sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT

In order to enable PREEMPT_DYNAMIC for PREEMPT_RT, remove PREEMPT_RT
from the 'Preemption Model' choice. Strictly speaking PREEMPT_RT is
not a change in how preemption works, but rather it makes a ton more
code preemptible.

Notably, take away NONE and VOLUNTARY options for PREEMPT_RT, they make
no sense (but are techincally possible).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20241007075055.441622332@infradead.org

kernel/Kconfig.preempt

@@ -20,6 +20,7 @@ choice
 
 config PREEMPT_NONE
 	bool "No Forced Preemption (Server)"
+	depends on !PREEMPT_RT
 	select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This is the traditional Linux preemption model, geared towards
@@ -35,6 +36,7 @@ config PREEMPT_NONE
 config PREEMPT_VOLUNTARY
 	bool "Voluntary Kernel Preemption (Desktop)"
 	depends on !ARCH_NO_PREEMPT
+	depends on !PREEMPT_RT
 	select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This option reduces the latency of the kernel by adding more
@@ -54,7 +56,7 @@ config PREEMPT_VOLUNTARY
 config PREEMPT
 	bool "Preemptible Kernel (Low-Latency Desktop)"
 	depends on !ARCH_NO_PREEMPT
-	select PREEMPT_BUILD
+	select PREEMPT_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This option reduces the latency of the kernel by making
 	  all kernel code (that is not executing in a critical section)
@@ -74,14 +76,16 @@ config PREEMPT_LAZY
 	bool "Scheduler controlled preemption model"
 	depends on !ARCH_NO_PREEMPT
 	depends on ARCH_HAS_PREEMPT_LAZY
-	select PREEMPT_BUILD
+	select PREEMPT_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This option provides a scheduler driven preemption model that
 	  is fundamentally similar to full preemption, but is less
 	  eager to preempt SCHED_NORMAL tasks in an attempt to
 	  reduce lock holder preemption and recover some of the performance
 	  gains seen from using Voluntary preemption.
 
+endchoice
+
 config PREEMPT_RT
 	bool "Fully Preemptible Kernel (Real-Time)"
 	depends on EXPERT && ARCH_SUPPORTS_RT
@@ -99,8 +103,6 @@ config PREEMPT_RT
 	  Select this if you are building a kernel for systems which
 	  require real-time guarantees.
 
-endchoice
-
 config PREEMPT_COUNT
        bool
 
@@ -110,7 +112,7 @@ config PREEMPTION
 
 config PREEMPT_DYNAMIC
 	bool "Preemption behaviour defined on boot"
-	depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT
+	depends on HAVE_PREEMPT_DYNAMIC
 	select JUMP_LABEL if HAVE_PREEMPT_DYNAMIC_KEY
 	select PREEMPT_BUILD
 	default y if HAVE_PREEMPT_DYNAMIC_CALL

kernel/sched/core.c

@@ -7450,11 +7450,13 @@ int preempt_dynamic_mode = preempt_dynamic_undefined;
 
 int sched_dynamic_mode(const char *str)
 {
+#ifndef CONFIG_PREEMPT_RT
 	if (!strcmp(str, "none"))
 		return preempt_dynamic_none;
 
 	if (!strcmp(str, "voluntary"))
 		return preempt_dynamic_voluntary;
+#endif
 
 	if (!strcmp(str, "full"))
 		return preempt_dynamic_full;

kernel/sched/debug.c

@@ -248,9 +248,9 @@ static int sched_dynamic_show(struct seq_file *m, void *v)
 		"none", "voluntary", "full", "lazy",
 	};
 	int j = ARRAY_SIZE(preempt_modes) - !IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY);
-	int i;
+	int i = IS_ENABLED(CONFIG_PREEMPT_RT) * 2;
 
-	for (i = 0; i < j; i++) {
+	for (; i < j; i++) {
 		if (preempt_dynamic_mode == i)
 			seq_puts(m, "(");
 		seq_puts(m, preempt_modes[i]);