Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Jinjie Ruan]

Gentle ping.

On 2024/6/20 17:00, Jinjie Ruan wrote:
> Enable support for PREEMPT_DYNAMIC on arm32, allowing the preemption model
> to be chosen at boot time.
> 
> Similar to arm64, arm32 does not yet use the generic entry code, we must
> define our own `sk_dynamic_irqentry_exit_cond_resched`, which will be
> enabled/disabled by the common code in kernel/sched/core.c.
> 
> And arm32 use generic preempt.h, so declare
> `sk_dynamic_irqentry_exit_cond_resched` if the arch do not use generic
> entry. Other architectures which use generic preempt.h but not use generic
> entry can benefit from it.
> 
> Test ok with the below cmdline parameters on Qemu versatilepb board:
> 	`preempt=none`
> 	`preempt=voluntary`
> 	`preempt=full`
> 
> Update preempt mode with debugfs interface on above Qemu board is also
> tested ok:
> 	# cd /sys/kernel/debug/sched
> 	# echo none > preempt
> 	# echo voluntary > preempt
> 	# echo full > preempt
> 
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> ---
>  arch/arm/Kconfig                 |  1 +
>  arch/arm/include/asm/exception.h |  2 ++
>  arch/arm/kernel/Makefile         |  2 +-
>  arch/arm/kernel/common.c         | 13 +++++++++++++
>  arch/arm/kernel/entry-armv.S     |  7 ++++++-
>  include/asm-generic/preempt.h    |  5 +++++
>  6 files changed, 28 insertions(+), 2 deletions(-)
>  create mode 100644 arch/arm/kernel/common.c
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 036381c5d42f..843f320dde7f 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -124,6 +124,7 @@ config ARM
>  	select HAVE_PERF_EVENTS
>  	select HAVE_PERF_REGS
>  	select HAVE_PERF_USER_STACK_DUMP
> +	select HAVE_PREEMPT_DYNAMIC_KEY
>  	select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
>  	select HAVE_REGS_AND_STACK_ACCESS_API
>  	select HAVE_RSEQ
> diff --git a/arch/arm/include/asm/exception.h b/arch/arm/include/asm/exception.h
> index 3c82975d46db..ac96b76b394e 100644
> --- a/arch/arm/include/asm/exception.h
> +++ b/arch/arm/include/asm/exception.h
> @@ -12,4 +12,6 @@
>  
>  #define __exception_irq_entry	__irq_entry
>  
> +bool need_irq_preemption(void);
> +
>  #endif /* __ASM_ARM_EXCEPTION_H */
> diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
> index 89a77e3f51d2..58acd62dc5e9 100644
> --- a/arch/arm/kernel/Makefile
> +++ b/arch/arm/kernel/Makefile
> @@ -17,7 +17,7 @@ CFLAGS_REMOVE_return_address.o = -pg
>  
>  # Object file lists.
>  
> -obj-y		:= elf.o entry-common.o irq.o opcodes.o \
> +obj-y		:= common.o elf.o entry-common.o irq.o opcodes.o \
>  		   process.o ptrace.o reboot.o io.o \
>  		   setup.o signal.o sigreturn_codes.o \
>  		   stacktrace.o sys_arm.o time.o traps.o
> diff --git a/arch/arm/kernel/common.c b/arch/arm/kernel/common.c
> new file mode 100644
> index 000000000000..52b0abcae07e
> --- /dev/null
> +++ b/arch/arm/kernel/common.c
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/jump_label.h>
> +#include <asm/exception.h>
> +
> +#ifdef CONFIG_PREEMPT_DYNAMIC
> +DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> +
> +bool need_irq_preemption(void)
> +{
> +	return static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched);
> +}
> +#endif
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index 6150a716828c..571e86433833 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -221,6 +221,11 @@ __irq_svc:
>  	irq_handler from_user=0
>  
>  #ifdef CONFIG_PREEMPTION
> +#ifdef CONFIG_PREEMPT_DYNAMIC
> +	bl	need_irq_preemption
> +	cmp	r0, #0
> +	beq	2f
> +#endif
>  	ldr	r8, [tsk, #TI_PREEMPT]		@ get preempt count
>  	ldr	r0, [tsk, #TI_FLAGS]		@ get flags
>  	teq	r8, #0				@ if preempt count != 0
> @@ -228,7 +233,7 @@ __irq_svc:
>  	tst	r0, #_TIF_NEED_RESCHED
>  	blne	svc_preempt
>  #endif
> -
> +2:
>  	svc_exit r5, irq = 1			@ return from exception
>   UNWIND(.fnend		)
>  ENDPROC(__irq_svc)
> diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
> index 51f8f3881523..2db7a3e86303 100644
> --- a/include/asm-generic/preempt.h
> +++ b/include/asm-generic/preempt.h
> @@ -2,6 +2,7 @@
>  #ifndef __ASM_PREEMPT_H
>  #define __ASM_PREEMPT_H
>  
> +#include <linux/jump_label.h>
>  #include <linux/thread_info.h>
>  
>  #define PREEMPT_ENABLED	(0)
> @@ -89,6 +90,10 @@ void dynamic_preempt_schedule_notrace(void);
>  #define __preempt_schedule()		dynamic_preempt_schedule()
>  #define __preempt_schedule_notrace()	dynamic_preempt_schedule_notrace()
>  
> +#ifndef CONFIG_GENERIC_ENTRY
> +DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> +#endif
> +
>  #else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_KEY*/
>  
>  #define __preempt_schedule() preempt_schedule()