The man page for sched_setscheduler(), which tuna uses to set the RT priority, says the
allowed range is specified by sched_get_priority_min() and sched_get_priority_max().
The man page for those 2 functions says the range is 1 to 99 for FIFO and RR.
>> import os
>> os.sched_get_priority_min(2)
1
>> os.sched_get_priority_max(2)
99