[RFC PATCH 4/8] preempt: Introduce CONFIG_PREEMPT_DYNAMIC

[Frederic Weisbecker]

From: Michal Hocko <mhocko@xxxxxxxxxx>

Preemption mode selection is currently hardcoded on Kconfig choices.
Introduce a dedicated option to tune preemption flavour at boot time,

This will be only available on architectures efficiently supporting
static calls in order not to tempt with the feature against additional
overhead that might be prohibitive or undesirable.

CONFIG_PREEMPT_DYNAMIC is automatically selected by CONFIG_PREEMPT if
the architecture provides the necessary support (CONFIG_STATIC_CALL_INLINE,
CONFIG_GENERIC_ENTRY, and provide with __preempt_schedule_function() /
__preempt_schedule_notrace_function()).

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  7 +++++++
 arch/Kconfig                                  |  9 +++++++++
 arch/x86/Kconfig                              |  1 +
 kernel/Kconfig.preempt                        | 19 +++++++++++++++++++
 4 files changed, 36 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9e3cdb271d06..75e6c5d736fb 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3916,6 +3916,13 @@
 			Format: {"off"}
 			Disable Hardware Transactional Memory
 
+	preempt=	[KNL]
+			Select preemption mode if you have CONFIG_PREEMPT_DYNAMIC
+			none - Limited to cond_resched() calls
+			voluntary - Limited to cond_resched() and might_sleep() calls
+			full - Any section that isn't explicitly preempt disabled
+			       can be preempted anytime.
+
 	print-fatal-signals=
 			[KNL] debug: print fatal signals
 
diff --git a/arch/Kconfig b/arch/Kconfig
index 24862d15f3a3..84db237bdb67 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1090,6 +1090,15 @@ config HAVE_STATIC_CALL_INLINE
 	bool
 	depends on HAVE_STATIC_CALL
 
+config HAVE_PREEMPT_DYNAMIC
+	bool
+	depends on HAVE_STATIC_CALL_INLINE
+	depends on GENERIC_ENTRY
+	help
+	   Select this if the architecture support boot time preempt setting
+	   on top of static calls. It is strongly advised to support inline
+	   static call to avoid any overhead.
+
 config ARCH_WANT_LD_ORPHAN_WARN
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 21f851179ff0..cdae92ec29d2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -224,6 +224,7 @@ config X86
 	select HAVE_STACK_VALIDATION		if X86_64
 	select HAVE_STATIC_CALL
 	select HAVE_STATIC_CALL_INLINE		if HAVE_STACK_VALIDATION
+	select HAVE_PREEMPT_DYNAMIC		if HAVE_STATIC_CALL_INLINE
 	select HAVE_RSEQ
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UNSTABLE_SCHED_CLOCK
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index bf82259cff96..416017301660 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -40,6 +40,7 @@ config PREEMPT
 	depends on !ARCH_NO_PREEMPT
 	select PREEMPTION
 	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
+	select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
 	help
 	  This option reduces the latency of the kernel by making
 	  all kernel code (that is not executing in a critical section)
@@ -80,3 +81,21 @@ config PREEMPT_COUNT
 config PREEMPTION
        bool
        select PREEMPT_COUNT
+
+config PREEMPT_DYNAMIC
+	bool
+	help
+	  This option allows to define the preemption model on the kernel
+	  command line parameter and thus override the default preemption
+	  model defined during compile time.
+
+	  The feature is primarily interesting for Linux distributions which
+	  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.
+
+	  Interesting if you want the same pre-built kernel should be used for
+	  both Server and Desktop workloads.
-- 
2.25.1