How to ensure that a thread is prioritized for scheduling without interrupting the currently running thread on the Linux system

Question

I want to develop a deterministic scheduling scheme based on the Linux system, where all threads are not preempted during runtime, and certain threads are prioritized for scheduling. Both SCHED_FIFO and SCHED_RR policies involve preemption.

I tried disabling preemption for a thread after it is scheduled, so that it won't be preempted, but I am not sure how to achieve this in user space