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2.4. Post-Installation Instructions

Choosing Which Kernel to Boot

Any installed kernel, both standard and Real Time, can be booted. One way is to select the desired kernel manually in the GRUB menu during booting. Another way is to set the default kernel to the desired one as shown in this section.

When the Red Hat Enterprise Linux for Real Time kernel is installed, it is automatically set to be the default kernel and is used on the next boot.

To verify that the Real Time kernel is the default kernel, run as root:
```
~]# grubby --default-kernel
/boot/vmlinuz-3.10.0-327.18.2.rt56.223.el7_2.x86_64
```
The rt designation in the above example shows that the default kernel is the Real Time kernel.

To show which kernel the system is currently running, use this command:

~]$ uname -a
Linux rt-server.example.com 3.10.0-327.18.2.rt56.223.el7_2.x86_64 …

However, when the system receives a minor update, for example, from 7.1 to 7.2, the default kernel might automatically change from the Real Time kernel back to the standard kernel.

To set the default kernel to the Real Time kernel again, follow this procedure:

List the installed Real Time kernels:

~]# ls /boot/vmlinuz*rt*
/boot/vmlinuz-3.10.0-327.18.2.rt56.223.el7_2.x86_64

Set the default kernel to the listed Real Time kernel by running the following command as root:
```
~]# grubby --set-default realtime-kernel
```
Substitute realtime-kernel with the Real Time kernel version, for example:
```
~]# grubby --set-default /boot/vmlinuz-3.10.0-327.18.2.rt56.223.el7_2.x86_64
```

Module Compatibility of the Real Time Kernel and the Standard Kernel

Red Hat Enterprise Linux for Real Time differs substantially from the standard Red Hat Enterprise Linux 7 kernel. As a consequence, third-party kernel modules are incompatible with Red Hat Enterprise Linux for Real Time.

Kernel modules are inherently specific to the kernel they are built for. The realtime kernel is substantially different from the standard kernel, and so are the modules. In other words, you cannot take third-party modules from Red Hat Enterprise Linux 7 and use them as-is on Red Hat Enterprise Linux for Real Time. If you must use a third-party module, you must recompile it with the Red Hat Enterprise Linux for Real Time header files, which are available in the -devel subpackage. See Table 2.2, “Red Hat Enterprise Linux for Real Time Development and Test Packages” for more information about all the subpackages.

The following are some example third-party drivers which ship for standard Red Hat Enterprise Linux 7 but do not currently have a custom build for Red Hat Enterprise Linux for Real Time:

EMC Powerpath
NVidia graphics
Advanced storage adapter configuration utilities from Qlogic

The user space syscall interface is compatible with Red Hat Enterprise Linux for Real Time.

Enabling Kdump

You can configure Red Hat Enterprise Linux for Real Time to provide crash dump information by enabling kexec/kdump, if you have not done so for the standard kernel during the installation of Red Hat Enterprise Linux 7. Further information and instructions on how to configure your system to obtain kernel crash information can be found in the Red Hat Enterprise Linux for Real Time Tuning Guide.

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2.4. Post-Installation Instructions

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