sched: Provide Kconfig support for default dynamic preempt mode

Currently the boot defined preempt behaviour (aka dynamic preempt)
selects full preemption by default when the "preempt=" boot parameter
is omitted. However distros may rather want to default to either
no preemption or voluntary preemption.

To provide with this flexibility, make dynamic preemption a visible
Kconfig option and adapt the preemption behaviour selected by the user
to either static or dynamic preemption.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210914103134.11309-1-frederic@kernel.org

kernel/Kconfig.preempt

@@ -2,10 +2,11 @@
 
 choice
 	prompt "Preemption Model"
-	default PREEMPT_NONE
+	default PREEMPT_NONE_BEHAVIOUR
 
-config PREEMPT_NONE
+config PREEMPT_NONE_BEHAVIOUR
 	bool "No Forced Preemption (Server)"
+	select PREEMPT_NONE if !PREEMPT_DYNAMIC
 	help
 	  This is the traditional Linux preemption model, geared towards
 	  throughput. It will still provide good latencies most of the
@@ -17,9 +18,10 @@ config PREEMPT_NONE
 	  raw processing power of the kernel, irrespective of scheduling
 	  latencies.
 
-config PREEMPT_VOLUNTARY
+config PREEMPT_VOLUNTARY_BEHAVIOUR
 	bool "Voluntary Kernel Preemption (Desktop)"
 	depends on !ARCH_NO_PREEMPT
+	select PREEMPT_VOLUNTARY if !PREEMPT_DYNAMIC
 	help
 	  This option reduces the latency of the kernel by adding more
 	  "explicit preemption points" to the kernel code. These new
@@ -35,12 +37,10 @@ config PREEMPT_VOLUNTARY
 
 	  Select this if you are building a kernel for a desktop system.
 
-config PREEMPT
+config PREEMPT_BEHAVIOUR
 	bool "Preemptible Kernel (Low-Latency Desktop)"
 	depends on !ARCH_NO_PREEMPT
-	select PREEMPTION
-	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
-	select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
+	select PREEMPT
 	help
 	  This option reduces the latency of the kernel by making
 	  all kernel code (that is not executing in a critical section)
@@ -58,7 +58,7 @@ config PREEMPT
 
 config PREEMPT_RT
 	bool "Fully Preemptible Kernel (Real-Time)"
-	depends on EXPERT && ARCH_SUPPORTS_RT
+	depends on EXPERT && ARCH_SUPPORTS_RT && !PREEMPT_DYNAMIC
 	select PREEMPTION
 	help
 	  This option turns the kernel into a real-time kernel by replacing
@@ -75,6 +75,17 @@ config PREEMPT_RT
 
 endchoice
 
+config PREEMPT_NONE
+	bool
+
+config PREEMPT_VOLUNTARY
+	bool
+
+config PREEMPT
+	bool
+	select PREEMPTION
+	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
+
 config PREEMPT_COUNT
        bool
 
@@ -83,7 +94,10 @@ config PREEMPTION
        select PREEMPT_COUNT
 
 config PREEMPT_DYNAMIC
-	bool
+	bool "Preemption behaviour defined on boot"
+	depends on HAVE_PREEMPT_DYNAMIC
+	select PREEMPT
+	default y
 	help
 	  This option allows to define the preemption model on the kernel
 	  command line parameter and thus override the default preemption

kernel/sched/core.c

@@ -6520,12 +6520,13 @@ EXPORT_STATIC_CALL_TRAMP(preempt_schedule_notrace);
  */
 
 enum {
-	preempt_dynamic_none = 0,
+	preempt_dynamic_undefined = -1,
+	preempt_dynamic_none,
 	preempt_dynamic_voluntary,
 	preempt_dynamic_full,
 };
 
-int preempt_dynamic_mode = preempt_dynamic_full;
+int preempt_dynamic_mode = preempt_dynamic_undefined;
 
 int sched_dynamic_mode(const char *str)
 {
@@ -6598,7 +6599,27 @@ static int __init setup_preempt_mode(char *str)
 }
 __setup("preempt=", setup_preempt_mode);
 
-#endif /* CONFIG_PREEMPT_DYNAMIC */
+static void __init preempt_dynamic_init(void)
+{
+	if (preempt_dynamic_mode == preempt_dynamic_undefined) {
+		if (IS_ENABLED(CONFIG_PREEMPT_NONE_BEHAVIOUR)) {
+			sched_dynamic_update(preempt_dynamic_none);
+		} else if (IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY_BEHAVIOUR)) {
+			sched_dynamic_update(preempt_dynamic_voluntary);
+		} else {
+			/* Default static call setting, nothing to do */
+			WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_BEHAVIOUR));
+			preempt_dynamic_mode = preempt_dynamic_full;
+			pr_info("Dynamic Preempt: full\n");
+		}
+	}
+}
+
+#else /* !CONFIG_PREEMPT_DYNAMIC */
+
+static inline void preempt_dynamic_init(void) { }
+
+#endif /* #ifdef CONFIG_PREEMPT_DYNAMIC */
 
 /*
  * This is the entry point to schedule() from kernel preemption
@@ -9398,6 +9419,8 @@ void __init sched_init(void)
 
 	init_uclamp();
 
+	preempt_dynamic_init();
+
 	scheduler_running = 1;
 }