[PATCH 5/5] sched: dynamic: Remove HAVE_PREEMPT_DYNAMIC_{CALL,KEY}

[Mark Rutland <mark.rutland@arm.com>]

PREEMPT_DYNAMIC is now limited to the FULL and LAZY preemption models.
Switching model only changes the behaviour of dynamic_preempt_lazy(),
which uses a static key. There are no other static calls or static keys,
and so there's no need for HAVE_PREEMPT_DYNAMIC_CALL or
HAVE_PREEMPT_DYNAMIC_KEY.

The static key used by dynamic_preempt_lazy() is entirely local to
kernel/sched/core.c, and any architecture which selects
ARCH_HAS_PREEMPT_LAZY implements the necessary support. Remove
PREEMPT_DYNAMIC's dependencies on HAVE_PREEMPT_DYNAMIC_CALL and
HAVE_PREEMPT_DYNAMIC_KEY entirely, leaving the depenency on
ARCH_HAS_PREEMPT_LAZY.

For architectures which previously selected HAVE_PREEMPT_DYNAMIC_KEY,
PREEMPT_DYNAMIC will now be selected by default, matching x86. As the
runtime impact is limited to dynamic_preempt_lazy(), this shouldn't be
as concerning as previously (e.g. where calls to {cond,might}_resched()
stubs could introduce a measurable penalty).

As all of this can work without jump labels, JUMP_LABEL is not selected
explicitly, though it is obviously preferable to have JUMP_LABEL
enabled.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: John Stultz <jstultz@google.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
---
 arch/Kconfig           | 38 --------------------------------------
 arch/arm64/Kconfig     |  1 -
 arch/loongarch/Kconfig |  1 -
 arch/powerpc/Kconfig   |  1 -
 arch/riscv/Kconfig     |  1 -
 arch/s390/Kconfig      |  1 -
 arch/x86/Kconfig       |  1 -
 kernel/Kconfig.preempt |  8 ++------
 kernel/sched/core.c    | 10 ----------
 9 files changed, 2 insertions(+), 60 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index fa7507ac8e13e..16e46010922a5 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1696,44 +1696,6 @@ config HAVE_STATIC_CALL_INLINE
 	depends on HAVE_STATIC_CALL
 	select OBJTOOL
 
-config HAVE_PREEMPT_DYNAMIC
-	bool
-
-config HAVE_PREEMPT_DYNAMIC_CALL
-	bool
-	depends on HAVE_STATIC_CALL
-	select HAVE_PREEMPT_DYNAMIC
-	help
-	  An architecture should select this if it can handle the preemption
-	  model being selected at boot time using static calls.
-
-	  Where an architecture selects HAVE_STATIC_CALL_INLINE, any call to a
-	  preemption function will be patched directly.
-
-	  Where an architecture does not select HAVE_STATIC_CALL_INLINE, any
-	  call to a preemption function will go through a trampoline, and the
-	  trampoline will be patched.
-
-	  It is strongly advised to support inline static call to avoid any
-	  overhead.
-
-config HAVE_PREEMPT_DYNAMIC_KEY
-	bool
-	depends on HAVE_ARCH_JUMP_LABEL
-	select HAVE_PREEMPT_DYNAMIC
-	help
-	  An architecture should select this if it can handle the preemption
-	  model being selected at boot time using static keys.
-
-	  Each preemption function will be given an early return based on a
-	  static key. This should have slightly lower overhead than non-inline
-	  static calls, as this effectively inlines each trampoline into the
-	  start of its callee. This may avoid redundant work, and may
-	  integrate better with CFI schemes.
-
-	  This will have greater overhead than using inline static calls as
-	  the call to the preemption function cannot be entirely elided.
-
 config ARCH_WANT_LD_ORPHAN_WARN
 	bool
 	help
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b3afe0688919b..1504bc354b9e5 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -216,7 +216,6 @@ config ARM64
 	select HAVE_PERF_EVENTS_NMI if ARM64_PSEUDO_NMI
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
-	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index d8d2523250179..1889d808597da 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -170,7 +170,6 @@ config LOONGARCH
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
-	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE if UNWINDER_ORC
 	select HAVE_RETHOOK
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index f7ce5fff81f03..940f8fbea55c5 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -280,7 +280,6 @@ config PPC
 	select HAVE_PERF_EVENTS_NMI		if PPC64
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
-	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_RETHOOK			if KPROBES
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 3f0a647218e40..dfdf56f8542aa 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -193,7 +193,6 @@ config RISCV
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
-	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RETHOOK
 	select HAVE_RSEQ
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 84404e6778d50..ad527859694ab 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -239,7 +239,6 @@ config S390
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
-	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE
 	select HAVE_RETHOOK
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index bdad90f210e4b..b05ffa8f661b4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -294,7 +294,6 @@ config X86
 	select HAVE_STACK_VALIDATION		if HAVE_OBJTOOL
 	select HAVE_STATIC_CALL
 	select HAVE_STATIC_CALL_INLINE		if HAVE_OBJTOOL
-	select HAVE_PREEMPT_DYNAMIC_CALL
 	select HAVE_RSEQ
 	select HAVE_RUST			if X86_64
 	select HAVE_SYSCALL_TRACEPOINTS
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index fb49424003b2b..87d567680bc74 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -128,11 +128,9 @@ config PREEMPTION
 
 config PREEMPT_DYNAMIC
 	bool "Preemption behaviour defined on boot"
-	depends on HAVE_PREEMPT_DYNAMIC
 	depends on ARCH_HAS_PREEMPT_LAZY
-	select JUMP_LABEL if HAVE_PREEMPT_DYNAMIC_KEY
 	select PREEMPT_BUILD
-	default y if HAVE_PREEMPT_DYNAMIC_CALL
+	default y
 	help
 	  This option allows to define the preemption model on the kernel
 	  command line parameter and thus override the default preemption
@@ -142,9 +140,7 @@ config PREEMPT_DYNAMIC
 	  provide a pre-built kernel binary to reduce the number of kernel
 	  flavors they offer while still offering different usecases.
 
-	  The runtime overhead is negligible with HAVE_STATIC_CALL_INLINE enabled
-	  but if runtime patching is not available for the specific architecture
-	  then the potential overhead should be considered.
+	  The runtime overhead is negligible.
 
 	  Interesting if you want the same pre-built kernel should be used for
 	  both Server and Desktop workloads.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4f754f4a472f8..7b815d8ce67d3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7825,16 +7825,6 @@ int sched_dynamic_mode(const char *str)
 # define preempt_dynamic_key_enable(f)	static_key_enable(&sk_dynamic_##f.key)
 # define preempt_dynamic_key_disable(f)	static_key_disable(&sk_dynamic_##f.key)
 
-# if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
-#  define preempt_dynamic_enable(f)	static_call_update(f, f##_dynamic_enabled)
-#  define preempt_dynamic_disable(f)	static_call_update(f, f##_dynamic_disabled)
-# elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
-#  define preempt_dynamic_enable(f)	preempt_dynamic_key_enable(f)
-#  define preempt_dynamic_disable(f)	preempt_dynamic_key_disable(f)
-# else
-#  error "Unsupported PREEMPT_DYNAMIC mechanism"
-# endif
-
 static DEFINE_MUTEX(sched_dynamic_mutex);
 
 static void __sched_dynamic_update(int mode)
-- 
2.30.2