 core system, and “16-N” becomes “16-3” and now the same boot input will be flagged as invalid (start > end).

The special case-tolerant group name “all” has a meaning of selecting all CPUs, so that “nohz_full=all” is the equivalent of “nohz_full=0-N”.

The semantics of “N” and “all” is supported on a level of bitmaps and holds for all users of bitmap_parse() .

This document may not be entirely up to date and comprehensive. The command “modinfo -p $” shows a current list of all parameters of a loadable module. Loadable modules, after being loaded into the running kernel, also reveal their parameters in /sys/module/$/parameters/. Some of these parameters may be changed at runtime by the command echo -n $ > /sys/module/$/parameters/$ .

The parameters listed below are only valid if certain kernel build options were enabled and if respective hardware is present. The text in square brackets at the beginning of each description states the restrictions within which a parameter is applicable:

In addition, the following text indicates that the option:

Note that ALL kernel parameters listed below are CASE SENSITIVE, and that a trailing = on the name of any parameter states that that parameter will be entered as an environment variable, whereas its absence indicates that it will appear as a kernel argument readable via /proc/cmdline by programs running once the system is up.

The number of kernel parameters is not limited, but the length of the complete command line (parameters including spaces etc.) is limited to a fixed number of characters. This limit depends on the architecture and is between 256 and 4096 characters. It is defined in the file ./include/asm/setup.h as COMMAND_LINE_SIZE.

Finally, the [KMG] suffix is commonly described after a number of kernel parameter values. These ‘K’, ‘M’, and ‘G’ letters represent the _binary_ multipliers ‘Kilo’, ‘Mega’, and ‘Giga’, equaling 2^10, 2^20, and 2^30 bytes respectively. Such letter suffixes can also be entirely omitted:

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The Linux Kernel documentation¶

This is the top level of the kernel’s documentation tree. Kernel documentation, like the kernel itself, is very much a work in progress; that is especially true as we work to integrate our many scattered documents into a coherent whole. Please note that improvements to the documentation are welcome; join the linux-doc list at vger.kernel.org if you want to help out.

Licensing documentation¶

The following describes the license of the Linux kernel source code (GPLv2), how to properly mark the license of individual files in the source tree, as well as links to the full license text.

User-oriented documentation¶

The following manuals are written for users of the kernel — those who are trying to get it to work optimally on a given system.

Firmware-related documentation¶

The following holds information on the kernel’s expectations regarding the platform firmwares.

Application-developer documentation¶

The user-space API manual gathers together documents describing aspects of the kernel interface as seen by application developers.

Introduction to kernel development¶

These manuals contain overall information about how to develop the kernel. The kernel community is quite large, with thousands of developers contributing over the course of a year. As with any large community, knowing how things are done will make the process of getting your changes merged much easier.

Kernel API documentation¶

These books get into the details of how specific kernel subsystems work from the point of view of a kernel developer. Much of the information here is taken directly from the kernel source, with supplemental material added as needed (or at least as we managed to add it — probably not all that is needed).

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Kernel/Traditional compilation

This article is an introduction to building custom kernels from kernel.org sources. This method of compiling kernels is the traditional method common to all distributions. It can be, depending on your background, more complicated than using the Kernels/Arch Build System. Consider the Arch Build System tools are developed and maintained to make repeatable compilation tasks efficient and safe.

Contents

Preparation

It is not necessary (or recommended) to use the root account or root privileges (i.e. via Sudo) for kernel preparation.

Install the core packages

Install the base-devel package group, which contains necessary packages such as make and gcc . It is also recommended to install the following packages, as listed in the default Arch kernel PKGBUILD: xmlto , kmod , inetutils , bc , libelf , git , cpio , perl , tar , xz .

Create a kernel compilation directory

It is recommended to create a separate build directory for your kernel(s). In this example, the directory kernelbuild will be created in the home directory:

Download the kernel source

Download the kernel source from https://www.kernel.org. This should be the tarball ( tar.xz ) file for your chosen kernel.

It can be downloaded by simply right-clicking the tar.xz link in your browser and selecting Save Link As. , or any other number of ways via alternative graphical or command-line tools that utilise HTTP, TFTP, Rsync, or Git.

In the following command-line example, wget has been installed and is used inside the

/kernelbuild directory to obtain kernel 4.8.6:

You should also verify the correctness of the download before trusting it. First grab the signature, then use that to grab the fingerprint of the signing key, then use the fingerprint to obtain the actual signing key:

Note the signature was generated for the tar archive (i.e. extension .tar ), not the compressed .tar.xz file that you have downloaded. You need to decompress the latter without untarring it. Verify that you have xz installed, then you can proceed like so:

Do not proceed if this does not result in output that includes the string «Good signature».

If wget was not used inside the build directory, it will be necessary to move the tarball into it, e.g.

Unpack the kernel source

Within the build directory, unpack the kernel tarball:

To finalise the preparation, ensure that the kernel tree is absolutely clean; do not rely on the source tree being clean after unpacking. To do so, first change into the new kernel source directory created, and then run the make mrproper command:

Kernel configuration

This is the most crucial step in customizing the default kernel to reflect your computer’s precise specifications. Kernel configuration is set in its .config file, which includes the use of Kernel modules. By setting the options in .config properly, your kernel and computer will function most efficiently.

You can do a mixture of two things:

• Use the default Arch settings from an official kernel (recommended)
• Manually configure the kernel options (optional, advanced and not recommended)

Default Arch configuration

This method will create a .config file for the custom kernel using the default Arch kernel settings. If a stock Arch kernel is running, you can use the following command inside the custom kernel source directory:

Otherwise, the default configuration can be found online in the official Arch Linux kernel package.

Advanced configuration

There are several tools available to fine-tune the kernel configuration, which provide an alternative to otherwise spending hours manually configuring each and every one of the options available during compilation.

Those tools are:

• make menuconfig : Command-line ncurses interface superseded by nconfig
• make nconfig : Newer ncurses interface for the command-line
• make xconfig : User-friendly graphical interface that requires packagekit-qt5 to be installed as a dependency. This is the recommended method — especially for less experienced users — as it is easier to navigate, and information about each option is also displayed.
• make gconfig : Graphical configuration similar to xconfig but using gtk. This requires gtk2 , glib2 and libgladeAUR .

The chosen method should be run inside the kernel source directory, and all will either create a new .config file, or overwrite an existing one where present. All optional configurations will be automatically enabled, although any newer configuration options (i.e. with an older kernel .config ) may not be automatically selected.

Once the changes have been made save the .config file. It is a good idea to make a backup copy outside the source directory. You may need to do this multiple times before you get all the options right.

If unsure, only change a few options between compilations. If you cannot boot your newly built kernel, see the list of necessary config items here.

Running lspci -k # from liveCD lists names of kernel modules in use. Most importantly, you must maintain cgroups support. This is necessary for systemd. For more detailed information, see Gentoo:Kernel/Gentoo Kernel Configuration Guide and Gentoo:Intel#Kernel or Gentoo:Ryzen#Kernel for Intel or AMD Ryzen processors.

Compilation

Compilation time will vary from as little as fifteen minutes to over an hour, depending on your kernel configuration and processor capability. Once the .config file has been set for the custom kernel, within the source directory run the following command to compile:

Installation

Install the modules

Once the kernel has been compiled, the modules for it must follow. First build the modules:

Then install the modules. As root or with root privileges, run the following command to do so:

This will copy the compiled modules into /lib/modules/ — . For example, for kernel version 4.8 installed above, they would be copied to /lib/modules/4.8.6-ARCH . This keeps the modules for individual kernels used separated.

Copy the kernel to /boot directory

The kernel compilation process will generate a compressed bzImage (big zImage) of that kernel, which must be copied to the /boot directory and renamed in the process. Provided the name is prefixed with vmlinuz- , you may name the kernel as you wish. In the examples below, the installed and compiled 4.8 kernel has been copied over and renamed to vmlinuz-linux48 :

Make initial RAM disk

If you do not know what making an initial RAM disk is, see Initramfs on Wikipedia and mkinitcpio.

Automated preset method

An existing mkinitcpio preset can be copied and modified so that the custom kernel initramfs images can be generated in the same way as for an official kernel. This is useful where intending to recompile the kernel (e.g. where updated). In the example below, the preset file for the stock Arch kernel will be copied and modified for kernel 4.8, installed above.

First, copy the existing preset file, renaming it to match the name of the custom kernel specified as a suffix to /boot/vmlinuz- when copying the bzImage (in this case, linux48 ):

Second, edit the file and amend for the custom kernel. Note (again) that the ALL_kver= parameter also matches the name of the custom kernel specified when copying the bzImage :

Finally, generate the initramfs images for the custom kernel in the same way as for an official kernel:

Manual method

Rather than use a preset file, mkinitcpio can also be used to generate an initramfs file manually. The syntax of the command is:

• -k ( —kernel ): Specifies the modules to use when generating the initramfs image. The name will be the same as the name of the custom kernel source directory (and the modules directory for it, located in /usr/lib/modules/ ).
• -g ( —generate ): Specifies the name of the initramfs file to generate in the /boot directory. Again, using the naming convention mentioned above is recommended.

For example, the command for the 4.8 custom kernel installed above would be:

Copy System.map

The System.map file is not required for booting Linux. It is a type of «phone directory» list of functions in a particular build of a kernel. The System.map contains a list of kernel symbols (i.e function names, variable names etc) and their corresponding addresses. This «symbol-name to address mapping» is used by:

• Some processes like klogd, ksymoops, etc.
• By OOPS handler when information has to be dumped to the screen during a kernel crash (i.e info like in which function it has crashed).

If your /boot is on a filesystem which supports symlinks (i.e., not FAT32), copy System.map to /boot , appending your kernel’s name to the destination file. Then create a symlink from /boot/System.map to point to /boot/System.map- :

After completing all steps above, you should have the following 3 files and 1 soft symlink in your /boot directory along with any other previously existing files:

• Kernel: vmlinuz-
• Initramfs: Initramfs- .img
• System Map: System.map-
• System Map kernel symlink

Bootloader configuration

Add an entry for your new kernel in your bootloader’s configuration file. See Arch boot process#Feature comparison for possible boot loaders, their wiki articles and other information.

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