The login.conf's "priority" capability allows to set priorities in the idle or
realtime classes in addition to the classical nice values (-20 to 20), through
a natural extension where values greater than 20 put the processes in the idle
class (with priority adjusted within RTP_PRIO_MIN and RTP_PRIO_MAX, 21 being
converted to 0, 22 to 1, etc.) and values lower than -20 put the process in the
realtime class (with priority adjusted within RTP_PRIO_MIN and RTP_PRIO_MAX, -21
being converted to RTP_PRIO_MAX (31), -22 to 30, etc.).
Before this fix, in the latter case (realtime class), -21 was converted to 30,
and RTP_PRIO_MAX (31) could never be specified.
While here, change the priority computation for the idle-class case to be
symmetrical and use RTP_PRIO_MIN (in practice, this changes nothing at all,
since RTP_PRIO_MIN is 0; but this is the correct theoretical formula, which
would work as well with other values of RTP_PRIO_MIN).
PR: 271727