sched/cpupri: Remap CPUPRI_NORMAL to MAX_RT_PRIO-1

This makes the mapping continuous and frees up 100 for other usage.

Prev mapping:

p->rt_priority   p->prio   newpri   cpupri

                               -1       -1 (CPUPRI_INVALID)

                              100        0 (CPUPRI_NORMAL)

             1        98       98        1
           ...
            49        50       50       49
            50        49       49       50
           ...
            99         0        0       99

New mapping:

p->rt_priority   p->prio   newpri   cpupri

                               -1       -1 (CPUPRI_INVALID)

                               99        0 (CPUPRI_NORMAL)

             1        98       98        1
           ...
            49        50       50       49
            50        49       49       50
           ...
            99         0        0       99
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

kernel/sched/cpupri.c

@@ -24,17 +24,37 @@
  */
 #include "sched.h"
 
-/* Convert between a 140 based task->prio, and our 100 based cpupri */
+/*
+ * p->rt_priority   p->prio   newpri   cpupri
+ *
+ *				  -1       -1 (CPUPRI_INVALID)
+ *
+ *				  99        0 (CPUPRI_NORMAL)
+ *
+ *		1        98       98        1
+ *	      ...
+ *	       49        50       50       49
+ *	       50        49       49       50
+ *	      ...
+ *	       99         0        0       99
+ */
 static int convert_prio(int prio)
 {
 	int cpupri;
 
-	if (prio == CPUPRI_INVALID)
-		cpupri = CPUPRI_INVALID;
-	else if (prio >= MAX_RT_PRIO)
-		cpupri = CPUPRI_NORMAL;
-	else
-		cpupri = MAX_RT_PRIO - prio - 1;
+	switch (prio) {
+	case CPUPRI_INVALID:
+		cpupri = CPUPRI_INVALID;	/* -1 */
+		break;
+
+	case 0 ... 98:
+		cpupri = MAX_RT_PRIO-1 - prio;	/* 1 ... 99 */
+		break;
+
+	case MAX_RT_PRIO-1:
+		cpupri = CPUPRI_NORMAL;		/*  0 */
+		break;
+	}
 
 	return cpupri;
 }

kernel/sched/rt.c

@@ -89,8 +89,8 @@ void init_rt_rq(struct rt_rq *rt_rq)
 	__set_bit(MAX_RT_PRIO, array->bitmap);
 
 #if defined CONFIG_SMP
-	rt_rq->highest_prio.curr = MAX_RT_PRIO;
-	rt_rq->highest_prio.next = MAX_RT_PRIO;
+	rt_rq->highest_prio.curr = MAX_RT_PRIO-1;
+	rt_rq->highest_prio.next = MAX_RT_PRIO-1;
 	rt_rq->rt_nr_migratory = 0;
 	rt_rq->overloaded = 0;
 	plist_head_init(&rt_rq->pushable_tasks);
@@ -161,7 +161,7 @@ void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
 {
 	struct rq *rq = cpu_rq(cpu);
 
-	rt_rq->highest_prio.curr = MAX_RT_PRIO;
+	rt_rq->highest_prio.curr = MAX_RT_PRIO-1;
 	rt_rq->rt_nr_boosted = 0;
 	rt_rq->rq = rq;
 	rt_rq->tg = tg;
@@ -393,8 +393,9 @@ static void dequeue_pushable_task(struct rq *rq, struct task_struct *p)
 		p = plist_first_entry(&rq->rt.pushable_tasks,
 				      struct task_struct, pushable_tasks);
 		rq->rt.highest_prio.next = p->prio;
-	} else
-		rq->rt.highest_prio.next = MAX_RT_PRIO;
+	} else {
+		rq->rt.highest_prio.next = MAX_RT_PRIO-1;
+	}
 }
 
 #else
@@ -1147,8 +1148,9 @@ dec_rt_prio(struct rt_rq *rt_rq, int prio)
 				sched_find_first_bit(array->bitmap);
 		}
 
-	} else
-		rt_rq->highest_prio.curr = MAX_RT_PRIO;
+	} else {
+		rt_rq->highest_prio.curr = MAX_RT_PRIO-1;
+	}
 
 	dec_rt_prio_smp(rt_rq, prio, prev_prio);
 }