When applying the preempt_rt patch to the Linux kernel, EFI variables at runtime are disabled by default, seemingly due to high latencies (https://www.spinics.net/lists/linux-rt-users/msg19980.html). If you still need EFI variables accessible for userspace applications like efibootmgr, add “efi=runtime” to the kernel parameters/arguments. This will re-enable the possibility to mount efivarfs with “mount -t efivarfs none /sys/firmware/efi/efivars”. This will of course not magically make the above mentioned latencies disappear, so use this at your own discretion.