I am using mainline (torvalds/linux) kernel, forcing ARCH_SUPPORTS_RT:
diff --git a/arch/Kconfig b/arch/Kconfig
index 5ca66aad0d08..879c34398cb7 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1195,7 +1195,7 @@ config ARCH_NO_PREEMPT
bool
config ARCH_SUPPORTS_RT
- bool
+ def_bool y
Since I was not aware of a PREEMPT_RT-patched tree, I did this so I could
compile a PREEMPT_RT kernel.