[johnr2]

> Presumably not on mainline kernel.

At first it was necessary to compile the realtime-lsm module to allow user programs to make use of realtime scheduling. In late 2006 or so (IIRC) rlimits-aware PAM became available which made realtime-lsm redundant.

I've used CONFIG_PREEMPT=y (along with CONFIG_HZ=1000) when compiling mainline kernels all along. My current distro of choice (Void) enables these by default in their 5.4 series kernels. I've never needed PREEMPT_RT for my particular use case.

AFAIK FreeBSD still doesn't allow users to run programs with realtime scheduling privileges (SCHED_FIFO or SCHED_RR), only root.