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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: Michal Hocko <mhocko@suse.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
[peterz: relax requirement to HAVE_STATIC_CALL]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210118141223.123667-5-frederic@kernel.org
master
Michal Hocko 2021-01-18 15:12:19 +01:00 committed by Ingo Molnar
parent 29fd01944b
commit 6ef869e064
4 changed files with 36 additions and 0 deletions
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7
Documentation/admin-guide/kernel-parameters.txt
View File

@ -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

9
arch/Kconfig
View File

@ -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

1
arch/x86/Kconfig
View File

@ -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

19
kernel/Kconfig.preempt
View File

@ -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.