[PATCH 1/1] arm/syscalls: mark syscall invocation as likely in invoke_syscall

[PATCH 1/1] arm/syscalls: mark syscall invocation as likely in invoke_syscall

[Can Peng]

The invoke_syscall() function is overwhelmingly called for
valid system call entries. Annotate the main path with likely()
to help the compiler generate better branch prediction hints,
reducing CPU pipeline stalls due to mispredictions.

This is a micro-optimization targeting syscall-heavy workloads.

Signed-off-by: Can Peng <pengcan@kylinos.cn>
---
 arch/arm64/kernel/syscall.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index c442fcec6b9e..aba7ca6bca2d 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -43,7 +43,7 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 
 	add_random_kstack_offset();
 
-	if (scno < sc_nr) {
+	if (likely(scno < sc_nr)) {
 		syscall_fn_t syscall_fn;
 		syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
 		ret = __invoke_syscall(regs, syscall_fn);
-- 
2.25.1

Re: [PATCH 1/1] arm/syscalls: mark syscall invocation as likely in invoke_syscall

[Will Deacon]

On Fri, Sep 19, 2025 at 06:00:42PM +0800, Can Peng wrote:
> The invoke_syscall() function is overwhelmingly called for
> valid system call entries. Annotate the main path with likely()
> to help the compiler generate better branch prediction hints,
> reducing CPU pipeline stalls due to mispredictions.
> 
> This is a micro-optimization targeting syscall-heavy workloads.

Does it actually make a measurable difference?

Will

Re: [PATCH 1/1] arm/syscalls: mark syscall invocation as

[Can Peng]

Hello,

>> The invoke_syscall() function is overwhelmingly called for
>> valid system call entries. Annotate the main path with likely()
>> to help the compiler generate better branch prediction hints,
>> reducing CPU pipeline stalls due to mispredictions.
>> 
>> This is a micro-optimization targeting syscall-heavy workloads.
>
>Does it actually make a measurable difference?

This change implements a micro-optimization. Benchmark results
from the Kunpeng 920 machine are as follows:

before:
- lat_syscall result 
write call      0.2332  0.2337  0.2341
read call       0.2501  0.2493  0.2506

after:
- lat_syscall result
write call      0.2301  0.2316  0.2307
read call       0.2492  0.2498  0.2491


Best regards,
Can Peng

Re: [PATCH 1/1] arm/syscalls: mark syscall invocation as likely in invoke_syscall

[Will Deacon]

On Fri, 19 Sep 2025 18:00:42 +0800, Can Peng wrote:
> The invoke_syscall() function is overwhelmingly called for
> valid system call entries. Annotate the main path with likely()
> to help the compiler generate better branch prediction hints,
> reducing CPU pipeline stalls due to mispredictions.
> 
> This is a micro-optimization targeting syscall-heavy workloads.
> 
> [...]

Applied to arm64 (for-next/entry), thanks!

[1/1] arm/syscalls: mark syscall invocation as likely in invoke_syscall
      https://git.kernel.org/arm64/c/da9e5c04be58

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev