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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 --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