sched/core: Provide a method to check if a task is PI-boosted.

Provide a method to check if a task inherited the priority from another
task. This happens if a task owns a lock which is requested by a task
with higher priority. This can be used as a hint to add a preemption
point to the critical section.

Provide a function which reports true if the task is PI-boosted.

Link: https://lore.kernel.org/r/20230804113039.419794-2-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

include/linux/sched.h

@@ -1905,6 +1905,7 @@ static inline int dl_task_check_affinity(struct task_struct *p, const struct cpu
 }
 #endif
 
+extern bool task_is_pi_boosted(const struct task_struct *p);
 extern int yield_to(struct task_struct *p, bool preempt);
 extern void set_user_nice(struct task_struct *p, long nice);
 extern int task_prio(const struct task_struct *p);

kernel/sched/core.c

@@ -8922,6 +8922,21 @@ static inline void preempt_dynamic_init(void) { }
 
 #endif /* #ifdef CONFIG_PREEMPT_DYNAMIC */
 
+/*
+ * task_is_pi_boosted - Check if task has been PI boosted.
+ * @p:	Task to check.
+ *
+ * Return true if task is subject to priority inheritance.
+ */
+bool task_is_pi_boosted(const struct task_struct *p)
+{
+	int prio = p->prio;
+
+	if (!rt_prio(prio))
+		return false;
+	return prio != p->normal_prio;
+}
+
 /**
  * yield - yield the current processor to other threads.
  *