[tip: sched/core] sched: Provide Kconfig support for default dynamic preempt mode

From: tip-bot2 for Frederic Weisbecker
Date: Thu Sep 16 2021 - 07:59:33 EST


The following commit has been merged into the sched/core branch of tip:

Commit-ID: 5fa8f86b5819f07ee8601244e26ffa5a622f2bf4
Gitweb: https://git.kernel.org/tip/5fa8f86b5819f07ee8601244e26ffa5a622f2bf4
Author: Frederic Weisbecker <frederic@xxxxxxxxxx>
AuthorDate: Tue, 14 Sep 2021 12:31:34 +02:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 15 Sep 2021 17:49:01 +02:00

sched: Provide Kconfig support for default dynamic preempt mode

Currently the boot defined preempt behaviour (aka dynamic preempt)
selects full preemption by default when the "preempt=" boot parameter
is omitted. However distros may rather want to default to either
no preemption or voluntary preemption.

To provide with this flexibility, make dynamic preemption a visible
Kconfig option and adapt the preemption behaviour selected by the user
to either static or dynamic preemption.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210914103134.11309-1-frederic@xxxxxxxxxx
---
kernel/Kconfig.preempt | 32 +++++++++++++++++++++++---------
kernel/sched/core.c | 29 ++++++++++++++++++++++++++---
2 files changed, 49 insertions(+), 12 deletions(-)

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index bd7c414..aa9f78e 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -2,10 +2,11 @@

choice
prompt "Preemption Model"
- default PREEMPT_NONE
+ default PREEMPT_NONE_BEHAVIOUR

-config PREEMPT_NONE
+config PREEMPT_NONE_BEHAVIOUR
bool "No Forced Preemption (Server)"
+ select PREEMPT_NONE if !PREEMPT_DYNAMIC
help
This is the traditional Linux preemption model, geared towards
throughput. It will still provide good latencies most of the
@@ -17,9 +18,10 @@ config PREEMPT_NONE
raw processing power of the kernel, irrespective of scheduling
latencies.

-config PREEMPT_VOLUNTARY
+config PREEMPT_VOLUNTARY_BEHAVIOUR
bool "Voluntary Kernel Preemption (Desktop)"
depends on !ARCH_NO_PREEMPT
+ select PREEMPT_VOLUNTARY if !PREEMPT_DYNAMIC
help
This option reduces the latency of the kernel by adding more
"explicit preemption points" to the kernel code. These new
@@ -35,12 +37,10 @@ config PREEMPT_VOLUNTARY

Select this if you are building a kernel for a desktop system.

-config PREEMPT
+config PREEMPT_BEHAVIOUR
bool "Preemptible Kernel (Low-Latency Desktop)"
depends on !ARCH_NO_PREEMPT
- select PREEMPTION
- select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
- select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
+ select PREEMPT
help
This option reduces the latency of the kernel by making
all kernel code (that is not executing in a critical section)
@@ -58,7 +58,7 @@ config PREEMPT

config PREEMPT_RT
bool "Fully Preemptible Kernel (Real-Time)"
- depends on EXPERT && ARCH_SUPPORTS_RT
+ depends on EXPERT && ARCH_SUPPORTS_RT && !PREEMPT_DYNAMIC
select PREEMPTION
help
This option turns the kernel into a real-time kernel by replacing
@@ -75,6 +75,17 @@ config PREEMPT_RT

endchoice

+config PREEMPT_NONE
+ bool
+
+config PREEMPT_VOLUNTARY
+ bool
+
+config PREEMPT
+ bool
+ select PREEMPTION
+ select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
+
config PREEMPT_COUNT
bool

@@ -83,7 +94,10 @@ config PREEMPTION
select PREEMPT_COUNT

config PREEMPT_DYNAMIC
- bool
+ bool "Preemption behaviour defined on boot"
+ depends on HAVE_PREEMPT_DYNAMIC
+ select PREEMPT
+ default y
help
This option allows to define the preemption model on the kernel
command line parameter and thus override the default preemption
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 85e212d..b36b5d7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6354,12 +6354,13 @@ EXPORT_STATIC_CALL_TRAMP(preempt_schedule_notrace);
*/

enum {
- preempt_dynamic_none = 0,
+ preempt_dynamic_undefined = -1,
+ preempt_dynamic_none,
preempt_dynamic_voluntary,
preempt_dynamic_full,
};

-int preempt_dynamic_mode = preempt_dynamic_full;
+int preempt_dynamic_mode = preempt_dynamic_undefined;

int sched_dynamic_mode(const char *str)
{
@@ -6432,7 +6433,27 @@ static int __init setup_preempt_mode(char *str)
}
__setup("preempt=", setup_preempt_mode);

-#endif /* CONFIG_PREEMPT_DYNAMIC */
+static void __init preempt_dynamic_init(void)
+{
+ if (preempt_dynamic_mode == preempt_dynamic_undefined) {
+ if (IS_ENABLED(CONFIG_PREEMPT_NONE_BEHAVIOUR)) {
+ sched_dynamic_update(preempt_dynamic_none);
+ } else if (IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY_BEHAVIOUR)) {
+ sched_dynamic_update(preempt_dynamic_voluntary);
+ } else {
+ /* Default static call setting, nothing to do */
+ WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_BEHAVIOUR));
+ preempt_dynamic_mode = preempt_dynamic_full;
+ pr_info("Dynamic Preempt: full\n");
+ }
+ }
+}
+
+#else /* !CONFIG_PREEMPT_DYNAMIC */
+
+static inline void preempt_dynamic_init(void) { }
+
+#endif /* #ifdef CONFIG_PREEMPT_DYNAMIC */

/*
* This is the entry point to schedule() from kernel preemption
@@ -9236,6 +9257,8 @@ void __init sched_init(void)

init_uclamp();

+ preempt_dynamic_init();
+
scheduler_running = 1;
}