Installing RHEL for Real Time

Procedure

1. Enable the RHEL for Real Time repository.

   # subscription-manager repos --enable rhel-10-for-x86_64-rt-rpms

   Note

   If you are using an Extended Update Support (EUS) subscription, enable the RHEL for Real Time EUS repository instead:

   # subscription-manager repos --enable rhel-10-for-x86_64-rt-eus-rpms

   For more information about EUS, see Red Hat Enterprise Linux Life Cycle.

2. Install the RHEL for Real Time package group.

   # dnf groupinstall RT

   This group installs several packages:

   • kernel-rt includes meta-package that carries the list of package dependencies to have a functional kernel-rt installed..
   • kernel-rt-core includes the core RHEL for Real Time kernel package.
   • kernel-rt-devel includes the proper kernel binary, compiled with PREEMPT_RT enabled, the package also contains configuration and scripts to drive the kernel installation.
   • kernel-rt-modules-core includes the essential kernel modules required for a functional kernel. kernel-rt-modules provides commonly used kernel modules, a complement to kernel-modules-core.
   • kernel-rt-modules-extra includes the RHEL for Real Time kernel extra modules package.
   • realtime-setup sets up the basic environment required by RHEL for Real Time.
   • rteval evaluates system suitability for RHEL for Real Time.
   • rteval-loads provides source code for rteval loads.
   • tuned-profiles-realtime includes the additional TuneD profiles targeted to real-time.

3. Optional: Additionally, the tuna package contains a tool that helps tune the real-time kernel workload, greatly automating CPU isolation and thread affinity operations from the command line or the GUI. This package is available in the base RHEL 10 repository.

   # dnf install tuna

   Note

   When the RHEL for Real Time kernel is installed, it is automatically set to be the default kernel and is used on the next boot. You can also configure other existing kernels variants, such as, kernel, kernel-debug, or kernel-rt-debug as the default boot kernel. For more information, see Configuring kernel-rt as the default boot kernel.

Procedure

1. To configure kernel-rt as the default boot kernel, enter the following command:

   # grubby --set-default=<RT_VMLINUZ>

   RT_VMLINUZ is the name of the vmlinux file that is associated with the kernel-rt kernel. For example:

   # grubby --set-default=/boot/vmlinuz-6.12.0-284.11.1.rt14.296.el0_0_2.x86_64+rt

2. To configure kernel-rt as default boot kernel on system updates, enter the following command:

   # sed -i 's/UPDATEDEFAULT=.*/UPDATEDEFAULT=yes/g' /etc/sysconfig/kernel
   # sed -i 's/DEFAULTKERNEL=.*/DEFAULTKERNEL=kernel-rt-core/g' /etc/sysconfig/kernel

   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.

   1. Optional: If you need to determine the path to the kernel from a package, first list the installed packages:

      # rpm -q kernel-rt-core
        kernel-rt-core-6.12.0-55.14.1.el10_0.x86_64
        kernel-rt-core-6.12.0-55.13.1.el10_0.x86_64
        kernel-rt-core-6.12.0-55.12.1.el10_0.x86_64

   2. To use the latest installed package as the default, enter the following command to find the path to the boot image from the package:

      # rpm -ql kernel-rt-core-6.12.0-55.14.1.el10_0.x86_64 | grep '^/boot/vmlinu'
      /boot/vmlinuz-6.12.0-55.14.1.el10_0.x86_64+rt

   3. To configure kernel-rt as the default boot kernel, enter the following command:

      # grubby --set-default=/boot/vmlinuz-6.12.0-55.14.1.el10_0.x86_64+rt

Procedure

1. On the Anaconda installer, click KDUMP and enable kdump.
2. In Kdump Memory Reservation, select Manual if you must customize the memory reserve.
3. In KDUMP > Memory To Be Reserved (MB), set the required memory reserve for kdump.

Procedure

1. Check if kdump is installed on your system:

   # rpm -q kexec-tools

   Output if the package is installed:

   kexec-tools-2.0.29-21.el10.x86_64

   Output if the package is not installed:

   package kexec-tools is not installed

2. Install kdump and other necessary packages:

   # dnf install kexec-tools

Procedure

1. Configure the default value for crash kernel:

   # kdumpctl reset-crashkernel --kernel=ALL

   When configuring the crashkernel= value, test the configuration by rebooting the system with kdump enabled. If the kdump kernel fails to boot, increase the memory size gradually to set an acceptable value.

2. To use a custom crashkernel= value:

   1. Configure the required memory reserve.

      crashkernel=192M

      Optionally, you can set the amount of reserved memory to a variable depending on the total amount of installed memory by using the syntax crashkernel=<range1>:<size1>,<range2>:<size2>. For example:

      crashkernel=1G-4G:192M,2G-64G:256M

      The example reserves 192 MB of memory if the total amount of system memory is 1 GB or higher and lower than 4 GB. If the total amount of memory is more than 4 GB, 256 MB is reserved for kdump.

   2. Optional: Offset the reserved memory.

      Some systems require to reserve memory with a certain fixed offset since crashkernel reservation is very early, and it wants to reserve some area for special usage. If the offset is set, the reserved memory begins there. To offset the reserved memory, use the following syntax:

      crashkernel=192M@16M

      The example reserves 192 MB of memory starting at 16 MB (physical address 0x01000000). If you offset to 0 or do not specify a value, kdump offsets the reserved memory automatically. You can also offset memory when setting a variable memory reservation by specifying the offset as the last value. For example, crashkernel=1G-4G:192M,2G-64G:256M@16M.

   3. Update the boot loader configuration:

      # grubby --update-kernel ALL --args "crashkernel=<custom-value>"

      The <custom-value> must contain the custom crashkernel= value that you have configured for the crash kernel.

3. Reboot for changes to take effect:

   # reboot

1. Activate the sysrq key to boot into the kdump kernel:

   # echo c > /proc/sysrq-trigger

   The command causes kernel to crash and reboots the kernel if required.

2. Display the /etc/kdump.conf file and check if the vmcore file is saved in the target destination.

   See the grubby(8) man page on your system for more information.

Procedure

1. As a root, edit the /etc/kdump.conf configuration file and remove the hash sign ("#") from the beginning of the #core_collector makedumpfile -l --message-level 1 -d 31.

2. Enter the following command to enable crash dump file compression:

   core_collector makedumpfile -l --message-level 1 -d 31

   The -l option sets the compressed file format to LZO. The -d option sets the dump level to 31. The --message-level option sets the message level to 1. You can also use the -c, -p, or -z options to specify other compression formats.

   See makedumpfile(8) man page on your system for more information.

Procedure

1. As a root user, remove the hash sign (#) from the beginning of the #failure_action line in the /etc/kdump.conf configuration file.

2. Replace the value with a required action.

   failure_action poweroff

Procedure

1. Enable the kdump service:

   # kdumpctl restart

2. Check the status of the kdump service with the kdumpctl:

   # kdumpctl status

   kdump:Kdump is operational

   Optionally, if you use the systemctl command, the output prints in the systemd journal.

3. Start a kernel crash to test the kdump configuration. The sysrq-trigger key combination causes the kernel to crash and might reboot the system if required.

   # echo c > /proc/sysrq-trigger

   On a kernel reboot, the address-YYYY-MM-DD-HH:MM:SS/vmcore file is created at the location you have specified in the /etc/kdump.conf file. The default is /var/crash/.

Procedure

1. Enable the kdump service for multi-user.target:

   # systemctl enable kdump.service

2. Start the service in the current session:

   # systemctl start kdump.service

3. Stop the kdump service:

   # systemctl stop kdump.service

4. Disable the kdump service:

   # systemctl disable kdump.service

   Warning

   It is recommended to set kptr_restrict=1 as default. When kptr_restrict is set to (1) as default, the kdumpctl service loads the crash kernel regardless of whether the Kernel Address Space Layout (KASLR) is enabled.

   If kptr_restrict is not set to 1 and KASLR is enabled, the contents of /proc/kore file are generated as all zeros. The kdumpctl service fails to access the /proc/kcore file and load the crash kernel. The kexec-kdump-howto.txt file displays a warning message, which recommends you to set kptr_restrict=1. Verify for the following in the sysctl.conf file to ensure that kdumpctl service loads the crash kernel:

   • The kernel kptr_restrict=1 setting in the sysctl.conf file.

Procedure

1. Display the list of modules that are loaded to the currently running kernel. Select the kernel module that you intend to block from loading:

   $ lsmod

   Module                  Size  Used by
   fuse                  126976  3
   xt_CHECKSUM            16384  1
   ipt_MASQUERADE         16384  1
   uinput                 20480  1
   xt_conntrack           16384  1

2. Update the KDUMP_COMMANDLINE_APPEND= variable in the /etc/sysconfig/kdump file. For example:

   KDUMP_COMMANDLINE_APPEND="rd.driver.blacklist=hv_vmbus,hv_storvsc,hv_utils,hv_netvsc,hid-hyperv"

   Also, consider the following example by using the modprobe.blacklist=<modules> configuration option:

   KDUMP_COMMANDLINE_APPEND="modprobe.blacklist=emcp modprobe.blacklist=bnx2fc modprobe.blacklist=libfcoe modprobe.blacklist=fcoe"

3. Restart the kdump service:

   # systemctl restart kdump

   See the dracut.cmdline man page on your system for more information.

Procedure

1. Print the estimate crashkernel= value:

   # *kdumpctl estimate*
   
   Encrypted kdump target requires extra memory, assuming using the keyslot with minimum memory requirement
      Reserved crashkernel:    256M
      Recommended crashkernel: 652M
   
      Kernel image size:   47M
      Kernel modules size: 8M
      Initramfs size:      20M
      Runtime reservation: 64M
      LUKS required size:  512M
      Large modules: <none>
      WARNING: Current crashkernel size is lower than recommended size 652M.

2. Configure the amount of required memory by increasing the crashkernel= value.

3. Reboot the system.

   Note

   If the kdump service still fails to save the dump file to the encrypted target, increase the crashkernel= value as required.

Procedure

1. Add the crashkernel= command-line parameter to all installed kernels:

   # grubby --update-kernel=ALL --args="crashkernel=xxM"

   xxM is the required memory in megabytes.

2. In terms of UKI, add the crashkernel= command-line parameter to all installed UKIs by copying an add-on associated with the required argument from /lib/modules/$(uname -r)/vmlinuz-virt.efi.extra.d/ to /boot/efi/loader/addons/.

3. Reboot the system:

   # reboot

4. Enable the kdump service:

   # systemctl enable --now kdump.service

Procedure

1. List the kernels installed on the machine:

   # ls -a /boot/vmlinuz-*

   /boot/vmlinuz-0-rescue-2930657cd0dc43c2b75db480e5e5b4a9
   /boot/vmlinuz-6.12.0-55.9.1.el10_0.x86_64
   /boot/vmlinuz-6.12.0-55.9.1.el10_0.x86_64

2. Add a specific kdump kernel to the system’s Grand Unified Boot Loader (GRUB) configuration:

   For example:

   # grubby --update-kernel=vmlinuz-6.12.0-55.9.1.el10_0.x86_64 --args="crashkernel=xxM"

   xxM is the required memory reserve in megabytes.

3. In terms of UKI, add the crashkernel= command-line parameter to a specific UKI by copying an add-on associated with the required argument from /lib/modules/$(uname -r)/vmlinuz-virt.efi.extra.d/ to /boot/efi/EFI/Linux/<machine-id>-<kernel-version>.efi.extra.d/.

4. Enable the kdump service:

   # systemctl enable --now kdump.service

Procedure

1. To stop the kdump service in the current session:

   # systemctl stop kdump.service

2. To disable the kdump service:

   # systemctl disable kdump.service

   Warning

   It is recommended to set kptr_restrict=1 as default. When kptr_restrict is set to (1) as default, the kdumpctl service loads the crash kernel regardless of whether the Kernel Address Space Layout (KASLR) is enabled.

   If kptr_restrict is not set to 1 and KASLR is enabled, the contents of /proc/kore file are generated as all zeros. The kdumpctl service fails to access the /proc/kcore file and load the crash kernel. The kexec-kdump-howto.txt file displays a warning message, which recommends you to set kptr_restrict=1. Verify for the following in the sysctl.conf file to ensure that kdumpctl service loads the crash kernel:

   • The kernel kptr_restrict=1 setting in the sysctl.conf file.