pri_to_cpu[CPUPRI_IDLE=0] isn't used since cpupri_set(..., newpri) is
never called with newpri = MAX_PRIO (140).
Current mapping:
p->rt_priority p->prio newpri cpupri
-1 -1 (CPUPRI_INVALID)
140 0 (CPUPRI_IDLE)
100 1 (CPUPRI_NORMAL)
1 98 98 3
...
49 50 50 51
50 49 49 52
...
99 0 0 101
Even when cpupri was introduced with commit 6e0534f278 ("sched: use a
2-d bitmap for searching lowest-pri CPU") in v2.6.27, only
(1) CPUPRI_INVALID (-1),
(2) MAX_RT_PRIO (100),
(3) an RT prio (RT1..RT99)
were used as newprio in cpupri_set(..., newpri) -> convert_prio(newpri).
MAX_RT_PRIO is used only in dec_rt_tasks() -> dec_rt_prio() ->
dec_rt_prio_smp() -> cpupri_set() in case of !rt_rq->rt_nr_running.
I.e. it stands for a non-rt task, including the IDLE task.
Commit 57785df5ac ("sched: Fix task priority bug") removed code in
v2.6.33 which did set the priority of the IDLE task to MAX_PRIO.
Although this happened after the introduction of cpupri, it didn't have
an effect on the values used for cpupri_set(..., newpri).
Remove CPUPRI_IDLE and adapt the cpupri implementation accordingly.
This will save a useless for loop with an atomic_read in
cpupri_find_fitness() calling __cpupri_find().
New mapping:
p->rt_priority p->prio newpri cpupri
-1 -1 (CPUPRI_INVALID)
100 0 (CPUPRI_NORMAL)
1 98 98 2
...
49 50 50 50
50 49 49 51
...
99 0 0 100
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200922083934.19275-2-dietmar.eggemann@arm.com
5e054bca44