riscv: support PREEMPT_DYNAMIC with static keys

PATCH Applied Jisheng Zhang 3 participants 3 messages Jul 16, 2023 – Aug 30, 2023

Subsystems (2)

RISC-V Generic include/asm header files

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Jisheng Zhang

author

2023-07-16 17:01 UTC #0 lore raw

PATCH v2 1/1 riscv: support PREEMPT_DYNAMIC with static keys

Currently, each architecture can support PREEMPT_DYNAMIC through either static calls or static keys. To support PREEMPT_DYNAMIC on riscv, we face three choices:

1. only add static calls support to riscv
As Mark pointed out in commit 99cf983cc8bc ("sched/preempt: Add PREEMPT_DYNAMIC using static keys"), static keys "...should have slightly lower overhead than non-inline static calls, as this effectively inlines each trampoline into the start of its callee. This may avoid redundant work, and may integrate better with CFI schemes." So even we add static calls(without inline static calls) to riscv, static keys is still a better choice.

2. add static calls and inline static calls to riscv Per my understanding, inline static calls requires objtool support which is not easy.

3. use static keys

While riscv doesn't have static calls support, it supports static keys perfectly. So this patch selects HAVE_PREEMPT_DYNAMIC_KEY to enable support for PREEMPT_DYNAMIC on riscv, so that the preemption model can be chosen at boot time. It also patches asm-generic/preempt.h, mainly to add __preempt_schedule() and __preempt_schedule_notrace() macros for PREEMPT_DYNAMIC case. Other architectures which use generic preempt.h can also benefit from this patch by simply selecting HAVE_PREEMPT_DYNAMIC_KEY to enable PREEMPT_DYNAMIC if they supports static keys.

Signed-off-by Jisheng Zhang <[email protected]>

---
since v1:
- keep Kconfig entries sorted
- group asm-generic modifications under CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)

 arch/riscv/Kconfig            |  1 +
 include/asm-generic/preempt.h | 14 +++++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

arch/riscv/Kconfig +1 -0

index 4c07b9189c86..686df6902947 100644
@@ -130,6 +130,7 @@ config RISCV
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
+	select HAVE_PREEMPT_DYNAMIC_KEY if !XIP_KERNEL
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RETHOOK if !XIP_KERNEL
 	select HAVE_RSEQ

include/asm-generic/preempt.h +13 -1

index b4d43a4af5f7..51f8f3881523 100644
@@ -80,9 +80,21 @@ static __always_inline bool should_resched(int preempt_offset)
 
 #ifdef CONFIG_PREEMPTION
 extern asmlinkage void preempt_schedule(void);
-#define __preempt_schedule() preempt_schedule()
 extern asmlinkage void preempt_schedule_notrace(void);
+
+#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
+
+void dynamic_preempt_schedule(void);
+void dynamic_preempt_schedule_notrace(void);
+#define __preempt_schedule()		dynamic_preempt_schedule()
+#define __preempt_schedule_notrace()	dynamic_preempt_schedule_notrace()
+
+#else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_KEY*/
+
+#define __preempt_schedule() preempt_schedule()
 #define __preempt_schedule_notrace() preempt_schedule_notrace()
+
+#endif /* CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_KEY*/
 #endif /* CONFIG_PREEMPTION */
 
 #endif /* __ASM_PREEMPT_H */

Signature · 1 line

2.40.1

Conor Dooley

Reviewed-by on v2 1/1

2023-07-24 10:08 UTC #1 lore raw

Quoted 45 lines · Jisheng Zhang

Currently, each architecture can support PREEMPT_DYNAMIC through either static calls or static keys. To support PREEMPT_DYNAMIC on riscv, we face three choices:
1. only add static calls support to riscv
As Mark pointed out in commit 99cf983cc8bc ("sched/preempt: Add PREEMPT_DYNAMIC using static keys"), static keys "...should have slightly lower overhead than non-inline static calls, as this effectively inlines each trampoline into the start of its callee. This may avoid redundant work, and may integrate better with CFI schemes." So even we add static calls(without inline static calls) to riscv, static keys is still a better choice.
2. add static calls and inline static calls to riscv Per my understanding, inline static calls requires objtool support which is not easy.
3. use static keys
While riscv doesn't have static calls support, it supports static keys perfectly. So this patch selects HAVE_PREEMPT_DYNAMIC_KEY to enable support for PREEMPT_DYNAMIC on riscv, so that the preemption model can be chosen at boot time. It also patches asm-generic/preempt.h, mainly to add __preempt_schedule() and __preempt_schedule_notrace() macros for PREEMPT_DYNAMIC case. Other architectures which use generic preempt.h can also benefit from this patch by simply selecting HAVE_PREEMPT_DYNAMIC_KEY to enable PREEMPT_DYNAMIC if they supports static keys.
Signed-off-by Jisheng Zhang <[email protected]>
---
since v1:
- keep Kconfig entries sorted
- group asm-generic modifications under CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
 arch/riscv/Kconfig            |  1 +
 include/asm-generic/preempt.h | 14 +++++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)
arch/riscv/Kconfig +0 -0
index 4c07b9189c86..686df6902947 100644

arch/riscv/Kconfig config RISCV · line 133

Quoted 4 lines

arch/riscv/Kconfig +1 -0

@@ -130,6 +130,7 @@ config RISCV
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
+	select HAVE_PREEMPT_DYNAMIC_KEY if !XIP_KERNEL

Had a go of this, and it seems fine to me, as do the asm-generic bits seem fine from a single arch perspective.

Reviewed-by Conor Dooley <[email protected]>

Thanks,
Conor.

[email protected]

CI Applied

2023-08-30 13:21 UTC #2 lore raw

In reply to Jisheng Zhang

This patch was applied to riscv/linux.git (for-next) by Palmer Dabbelt <[email protected]>:

Hello:

This patch was applied to riscv/linux.git (for-next) by Palmer Dabbelt <[email protected]>:

Quoted 13 lines · On Mon, 17 Jul 2023 00:49:25 +0800 you wrote:

Currently, each architecture can support PREEMPT_DYNAMIC through either static calls or static keys. To support PREEMPT_DYNAMIC on riscv, we face three choices:
1. only add static calls support to riscv
As Mark pointed out in commit 99cf983cc8bc ("sched/preempt: Add PREEMPT_DYNAMIC using static keys"), static keys "...should have slightly lower overhead than non-inline static calls, as this effectively inlines each trampoline into the start of its callee. This may avoid redundant work, and may integrate better with CFI schemes." So even we add static calls(without inline static calls) to riscv, static keys is still a better choice.

Quoted 1 line

[...]

Here is the summary with links:
- [v2] riscv: support PREEMPT_DYNAMIC with static keys

    https://git.kernel.org/riscv/c/15e062726f55

You are awesome, thank you!

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