Good overview, but missing any discussion (that I could see) of how non-SCHED_OTHER scheduling classes (such as SCHED_FIFO and SCHED_RR) interact with "normal" scheduling. The rules described here (e.g. "the task that has run least runs next") do not apply to tasks in these scheduling classes.
My understanding is that they don't. Realtime tasks have strict priority over non realtime tasks. As long as there are runnable rt task the kernel won't schedule any other task.
At least that's the theory. I think these days the kernel reserves (optionally?) 5% cpu to non rt tasks, enough to run a shell and kill a runaway rt process.
