[PATCH v2 2/5] preempt/dynamic: Introduce preempt mode accessors

[Valentin Schneider <valentin.schneider@arm.com>]

CONFIG_PREEMPT{_NONE, _VOLUNTARY} designate either:
o The build-time preemption model when !PREEMPT_DYNAMIC
o The default boot-time preemption model when PREEMPT_DYNAMIC

IOW, using those on PREEMPT_DYNAMIC kernels is meaningless - the actual
model could have been set to something else by the "preempt=foo" cmdline
parameter.

Introduce a set of helpers to determine the actual preemption mode used by
the live kernel.

Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 include/linux/sched.h | 16 ++++++++++++++++
 kernel/sched/core.c   | 11 +++++++++++
 2 files changed, 27 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5f8db54226af..0640d5622496 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2073,6 +2073,22 @@ static inline void cond_resched_rcu(void)
 #endif
 }
 
+#ifdef CONFIG_PREEMPT_DYNAMIC
+
+extern bool is_preempt_none(void);
+extern bool is_preempt_voluntary(void);
+extern bool is_preempt_full(void);
+
+#else
+
+#define is_preempt_none() IS_ENABLED(CONFIG_PREEMPT_NONE)
+#define is_preempt_voluntary() IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)
+#define is_preempt_full() IS_ENABLED(CONFIG_PREEMPT)
+
+#endif
+
+#define is_preempt_rt() IS_ENABLED(CONFIG_PREEMPT_RT)
+
 /*
  * Does a critical section need to be broken due to another
  * task waiting?: (technically does not depend on CONFIG_PREEMPTION,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 97047aa7b6c2..9db7f77e53c3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6638,6 +6638,17 @@ static void __init preempt_dynamic_init(void)
 	}
 }
 
+#define PREEMPT_MODE_ACCESSOR(mode) \
+	bool is_preempt_##mode(void)						 \
+	{									 \
+		WARN_ON_ONCE(preempt_dynamic_mode == preempt_dynamic_undefined); \
+		return preempt_dynamic_mode == preempt_dynamic_##mode;		 \
+	}
+
+PREEMPT_MODE_ACCESSOR(none)
+PREEMPT_MODE_ACCESSOR(voluntary)
+PREEMPT_MODE_ACCESSOR(full)
+
 #else /* !CONFIG_PREEMPT_DYNAMIC */
 
 static inline void preempt_dynamic_init(void) { }
-- 
2.25.1