*  If an unprivileged thread has a nonzero RLIMIT_RTPRIO soft limit, then it can change its scheduling policy and priority, subject to the restriction that the priority cannot be set to a value higher than the maximum of its current prior‐
      ity and its RLIMIT_RTPRIO soft limit.

   *  If the RLIMIT_RTPRIO soft limit is 0, then the only permitted changes are to lower the priority, or to switch to a non-real-time policy.

   *  Subject to the same rules, another unprivileged thread can also make these changes, as long as the effective user ID of the thread making the change matches the real or effective user ID of the target thread.

   *  Special  rules  apply for the SCHED_IDLE.  In Linux kernels before 2.6.39, an unprivileged thread operating under this policy cannot change its policy, regardless of the value of its RLIMIT_RTPRIO resource limit.  In Linux kernels since
      2.6.39, an unprivileged thread can switch to either the SCHED_BATCH or the SCHED_NORMAL policy so long as its nice value falls within the range permitted by its RLIMIT_NICE resource limit (see getrlimit(2)).*