Subject: Re: [PATCH v2 1/5] preempt: Restore preemption model selection configs

From: Marco Elver <[email-masked]>
Date: Thu, 11 Nov 2021 09:58:54 +0100

On Wed, Nov 10, 2021 at 08:24PM +0000, Valentin Schneider wrote:
> Commit c597bfddc9e9 ("sched: Provide Kconfig support for default dynamic
> preempt mode") changed the selectable config names for the preemption
> model. This means a config file must now select
> 
>   CONFIG_PREEMPT_BEHAVIOUR=y
> 
> rather than
> 
>   CONFIG_PREEMPT=y
> 
> to get a preemptible kernel. This means all arch config files would need to
> be updated - right now they'll all end up with the default
> CONFIG_PREEMPT_NONE_BEHAVIOUR.
> 
> Rather than touch a good hundred of config files, restore usage of
> CONFIG_PREEMPT{_NONE, _VOLUNTARY}. Make them configure:
> o The build-time preemption model when !PREEMPT_DYNAMIC
> o The default boot-time preemption model when PREEMPT_DYNAMIC
> 
> Add siblings of those configs with the _BUILD suffix to unconditionally
> designate the build-time preemption model (PREEMPT_DYNAMIC is built with
> the "highest" preemption model it supports, aka PREEMPT). Downstream
> configs should by now all be depending / selected by CONFIG_PREEMPTION
> rather than CONFIG_PREEMPT, so only a few sites need patching up.
> 
> Signed-off-by: Valentin Schneider <[email-masked]>

Acked-by: Marco Elver <[email-masked]>

Much better, thank you!

> ---
>  include/linux/kernel.h   |  2 +-
>  include/linux/vermagic.h |  2 +-
>  init/Makefile            |  2 +-
>  kernel/Kconfig.preempt   | 42 ++++++++++++++++++++--------------------
>  kernel/sched/core.c      |  6 +++---
>  5 files changed, 27 insertions(+), 27 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 2776423a587e..9c7d774ef809 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -88,7 +88,7 @@
>  struct completion;
>  struct user;
>  
> -#ifdef CONFIG_PREEMPT_VOLUNTARY
> +#ifdef CONFIG_PREEMPT_VOLUNTARY_BUILD
>  
>  extern int __cond_resched(void);
>  # define might_resched() __cond_resched()
> diff --git a/include/linux/vermagic.h b/include/linux/vermagic.h
> index 1eaaa93c37bf..329d63babaeb 100644
> --- a/include/linux/vermagic.h
> +++ b/include/linux/vermagic.h
> @@ -15,7 +15,7 @@
>  #else
>  #define MODULE_VERMAGIC_SMP ""
>  #endif
> -#ifdef CONFIG_PREEMPT
> +#ifdef CONFIG_PREEMPT_BUILD
>  #define MODULE_VERMAGIC_PREEMPT "preempt "
>  #elif defined(CONFIG_PREEMPT_RT)
>  #define MODULE_VERMAGIC_PREEMPT "preempt_rt "
> diff --git a/init/Makefile b/init/Makefile
> index 2846113677ee..04eeee12c076 100644
> --- a/init/Makefile
> +++ b/init/Makefile
> @@ -30,7 +30,7 @@ $(obj)/version.o: include/generated/compile.h
>  quiet_cmd_compile.h = CHK     $@
>        cmd_compile.h = \
>  	$(CONFIG_SHELL) $(srctree)/scripts/mkcompile_h $@	\
> -	"$(UTS_MACHINE)" "$(CONFIG_SMP)" "$(CONFIG_PREEMPT)"	\
> +	"$(UTS_MACHINE)" "$(CONFIG_SMP)" "$(CONFIG_PREEMPT_BUILD)"	\
>  	"$(CONFIG_PREEMPT_RT)" $(CONFIG_CC_VERSION_TEXT) "$(LD)"
>  
>  include/generated/compile.h: FORCE
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index 60f1bfc3c7b2..ce77f0265660 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -1,12 +1,23 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  
> +config PREEMPT_NONE_BUILD
> +	bool
> +
> +config PREEMPT_VOLUNTARY_BUILD
> +	bool
> +
> +config PREEMPT_BUILD
> +	bool
> +	select PREEMPTION
> +	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
> +
>  choice
>  	prompt "Preemption Model"
> -	default PREEMPT_NONE_BEHAVIOUR
> +	default PREEMPT_NONE
>  
> -config PREEMPT_NONE_BEHAVIOUR
> +config PREEMPT_NONE
>  	bool "No Forced Preemption (Server)"
> -	select PREEMPT_NONE if !PREEMPT_DYNAMIC
> +	select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
>  	help
>  	  This is the traditional Linux preemption model, geared towards
>  	  throughput. It will still provide good latencies most of the
> @@ -18,10 +29,10 @@ config PREEMPT_NONE_BEHAVIOUR
>  	  raw processing power of the kernel, irrespective of scheduling
>  	  latencies.
>  
> -config PREEMPT_VOLUNTARY_BEHAVIOUR
> +config PREEMPT_VOLUNTARY
>  	bool "Voluntary Kernel Preemption (Desktop)"
>  	depends on !ARCH_NO_PREEMPT
> -	select PREEMPT_VOLUNTARY if !PREEMPT_DYNAMIC
> +	select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
>  	help
>  	  This option reduces the latency of the kernel by adding more
>  	  "explicit preemption points" to the kernel code. These new
> @@ -37,10 +48,10 @@ config PREEMPT_VOLUNTARY_BEHAVIOUR
>  
>  	  Select this if you are building a kernel for a desktop system.
>  
> -config PREEMPT_BEHAVIOUR
> +config PREEMPT
>  	bool "Preemptible Kernel (Low-Latency Desktop)"
>  	depends on !ARCH_NO_PREEMPT
> -	select PREEMPT
> +	select PREEMPT_BUILD
>  	help
>  	  This option reduces the latency of the kernel by making
>  	  all kernel code (that is not executing in a critical section)
> @@ -58,7 +69,7 @@ config PREEMPT_BEHAVIOUR
>  
>  config PREEMPT_RT
>  	bool "Fully Preemptible Kernel (Real-Time)"
> -	depends on EXPERT && ARCH_SUPPORTS_RT && !PREEMPT_DYNAMIC
> +	depends on EXPERT && ARCH_SUPPORTS_RT
>  	select PREEMPTION
>  	help
>  	  This option turns the kernel into a real-time kernel by replacing
> @@ -75,17 +86,6 @@ 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
>  
> @@ -95,8 +95,8 @@ config PREEMPTION
>  
>  config PREEMPT_DYNAMIC
>  	bool "Preemption behaviour defined on boot"
> -	depends on HAVE_PREEMPT_DYNAMIC
> -	select PREEMPT
> +	depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT
> +	select PREEMPT_BUILD
>  	default y
>  	help
>  	  This option allows to define the preemption model on the kernel
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f2611b9cf503..97047aa7b6c2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6625,13 +6625,13 @@ __setup("preempt=", setup_preempt_mode);
>  static void __init preempt_dynamic_init(void)
>  {
>  	if (preempt_dynamic_mode == preempt_dynamic_undefined) {
> -		if (IS_ENABLED(CONFIG_PREEMPT_NONE_BEHAVIOUR)) {
> +		if (IS_ENABLED(CONFIG_PREEMPT_NONE)) {
>  			sched_dynamic_update(preempt_dynamic_none);
> -		} else if (IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY_BEHAVIOUR)) {
> +		} else if (IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)) {
>  			sched_dynamic_update(preempt_dynamic_voluntary);
>  		} else {
>  			/* Default static call setting, nothing to do */
> -			WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_BEHAVIOUR));
> +			WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT));
>  			preempt_dynamic_mode = preempt_dynamic_full;
>  			pr_info("Dynamic Preempt: full\n");
>  		}
> -- 
> 2.25.1
>