Linux Kernel

• controls everything on the system.
  • hardware
  • enforces security and access controls
  • runs, schedules, and manages processes and service daemons.
• comprised of several modules. A
• new kernel must be installed or an existing kernel must be upgraded when the need arises from an application or functionality standpoint.
• core of the Linux system. It
• manages
• hardware,
• enforces security,
• regulates access to the system, as well as

handles

• processes,

• services, and

• application workloads. It is a

• collection of software components called modules

  • Modules
    • device drivers that control hardware devices
      • processor
      • memory
      • storage
      • controller cards
      • peripheral equipment
    • interact with software subsystems
      • storage partitioning
      • file systems
      • networking
      • virtualization

• Some modules are static to the kernel and are integral to system functionality,

• Some modules are loaded dynamically as needed

• RHEL 8.0 and RHEL 8.2 are shipped with kernel version 4.18.0 (4.18.0-80 and 4.18.0-193 to be specific) for the 64-bit Intel/AMD processor architecture computers with single, multi-core, and multi-processor configurations.

• uname -m shows the architecture of the system.

• Kernel requires a rebuild when a new functionality is added or removed.

• functionality may be introduced by:

  • installing a new kernel
  • upgrading an existing one
  • installing a new hardware device, or
  • changing a critical system component.

• existing functionality that is no longer needed may be removed to make the overall footprint of the kernel smaller for improved performance and reduced memory utilization.

• tunable parameters are set that define a baseline for kernel functionality.

• Some parameters must be tuned for some applications and database software to be installed smoothly and operate properly.

• You can generate and store several custom kernels with varied configuration and required modules, but

• only one of them can be active at a time.

• different kernel may be loaded by interacting with GRUB2.

Kernel Packages

• set of core kernel packages that must be installed on the system at a minimum to make it work.
• Additional packages providing supplementary kernel support are also available.

Core and some add-on kernel packages.

Kernel Packages

• packages containing the source code for RHEL 8 are also available for those who wish to customize and recompile the code

List kernel packages installed on the system: dnf list installed kernel*

• Shows six kernel packages that were loaded during the OS installation.

Analyzing Kernel Version

Check the version of the kernel running on the system to check for compatibility with an application or database:

uname -r
5.14.0-362.24.1.el9_3.x86_64

5 - Major version 14 - Major revision 0 - Kernel patch version 362 - Red Hat version el9 - Enterprise Linux 9 x86_64 - Processor architecture

Kernel Directory Structure

Kernel and its support files (noteworthy locations)

• /boot
• /proc
• /usr/lib/modules

/boot

• Created at system installation.
• Linux kernel
• GRUB2 configuration
• other kernel and boot support files.

View the /boot filesystem: ls -l /boot

• four files are for the kernel and
  • vmlinuz - main kernel file
  • initramfs - main kernel’s boot image
  • config - configuration
  • System.map - mapping
• two files for kernel rescue version
  • Have the current kernel version appended to their names.
  • have the string “rescue” embedded within their names

/boot/efi/ and /boot/grub2/

• hold bootloader information specific to firmware type used on the system: UEFI or BIOS.

List /boot/Grub2:

[root@localhost ~]# ls -l /boot/grub2
total 32
-rw-r--r--. 1 root root   64 Feb 25 05:13 device.map
drwxr-xr-x. 2 root root   25 Feb 25 05:13 fonts
-rw-------. 1 root root 7049 Mar 21 04:47 grub.cfg
-rw-------. 1 root root 1024 Mar 21 05:12 grubenv
drwxr-xr-x. 2 root root 8192 Feb 25 05:13 i386-pc
drwxr-xr-x. 2 root root 4096 Feb 25 05:13 locale

• grub.cfg
  • bootable kernel information
• grub.env
  • environment information that the kernel uses.

/boot/loader

• storage location for configuration of the running and rescue kernels.
• Configuration is stored in files under the /boot/loader/entries/

[root@localhost ~]# ls -l /boot/loader/entries/
total 12
-rw-r--r--. 1 root root 484 Feb 25 05:13 8215ac7e45d34823b4dce2e258c3cc47-0-rescue.conf
-rw-r--r--. 1 root root 460 Mar 16 06:17 8215ac7e45d34823b4dce2e258c3cc47-5.14.0-362.18.1.el9_3.x86_64.conf
-rw-r--r--. 1 root root 459 Mar 16 06:17 8215ac7e45d34823b4dce2e258c3cc47-5.14.0-362.24.1.el9_3.x86_64.conf

• The files are named using the machine id of the system as stored in /etc/machine-id/ and the kernel version they are for.

content of the kernel file:

[root@localhost entries]# cat /boot/loader/entries/8215ac7e45d34823b4dce2e258c3cc47-5.14.0-362.18.1.el9_3.x86_64.conf
title Red Hat Enterprise Linux (5.14.0-362.18.1.el9_3.x86_64) 9.3 (Plow)
version 5.14.0-362.18.1.el9_3.x86_64
linux /vmlinuz-5.14.0-362.18.1.el9_3.x86_64
initrd /initramfs-5.14.0-362.18.1.el9_3.x86_64.img $tuned_initrd
options root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet  $tuned_params
grub_users $grub_users
grub_arg --unrestricted
grub_class rhel

• “title” is displayed on the bootloader screen
• “kernelopts” and “tuned_params” supply values to the booting kernel to control its behavior.

/proc

• Virtual, memory-based file system
• contents are created and updated in memory at system boot and during runtime
• destroyed at system shutdown
• current state of the kernel, which includes
  • hardware configuration
  • status information
    • processor,
    • memory, s
    • storage,
    • file systems,
    • swap,
    • processes
    • network interfaces
    • connections,
    • routing, *
    • etc.*
• Data kept in tens of thousands of zero-byte files organized in a hierarchy.

List /proc: ls -l /proc

• numerical subdirectories contain information about a specific process
  • process ID matches the subdirectory name.
• other files and subdirectories contain information, such as
  • memory segments for processes and
  • configuration data for system components. You
  • can view the configuration in vim

Show selections from the cpuinfo and meminfo files that hold processor and memory information: cat/proc/cpuinfo && cat /proc/meminfo

• data used by top, ps, uname, free, uptime and w, to display information.

/us/lib/modules/

• holds information about kernel modules.
• subdirectories are specific to the kernels installed on the system.

Long listing of /usr/lib/modules/ shows two installed kernels:

[root@localhost entries]# ls -l /usr/lib/modules
total 8
drwxr-xr-x. 7 root root 4096 Mar 16 06:18 5.14.0-362.18.1.el9_3.x86_64
drwxr-xr-x. 8 root root 4096 Mar 16 06:18 5.14.0-362.24.1.el9_3.x86_64

View /usr/lib/modules/5.14.0-362.18.1.el9_3.x86_64/: ls -l /usr/lib/modules/5.14.0-362.18.1.el9_3.x86_64

• Subdirectories hold module-specific information for the kernel version.

/lib/modules/4.18.0-80.el8.x86_64/kernel/drivers/

• stores modules for a variety of hardware and software components in various subdirectories: ls -l /usr/lib/modules/5.14.0-362.18.1.el9_3.x86_64/kernel/drivers

• Additional modules may be installed on the system to support more components.

Installing the Kernel

• requires extra care

• could leave your system in an unbootable or undesirable state.

• have the bootable medium handy prior to starting the kernel install process.

• By default, the dnf command adds a new kernel to the system, leaving the existing kernel(s) intact. It does not replace or overwrite existing kernel files.

• Always install a new version of the kernel instead of upgrading it.

• The upgrade process removes any existing kernel and replaces it with a new one.

• In case of a post-installation issue, you will not be able to revert to the old working kernel.

• Newer version of the kernel is typically required:

  • if an application needs to be deployed on the system that requires a different kernel to operate.
  • When deficiencies or bugs are identified in the existing kernel, it can hamper the kernel’s smooth operation.

• new kernel

  • addresses existing issues
  • adds bug fixes
  • security updates
  • new features
  • improved support for hardware devices.

• dnf is the preferred tool to install a kernel

• it resolves and installs any required dependencies automatically.

• rpm may be used but you must install any dependencies manually.

• Kernel packages for RHEL are available to subscribers on Red Hat’s Customer Portal.