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Subject[tip: sched/core] preempt: Introduce CONFIG_PREEMPT_DYNAMIC
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 5759bcdb871f7f73b033643cd27d6cec33280540
Gitweb: https://git.kernel.org/tip/5759bcdb871f7f73b033643cd27d6cec33280540
Author: Michal Hocko <mhocko@kernel.org>
AuthorDate: Mon, 18 Jan 2021 15:12:19 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 05 Feb 2021 17:19:56 +01:00

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: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Michal Hocko <mhocko@suse.com>
[peterz: relax requirement to HAVE_STATIC_CALL]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210118141223.123667-5-frederic@kernel.org
---
Documentation/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 c722ec1..e854863 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 78c6f05..94d3dde 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
+ 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 7b6dd10..d021da9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -223,6 +223,7 @@ 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
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index bf82259..4160173 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.
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