ANDROID: sched: Fix off-by-one with cpupri MAX_RT_PRIO evaluation

This patch addresses an issue seen where SCHED_FIFO prio 99
tasks were being woken up on a cpu where a long-running softirq
was executing, and the RT task was not being migrated, causing
long (10+ms wakeup latencies).

Prior to upstream commit 934fc3314b ("sched/cpupri: Remap
CPUPRI_NORMAL to MAX_RT_PRIO-1") the task->prio -> cpupri
mapping is a little ackward.

For RT tasks, its calculated as:
  cpupri = MAX_RT_PRIO - prio + 1;

See:
https://android.googlesource.com/kernel/common/+/refs/heads/android13-5.10/kernel/sched/cpupri.c#39

This is added ontop of the also ackward detail that the
task->prio is inverted (RT prio99 -> 0), means the cpupri
mapping for RT tasks goes from 2->101. This makes it easy to
evaluate the cpupri incorrectly.

Which it turns out happened In commit 3adfd8e344 ("ANDROID:
sched: avoid placing RT threads on cores handling softirqs"):
  3adfd8e344%5E%21/

With the lines:
  int task_pri = convert_prio(p->prio);
  bool drop_nopreempts = task_pri <= MAX_RT_PRIO;

Where the added logic to decide to migrate a rt task off of a
cpu depended on this drop_nopreempts being true.

This works properly for rt tasks from prio 99 to 1, but for the
case of task->prio == 0 (userland rt prio 99 tasks) it breaks,
as the cpupri will be MAX_RT_PRIO - 0 + 1, which then gets
checked as <= MAX_RT_PRIO.

This prevents the cpu from being dropped from the scheduling
set and prevents the rt user prio 99 task from migrating, which
results in high priority rt tasks being left on cpus where long
running softirqs are executing, causing long latencies.

This patch fixes the off by one by changing the evaulation
to MAX_RT_PRIO + 1, so that user-prio 99 tasks will also be
migrated if appropriate.

Luckilly this odd cpupri mapping has been fixed upstream, making
this patch no longer necessary in 5.15 and newer kernels.

Fixes: 3adfd8e344 ("ANDROID: sched: avoid placing RT threads on cores handling softirqs")
Signed-off-by: John Stultz <jstultz@google.com>
Change-Id: Ia2db7cd461eb4c90f5850b791de1ae95582f7530

kernel/sched/cpupri.c

@@ -150,7 +150,7 @@ int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p,
 {
 	int task_pri = convert_prio(p->prio);
 	int idx, cpu;
-	bool drop_nopreempts = task_pri <= MAX_RT_PRIO;
+	bool drop_nopreempts = task_pri <= MAX_RT_PRIO + 1;
 
 	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);