Gosh I was using the Real Time facilities in Solaris 2.5.1 more than 10 years ago - they were built into the Solaris kernel from the beginning. I wouldn't exactly call it Real Time - the despatch latency could be up to a millisecond, though usually far less - but for plant we were controlling that would easily good enough. Using a 140MHz microSPARC processor we could handle data samples coming in at 50Hz, comparable to the industrial controllers of the time.
For serious real time applications, of course, you need something designed for the purpose (is VxWorks still going?), but for the in-between stuff where a millisecond between friends can be forgiven, I found a Solaris kernel worked well back in the day. Whether the necessary gadgets (e.g. priority inheritance) can be successfully retrofitted into a Linux kernel/scheduler I have no idea.