Back in July, Linus Torvalds merged a patch
in the 5.3 merge window
that added the PREEMPT_RT option to the kernel build-time configuration.
That was meant as a
signal that the realtime patch set was moving from its longtime status as
out-of-tree code to a fully supported kernel feature. As the code behind
the configuration option makes its way into the mainline, some friction can
be expected; we are seeing a bit of that now with respect to the BPF subsystem.

Source: LWN.net – [$] BPF and the realtime patch set