Chapter 2. Specifying the RHEL kernel to run You can boot any installed kernel, standard or Real Time by selecting the required kernel manually in the GRUB menu during booting. You can also configure the kernel to boot by default. When the RHEL for Real Time kernel is installed, it is automatically set to be the default kernel and is used on the boot. 2.1. Displaying the default kernel You can display the kernel configured to boot by default. Procedure To view the default kernel: The rt in the output of the command shows that the default kernel is a real time kernel. 2.2. Displaying the running kernel You can display the currently running kernel Procedure To show which kernel the system is currently running. Note When the system receives a minor update, for example, from 8.3 to 8.4, the default kernel might automatically change from the Real Time kernel back to the standard kernel. 2.3. Configuring kernel-rt as the default boot kernel On a newly installed system, the stock RHEL kernel is set as the default boot kernel and is used as the default kernel on the boot and subsequent system updates. You can change this configuration and set kernel-rt as the default kernel to boot with and also make this configuration persistent across the system updates. Configuring kernel-rt is a one-time procedure, which you can change or revert to another kernel if necessary. You can also configure other existing kernels variants, such as, kernel , kernel-debug , or kernel-rt-debug , as the default boot kernel. Procedure To configure kernel-rt as the default boot kernel, enter the following command: RT_VMLINUZ is the name of the vmlinux file that is associated with the kernel-rt kernel. For example: To configure kernel-rt as default boot kernel on system updates, enter the following command: The UPDATEDEFAULT variable when specified as yes , sets the default kernel to change with system updates. In the example output, the path for the default kernel is specific to the kernel-rt-core package installed. You can determine the path to the kernel from a package by using the rpm -q kernel-rt-core command. Optional: If you need to determine the path to the kernel from a package, first list the installed packages: To use the latest installed package as the default, enter the following command to find the path to the boot image from that package: To configure kernel-rt as the default boot kernel, enter the following command: Verification To verify kernel-rt is the default kernel, enter the following command:

Commands:
grubby --default-kernel /boot/vmlinuz-kernel-rt-5.14.0-70.13.1.rt21.83.el9_0
~]# uname -a Linux rt-server.example.com 4.18.0-80.rt9.138.el8.x86_64 ...
grubby --set-default= <RT_VMLINUZ>
grubby --set-default=/boot/vmlinuz-5.14.0-284.11.1.rt14.296.el9_2.x86_64+rt
sed -i 's/UPDATEDEFAULT=.*/UPDATEDEFAULT=yes/g'/etc/sysconfig/kernel sed -i 's/DEFAULTKERNEL=.*/DEFAULTKERNEL=kernel-rt-core/g'/etc/sysconfig/kernel
rpm -q kernel-rt-core kernel-rt-core-5.14.0-284.11.1.rt14.296.el9_2.x86_64 kernel-rt-core-5.14.0-284.10.1.rt14.295.el9_2.x86_64 kernel-rt-core-5.14.0-284.9.1.rt14.294.el9_2.x86_64
rpm -ql kernel-rt-core-5.14.0-284.11.1.rt14.296.el9_2.x86_64|grep'^/boot/vmlinu' /boot/vmlinuz-5.14.0-284.11.1.rt14.296.el9_2.x86_64.x86_64+rt
grubby --set-default=/boot/vmlinuz-5.14.0-284.11.1.rt14.296.el9_2.x86_64.x86_64+rt
grubby --default-kernel /boot/vmlinuz-5.14.0-284.11.1.rt14.296.el9_2.x86_64.x86_64+rt