Re: [PATCH 3/3] arm64: use runtime constant to optimize handle_arch_irq access

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Feb 20, 2026 at 05:09:22PM +0800, Jisheng Zhang wrote:
> Currently, on arm64 platforms, the handle_arch_irq is a pointer which
> is set during booting, and every irq processing needs to access it,
> so it sits in hot code path. We can use the runtime constant mechanism
> which was introduced by Linus to speed up its accessing.
> 
> Tested on Quad CA55 platform, the perf sched benchmark is improved
> by ~6.5%

That is a surprisingly large impact. :/

Does this meaningfully actually affect any real workload?

> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/arm64/kernel/entry-common.c | 4 +++-
>  arch/arm64/kernel/irq.c          | 9 ++++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 3625797e9ee8..46a4c012e15f 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -25,6 +25,7 @@
>  #include <asm/kprobes.h>
>  #include <asm/mmu.h>
>  #include <asm/processor.h>
> +#include <asm/runtime-const.h>
>  #include <asm/sdei.h>
>  #include <asm/stacktrace.h>
>  #include <asm/sysreg.h>
> @@ -139,7 +140,8 @@ static void do_interrupt_handler(struct pt_regs *regs,
>  	set_irq_regs(old_regs);
>  }
>  
> -extern void (*handle_arch_irq)(struct pt_regs *);
> +extern void (*_handle_arch_irq)(struct pt_regs *);
> +#define handle_arch_irq runtime_const_ptr(_handle_arch_irq)
>  extern void (*handle_arch_fiq)(struct pt_regs *);

We should treat handle_arch_irq and handle_arch_fiq the same way. Either
both get this, or neither do.

>  static void noinstr __panic_unhandled(struct pt_regs *regs, const char *vector,
> diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
> index 15dedb385b9e..30629c183606 100644
> --- a/arch/arm64/kernel/irq.c
> +++ b/arch/arm64/kernel/irq.c
> @@ -23,6 +23,7 @@
>  #include <asm/daifflags.h>
>  #include <asm/exception.h>
>  #include <asm/numa.h>
> +#include <asm/runtime-const.h>
>  #include <asm/softirq_stack.h>
>  #include <asm/stacktrace.h>
>  #include <asm/vmap_stack.h>
> @@ -84,15 +85,17 @@ static void default_handle_fiq(struct pt_regs *regs)
>  	panic("FIQ taken without a root FIQ handler\n");
>  }
>  
> -void (*handle_arch_irq)(struct pt_regs *) __ro_after_init = default_handle_irq;
> +void (*_handle_arch_irq)(struct pt_regs *) __ro_after_init = default_handle_irq;
> +#define handle_arch_irq runtime_const_ptr(_handle_arch_irq)

This breaks the default case, since handle_arch_irq is initialized to a
bunch of garbage hex bytes (0x0123456789abcdef).

That means that if set_handle_irq() isn't called, an IRQ will result in
a call to that bogus address rather than default_handle_irq(), which'll
be more difficult to debug.

Mark.

>  void (*handle_arch_fiq)(struct pt_regs *) __ro_after_init = default_handle_fiq;
>  
>  int __init set_handle_irq(void (*handle_irq)(struct pt_regs *))
>  {
> -	if (handle_arch_irq != default_handle_irq)
> +	if (_handle_arch_irq != default_handle_irq)
>  		return -EBUSY;
>  
> -	handle_arch_irq = handle_irq;
> +	_handle_arch_irq = handle_irq;
> +	runtime_const_init(ptr, _handle_arch_irq);
>  	pr_info("Root IRQ handler: %ps\n", handle_irq);
>  	return 0;
>  }
> -- 
> 2.51.0
> 
>