Commit 6ef869e0 authored by Michal Hocko's avatar Michal Hocko Committed by Ingo Molnar
Browse files

preempt: Introduce CONFIG_PREEMPT_DYNAMIC 



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: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarMichal Hocko <mhocko@suse.com>
Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
[peterz: relax requirement to HAVE_STATIC_CALL]
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210118141223.123667-5-frederic@kernel.org
parent 29fd0194

Documentation/admin-guide/kernel-parameters.txt

+7 −0
Original line number Diff line number Diff line
@@ -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

arch/Kconfig

+9 −0
Original line number Diff line number Diff line
@@ -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

	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

arch/x86/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -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

	select HAVE_RSEQ

	select HAVE_SYSCALL_TRACEPOINTS

	select HAVE_UNSTABLE_SCHED_CLOCK

kernel/Kconfig.preempt

+19 −0
Original line number Diff line number Diff line
@@ -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.