[PATCH] ARM: support PREEMPT_DYNAMIC

[PATCH] ARM: support PREEMPT_DYNAMIC

[Jinjie Ruan]

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@huawei.com>
---
 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()
-- 
2.34.1

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@huawei.com>
> ---
>  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()

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Russell King (Oracle)]

On Wed, Jul 31, 2024 at 10:07:53AM +0800, Jinjie Ruan wrote:
> >  #ifdef CONFIG_PREEMPTION
> > +#ifdef CONFIG_PREEMPT_DYNAMIC
> > +	bl	need_irq_preemption
> > +	cmp	r0, #0
> > +	beq	2f
> > +#endif

Depending on the interrupt rate, this can be regarded as a fast path,
it would be nice if we could find a way to use static branches in
assembly code.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Jinjie Ruan]

On 2024/7/31 16:24, Russell King (Oracle) wrote:
> On Wed, Jul 31, 2024 at 10:07:53AM +0800, Jinjie Ruan wrote:
>>>  #ifdef CONFIG_PREEMPTION
>>> +#ifdef CONFIG_PREEMPT_DYNAMIC
>>> +	bl	need_irq_preemption
>>> +	cmp	r0, #0
>>> +	beq	2f
>>> +#endif
> 
> Depending on the interrupt rate, this can be regarded as a fast path,
> it would be nice if we could find a way to use static branches in
> assembly code.

You're right, it's more elegant to use dynamic keys in assembly, let me
think about how to do it.

> 

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Jinjie Ruan]

On 2024/7/31 16:24, Russell King (Oracle) wrote:
> On Wed, Jul 31, 2024 at 10:07:53AM +0800, Jinjie Ruan wrote:
>>>  #ifdef CONFIG_PREEMPTION
>>> +#ifdef CONFIG_PREEMPT_DYNAMIC
>>> +	bl	need_irq_preemption
>>> +	cmp	r0, #0
>>> +	beq	2f
>>> +#endif
> 
> Depending on the interrupt rate, this can be regarded as a fast path,
> it would be nice if we could find a way to use static branches in
> assembly code.
It seems to be hard to use static keys in assembly code.

By the way, currently, most architectures have simplified assembly code
and implemented its most functions in C functions. Does arm32 have this
plan?

> 

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Russell King (Oracle)]

On Wed, Jul 31, 2024 at 06:03:11PM +0800, Jinjie Ruan wrote:
> On 2024/7/31 16:24, Russell King (Oracle) wrote:
> > On Wed, Jul 31, 2024 at 10:07:53AM +0800, Jinjie Ruan wrote:
> >>>  #ifdef CONFIG_PREEMPTION
> >>> +#ifdef CONFIG_PREEMPT_DYNAMIC
> >>> +	bl	need_irq_preemption
> >>> +	cmp	r0, #0
> >>> +	beq	2f
> >>> +#endif
> > 
> > Depending on the interrupt rate, this can be regarded as a fast path,
> > it would be nice if we could find a way to use static branches in
> > assembly code.
> It seems to be hard to use static keys in assembly code.
> 
> By the way, currently, most architectures have simplified assembly code
> and implemented its most functions in C functions. Does arm32 have this
> plan?

arm32 is effectively in maintenance mode; very little active development
is occuring. So, there are no plans to change the code without good
reason (as code changes without reason will needlessly affect its
stability.)

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Jinjie Ruan]

On 2024/7/31 18:14, Russell King (Oracle) wrote:
> On Wed, Jul 31, 2024 at 06:03:11PM +0800, Jinjie Ruan wrote:
>> On 2024/7/31 16:24, Russell King (Oracle) wrote:
>>> On Wed, Jul 31, 2024 at 10:07:53AM +0800, Jinjie Ruan wrote:
>>>>>  #ifdef CONFIG_PREEMPTION
>>>>> +#ifdef CONFIG_PREEMPT_DYNAMIC
>>>>> +	bl	need_irq_preemption
>>>>> +	cmp	r0, #0
>>>>> +	beq	2f
>>>>> +#endif
>>>
>>> Depending on the interrupt rate, this can be regarded as a fast path,
>>> it would be nice if we could find a way to use static branches in
>>> assembly code.
>> It seems to be hard to use static keys in assembly code.
>>
>> By the way, currently, most architectures have simplified assembly code
>> and implemented its most functions in C functions. Does arm32 have this
>> plan?
> 
> arm32 is effectively in maintenance mode; very little active development
> is occuring. So, there are no plans to change the code without good
> reason (as code changes without reason will needlessly affect its
> stability.)

Thank you for helping me with my question.

> 

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Russell King (Oracle)]

On Wed, Jul 31, 2024 at 10:07:53AM +0800, Jinjie Ruan wrote:
> 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

Do you have a use case for this feature?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Jinjie Ruan]

On 2024/8/2 19:44, Russell King (Oracle) wrote:
> On Wed, Jul 31, 2024 at 10:07:53AM +0800, Jinjie Ruan wrote:
>> 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
> 
> Do you have a use case for this feature?

Yes, many of our ARM32 products use different preemption models, and it
would be much more convenient if we could configure it at startup.

> 

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Linus Walleij]

On Wed, Jul 31, 2024 at 12:03 PM Jinjie Ruan <ruanjinjie@huawei.com> wrote:

> By the way, currently, most architectures have simplified assembly code
> and implemented its most functions in C functions. Does arm32 have this
> plan?

I would turn it around, since I saw that Huawei contributed generic entry
code for Aarch64, do you folks have a plan to also do patches for ARM32?

I have many ARM32 systems and I am happy to help out with reviewing
and testing if you do.

Alternatively I might be able to have a look at it, because the entry code
is right in my work area all the time.

Yours,
Linus Walleij

Re: [PATCH] ARM: support PREEMPT_DYNAMIC

[Jinjie Ruan]

On 2024/8/23 23:56, Linus Walleij wrote:
> On Wed, Jul 31, 2024 at 12:03 PM Jinjie Ruan <ruanjinjie@huawei.com> wrote:
> 
>> By the way, currently, most architectures have simplified assembly code
>> and implemented its most functions in C functions. Does arm32 have this
>> plan?
> 
> I would turn it around, since I saw that Huawei contributed generic entry
> code for Aarch64, do you folks have a plan to also do patches for ARM32?

Hi, Linus，there are no plans to switch ARM32 to generic entry as it is
already in maintenance mode as Russell said, but we will try to do it if
Russell thinks it makes sense and necessary.

> 
> I have many ARM32 systems and I am happy to help out with reviewing
> and testing if you do.
> 
> Alternatively I might be able to have a look at it, because the entry code
> is right in my work area all the time.

Thank you very much, I'd love your help reviewing and testing the code.

> 
> Yours,
> Linus Walleij