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    Kernel Management

    Enterprise Linux kernel management encompasses installing new kernels, selecting the default boot kernel, cleaning up old versions, and optionally installing newer kernels from ELRepo.

    Viewing Kernel Information

    Section titled “Viewing Kernel Information”

    Check the Currently Running Kernel

    Section titled “Check the Currently Running Kernel”
    View the current kernel version
    uname -r

    View Detailed Kernel Information

    Section titled “View Detailed Kernel Information”
    View full kernel details
    uname -a

    List All Installed Kernels

    Section titled “List All Installed Kernels”
    List installed kernel packages
    rpm -qa kernel-core | sort -V

    List Kernels via dnf

    Section titled “List Kernels via dnf”
    List kernels through dnf
    dnf list installed kernel-core

    Check for Available Kernel Updates

    Section titled “Check for Available Kernel Updates”
    Check for kernel updates
    dnf check-update kernel

    Managing Boot Entries with grubby

    Section titled “Managing Boot Entries with grubby”

    grubby is the standard command-line tool for managing GRUB boot entries.

    List All Boot Entries

    Section titled “List All Boot Entries”
    List all available kernel boot entries
    grubby --info=ALL

    View the Current Default Kernel

    Section titled “View the Current Default Kernel”
    View the default boot kernel
    grubby --default-kernel

    View the Default Kernel Index

    Section titled “View the Default Kernel Index”
    View the default kernel index number
    grubby --default-index

    Set the Default Kernel

    Section titled “Set the Default Kernel”
    Set default kernel by path
    grubby --set-default /boot/vmlinuz-5.14.0-362.8.1.el9_3.x86_64
    Set default kernel by index (0 is newest)
    grubby --set-default-index 1

    Add Boot Parameters to a Specific Kernel

    Section titled “Add Boot Parameters to a Specific Kernel”
    Add a boot parameter to a specific kernel
    grubby --update-kernel /boot/vmlinuz-$(uname -r) --args="nouveau.modeset=0"

    Remove Kernel Boot Parameters

    Section titled “Remove Kernel Boot Parameters”
    Remove a boot parameter from a specific kernel
    grubby --update-kernel /boot/vmlinuz-$(uname -r) --remove-args="quiet"

    Add Parameters to All Kernels

    Section titled “Add Parameters to All Kernels”
    Add a boot parameter to all kernels
    grubby --update-kernel ALL --args="crashkernel=auto"

    Kernel Rollback

    Section titled “Kernel Rollback”

    When a new kernel causes system issues, you can quickly switch to a previous version.

    Method 1: GRUB Menu Selection

    Section titled “Method 1: GRUB Menu Selection”

    At boot time, select an older kernel from the GRUB menu. If the GRUB menu is not displayed, hold Shift or press Esc during boot.

    Method 2: Using grubby

    Section titled “Method 2: Using grubby”
    Switch the default to an older kernel
    # List available kernels
    grubby --info=ALL | grep -E "^kernel|^index"
    

    # Set the old version as default
    grubby --set-default /boot/vmlinuz-<old-version>
    

    # Reboot to apply
    reboot

    Method 3: Using dnf History

    Section titled “Method 3: Using dnf History”
    Roll back a kernel update via dnf history
    # Find the transaction ID for the kernel installation
    dnf history list | grep kernel
    

    # Undo that transaction
    dnf history undo <transaction-ID> -y

    Cleaning Up Old Kernels

    Section titled “Cleaning Up Old Kernels”

    Check the installonly_limit Setting

    Section titled “Check the installonly_limit Setting”

    installonly_limit controls how many kernel versions are retained.

    View the current installonly_limit
    grep installonly_limit /etc/dnf/dnf.conf

    The default is usually 3, meaning up to 3 kernel versions are kept.

    Modify installonly_limit

    Section titled “Modify installonly_limit”
    Set the number of kernels to retain
    # Change to keep 2 kernels (minimum recommended is 2)
    echo "installonly_limit=2" >> /etc/dnf/dnf.conf

    Manually Remove an Old Kernel

    Section titled “Manually Remove an Old Kernel”
    Remove a specific old kernel
    dnf remove kernel-core-<version> -y

    Use package-cleanup to Remove Old Kernels

    Section titled “Use package-cleanup to Remove Old Kernels”
    Keep only the 2 most recent kernels
    dnf install -y dnf-plugins-core
    # package-cleanup comes from yum-utils (dnf-plugins-core only ships dnf subcommand plugins)
    dnf install -y yum-utils
    package-cleanup --oldkernels --count=2

    Regenerate GRUB Configuration After Cleanup

    Section titled “Regenerate GRUB Configuration After Cleanup”
    Regenerate the GRUB configuration
    grub2-mkconfig -o /boot/grub2/grub.cfg

    For UEFI systems:

    Regenerate GRUB configuration for UEFI
    grub2-mkconfig -o /boot/efi/EFI/almalinux/grub.cfg

    Installing Newer Kernels from ELRepo

    Section titled “Installing Newer Kernels from ELRepo”

    ELRepo provides two kernel tracking branches:

    • kernel-ml — Mainline stable (latest features)
    • kernel-lt — Long Term support (more conservative)

    Install the ELRepo Repository

    Section titled “Install the ELRepo Repository”
    Import ELRepo GPG key and install the repository
    rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
    dnf install -y https://www.elrepo.org/elrepo-release-9.el9.elrepo.noarch.rpm

    For EL 8 systems:

    Install ELRepo on EL 8
    rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
    dnf install -y https://www.elrepo.org/elrepo-release-8.el8.elrepo.noarch.rpm

    View Available ELRepo Kernels

    Section titled “View Available ELRepo Kernels”
    List available kernels from ELRepo
    dnf --enablerepo=elrepo-kernel list available | grep kernel

    Install the Mainline Kernel

    Section titled “Install the Mainline Kernel”
    Install kernel-ml
    dnf --enablerepo=elrepo-kernel install -y kernel-ml

    Install the Long Term Kernel

    Section titled “Install the Long Term Kernel”
    Install kernel-lt
    dnf --enablerepo=elrepo-kernel install -y kernel-lt

    Set the ELRepo Kernel as Default

    Section titled “Set the ELRepo Kernel as Default”
    Set the newly installed ELRepo kernel as default
    grubby --default-kernel
    # If it is not the new kernel, set it manually
    grubby --set-default /boot/vmlinuz-<elrepo-kernel-version>

    Install Matching Kernel Development Tools

    Section titled “Install Matching Kernel Development Tools”
    Install kernel-ml development packages
    dnf --enablerepo=elrepo-kernel install -y kernel-ml-devel kernel-ml-headers
    Install kernel-lt development packages
    dnf --enablerepo=elrepo-kernel install -y kernel-lt-devel kernel-lt-headers

    Kernel Parameter Tuning

    Section titled “Kernel Parameter Tuning”

    View Current Kernel Parameters

    Section titled “View Current Kernel Parameters”
    View all sysctl parameters
    sysctl -a

    Temporarily Modify a Kernel Parameter

    Section titled “Temporarily Modify a Kernel Parameter”
    Temporarily adjust a parameter (lost on reboot)
    sysctl -w net.core.somaxconn=65535

    Permanently Modify Kernel Parameters

    Section titled “Permanently Modify Kernel Parameters”
    Create a persistent kernel parameter configuration
    cat > /etc/sysctl.d/99-custom.conf << 'EOF'
    # Network tuning
    net.core.somaxconn = 65535
    net.ipv4.tcp_max_syn_backlog = 65535
    

    # Memory management
    vm.swappiness = 10
    vm.dirty_ratio = 40
    vm.dirty_background_ratio = 10
    EOF
    

    # Apply immediately
    sysctl --system

    Kernel Module Management

    Section titled “Kernel Module Management”

    List Loaded Modules

    Section titled “List Loaded Modules”
    View loaded kernel modules
    lsmod

    View Module Information

    Section titled “View Module Information”
    View details about a specific module
    modinfo <module-name>

    Load and Unload Modules

    Section titled “Load and Unload Modules”
    Load a kernel module
    modprobe <module-name>
    Unload a kernel module
    modprobe -r <module-name>

    Auto-Load a Module at Boot

    Section titled “Auto-Load a Module at Boot”
    Configure a module to load at boot
    echo "<module-name>" > /etc/modules-load.d/<module-name>.conf

    Blacklist a Module

    Section titled “Blacklist a Module”
    Prevent a module from loading
    echo "blacklist <module-name>" > /etc/modprobe.d/blacklist-<module-name>.conf