Re: [PATCH 1/5] sched: dynamic: Make PREEMPT_DYNAMIC depend on ARCH_HAS_PREEMPT_LAZY

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 7/3/26 7:03 PM, Mark Rutland wrote:
> On architectures which select ARCH_HAS_PREEMPT_LAZY, it has not been
> possible to select the NONE and VOLUNTARY preemption models since
> commit:
> 
>    7dadeaa6e851 ("sched: Further restrict the preemption modes")
> 
> ... which was merged in v7.0.
> 
> All architectures which currently suppoort PREEMPT_DYNAMIC select

s/suppoort/support

> ARCH_HAS_PREEMPT_LAZY:
> 
>    [mark@lakrids:~/src/linux]% git describe HEAD
>    v7.2-rc1-1-g871a4586ea2e1
>    [mark@lakrids:~/src/linux]% git grep 'select HAVE_PREEMPT_DYNAMIC_' -- arch
>    arch/arm64/Kconfig:     select HAVE_PREEMPT_DYNAMIC_KEY
>    arch/loongarch/Kconfig: select HAVE_PREEMPT_DYNAMIC_KEY
>    arch/powerpc/Kconfig:   select HAVE_PREEMPT_DYNAMIC_KEY
>    arch/riscv/Kconfig:     select HAVE_PREEMPT_DYNAMIC_KEY
>    arch/s390/Kconfig:      select HAVE_PREEMPT_DYNAMIC_KEY
>    arch/x86/Kconfig:       select HAVE_PREEMPT_DYNAMIC_CALL
>    [mark@lakrids:~/src/linux]% git grep 'select ARCH_HAS_PREEMPT_LAZY' -- arch
>    arch/arm64/Kconfig:     select ARCH_HAS_PREEMPT_LAZY
>    arch/loongarch/Kconfig: select ARCH_HAS_PREEMPT_LAZY
>    arch/powerpc/Kconfig:   select ARCH_HAS_PREEMPT_LAZY
>    arch/riscv/Kconfig:     select ARCH_HAS_PREEMPT_LAZY
>    arch/s390/Kconfig:      select ARCH_HAS_PREEMPT_LAZY
>    arch/x86/Kconfig:       select ARCH_HAS_PREEMPT_LAZY
> 
> ... and hence in practice PREEMPT_DYNAMIC no longer supports the NONE or
> VOLUNTARY preemption models.
> 
> Make this official: have PREEMPT_DYNAMIC depend on
> ARCH_HAS_PREEMPT_LAZY, and remove the trivially unreachable code.
> Further simplifications will be made in subsequent patches.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: John Stultz <jstultz@google.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>   kernel/Kconfig.preempt |  1 +
>   kernel/sched/core.c    | 63 ++----------------------------------------
>   2 files changed, 4 insertions(+), 60 deletions(-)
> 
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index 88c594c6d7fcd..fb49424003b2b 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -129,6 +129,7 @@ config PREEMPTION
>   config PREEMPT_DYNAMIC
>   	bool "Preemption behaviour defined on boot"
>   	depends on HAVE_PREEMPT_DYNAMIC
> +	depends on ARCH_HAS_PREEMPT_LAZY
>   	select JUMP_LABEL if HAVE_PREEMPT_DYNAMIC_KEY
>   	select PREEMPT_BUILD
>   	default y if HAVE_PREEMPT_DYNAMIC_CALL


Can we update the help section bit?

"""
           The runtime overhead is negligible.

           Interesting if you want the same pre-built kernel should be used for
           both Server and Desktop workloads.
"""
I think server meant preempt=none and desktop meant preempt=full/lazy there. No?

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 96226707c2f61..2db78826a484b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7870,20 +7870,10 @@ EXPORT_SYMBOL(__cond_resched_rwlock_write);
>    *
>    *
>    * NONE:
> - *   cond_resched               <- __cond_resched
> - *   might_resched              <- RET0
> - *   preempt_schedule           <- NOP
> - *   preempt_schedule_notrace   <- NOP
> - *   irqentry_exit_cond_resched <- NOP
> - *   dynamic_preempt_lazy       <- false
> + *   (unselectable)
>    *
>    * VOLUNTARY:
> - *   cond_resched               <- __cond_resched
> - *   might_resched              <- __cond_resched
> - *   preempt_schedule           <- NOP
> - *   preempt_schedule_notrace   <- NOP
> - *   irqentry_exit_cond_resched <- NOP
> - *   dynamic_preempt_lazy       <- false
> + *   (unselectable)
>    *
>    * FULL:
>    *   cond_resched               <- RET0
> @@ -7914,21 +7904,11 @@ int preempt_dynamic_mode = preempt_dynamic_undefined;
>   
>   int sched_dynamic_mode(const char *str)
>   {
> -# if !(defined(CONFIG_PREEMPT_RT) || defined(CONFIG_ARCH_HAS_PREEMPT_LAZY))
> -	if (!strcmp(str, "none"))
> -		return preempt_dynamic_none;
> -
> -	if (!strcmp(str, "voluntary"))
> -		return preempt_dynamic_voluntary;
> -# endif
> -
>   	if (!strcmp(str, "full"))
>   		return preempt_dynamic_full;
>   
> -# ifdef CONFIG_ARCH_HAS_PREEMPT_LAZY
>   	if (!strcmp(str, "lazy"))
>   		return preempt_dynamic_lazy;
> -# endif
>   
>   	return -EINVAL;
>   }
> @@ -7950,40 +7930,7 @@ static DEFINE_MUTEX(sched_dynamic_mutex);
>   
>   static void __sched_dynamic_update(int mode)
>   {
> -	/*
> -	 * Avoid {NONE,VOLUNTARY} -> FULL transitions from ever ending up in
> -	 * the ZERO state, which is invalid.
> -	 */
> -	preempt_dynamic_enable(cond_resched);
> -	preempt_dynamic_enable(might_resched);
> -	preempt_dynamic_enable(preempt_schedule);
> -	preempt_dynamic_enable(preempt_schedule_notrace);
> -	preempt_dynamic_enable(irqentry_exit_cond_resched);
> -	preempt_dynamic_key_disable(preempt_lazy);
> -
>   	switch (mode) {
> -	case preempt_dynamic_none:
> -		preempt_dynamic_enable(cond_resched);
> -		preempt_dynamic_disable(might_resched);
> -		preempt_dynamic_disable(preempt_schedule);
> -		preempt_dynamic_disable(preempt_schedule_notrace);
> -		preempt_dynamic_disable(irqentry_exit_cond_resched);
> -		preempt_dynamic_key_disable(preempt_lazy);
> -		if (mode != preempt_dynamic_mode)
> -			pr_info("Dynamic Preempt: none\n");
> -		break;
> -
> -	case preempt_dynamic_voluntary:
> -		preempt_dynamic_enable(cond_resched);
> -		preempt_dynamic_enable(might_resched);
> -		preempt_dynamic_disable(preempt_schedule);
> -		preempt_dynamic_disable(preempt_schedule_notrace);
> -		preempt_dynamic_disable(irqentry_exit_cond_resched);
> -		preempt_dynamic_key_disable(preempt_lazy);
> -		if (mode != preempt_dynamic_mode)
> -			pr_info("Dynamic Preempt: voluntary\n");
> -		break;
> -
>   	case preempt_dynamic_full:
>   		preempt_dynamic_disable(cond_resched);
>   		preempt_dynamic_disable(might_resched);
> @@ -8033,11 +7980,7 @@ __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)) {
> -			sched_dynamic_update(preempt_dynamic_none);
> -		} else if (IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)) {
> -			sched_dynamic_update(preempt_dynamic_voluntary);
> -		} else if (IS_ENABLED(CONFIG_PREEMPT_LAZY)) {
> +		if (IS_ENABLED(CONFIG_PREEMPT_LAZY)) {
>   			sched_dynamic_update(preempt_dynamic_lazy);
>   		} else {
>   			/* Default static call setting, nothing to do */

other than that, it looks good to me.

Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>