Commit c597bfdd authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Peter Zijlstra
Browse files

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: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210914103134.11309-1-frederic@kernel.org
parent 32ed980c

kernel/Kconfig.preempt

+23 −9
Original line number Diff line number Diff line
@@ -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

kernel/sched/core.c

+26 −3
Original line number Diff line number Diff line
@@ -6520,12 +6520,13 @@ 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)

{
@@ -6598,7 +6599,27 @@ 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
@@ -9398,6 +9419,8 @@ void __init sched_init(void) 


	init_uclamp();



	preempt_dynamic_init();



	scheduler_running = 1;

}