After applying a kernel update on a Red Hat Enterprise Linux (RHEL) system, the server is still booting with an older kernel despite the new kernel being installed. This document explains how to diagnose and resolve this issue.

---

Diagnosis Steps

1. Checking Installed Kernels

To list all installed kernels:

rpm -qa | grep kernel

Observation:

The output indicates that multiple kernels are installed, including:

• kernel-4.18.0-553.34.1.el8_10.x86_64 (latest installed kernel)
• kernel-debug-4.18.0-553.32.1.el8_10.x86_64
• kernel-core-4.18.0-553.32.1.el8_10.x86_64

2. Checking the Boot Configuration

To verify the bootloader configuration:

ls -l /boot/

Observation:

• The /boot/ directory contains multiple kernel and initramfs images.

3. Checking GRUB Boot Entries

To verify the bootloader’s saved kernel entry:

cat /boot/efi/EFI/redhat/grubenv

Output Example:

###GRUB Environment Block#####
saved_entry=569c62490b9d4240afb066650e2120eb-4.18.0-553.34.1.el8_10.x86_64
kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet numa=off transparent_hugepage=never
boot_success=1
boot_indeterminate=0

Key Observations:

• saved_entry points to 4.18.0-553.34.1.el8_10.x86_64, which is the expected new kernel.
• boot_success=1 indicates that the last boot attempt was successful.

4. Checking GRUB Boot Entries

To list all available boot entries:

grubby --info=ALL

Output Example:

index=0
kernel="/boot/vmlinuz-4.18.0-553.34.1.el8_10.x86_64"
...
index=1
kernel="/boot/vmlinuz-4.18.0-553.32.1.el8_10.x86_64+debug"
...

Key Observations:

• The default boot kernel is set to index=0, which corresponds to the latest kernel.

5. Checking the Default Kernel

To confirm the default kernel:

grubby --default-kernel

Output:

/boot/vmlinuz-4.18.0-553.34.1.el8_10.x86_64

Key Observations:

• The system recognizes the correct default kernel.

---

Resolution Steps

1. Reinstall the Latest Kernel

To ensure the latest kernel is properly installed:

yum reinstall kernel-4.18.0-553.34.1.el8_10.x86_64 kernel-core-4.18.0-553.34.1.el8_10.x86_64

2. Set the Default Kernel Manually

If the system still boots with the older kernel, manually set the correct kernel:

grubby --set-default /boot/vmlinuz-4.18.0-553.34.1.el8_10.x86_64

3. Verify Default Kernel Index

To check which kernel index is set as default:

grubby --default-index

Expected Output:

0

4. Update GRUB Configuration

If needed, regenerate the GRUB configuration:

grub2-mkconfig -o /boot/grub2/grub.cfg

5. Reboot the System

reboot

6. Verify the Active Kernel After Boot

After rebooting, confirm the system is using the correct kernel:

uname -r

Expected Output:

4.18.0-553.34.1.el8_10.x86_64

---

Conclusion

By following the above steps, the system should boot with the latest installed kernel. If the issue persists, verify UEFI settings, check for BIOS/firmware updates, and inspect system logs for boot-related errors.

For further troubleshooting, review logs using:

dmesg | less
journalctl -xb