Re: [PATCH] x86/fred: Optimize the FRED entry by prioritizing high-probability event dispatching

[Nikolay Borisov <nik.borisov@suse.com>]

On 16.01.25 г. 8:51 ч., Ethan Zhao wrote:
> External interrupts (EVENT_TYPE_EXTINT) and system calls (EVENT_TYPE_OTHER)
> occur more frequently than other events in a typical system. Prioritizing
> these events saves CPU cycles and optimizes the efficiency of performance-
> critical paths.
> 
<snip>

Can you also include some performance numbers?

> Signed-off-by: Ethan Zhao <haifeng.zhao@linux.intel.com>
> ---
> base commit: 619f0b6fad524f08d493a98d55bac9ab8895e3a6
> ---
>   arch/x86/entry/entry_fred.c | 25 +++++++++++++++++++------
>   1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
> index f004a4dc74c2..591f47771ecf 100644
> --- a/arch/x86/entry/entry_fred.c
> +++ b/arch/x86/entry/entry_fred.c
> @@ -228,9 +228,18 @@ __visible noinstr void fred_entry_from_user(struct pt_regs *regs)
>   	/* Invalidate orig_ax so that syscall_get_nr() works correctly */
>   	regs->orig_ax = -1;
>   
> -	switch (regs->fred_ss.type) {
> -	case EVENT_TYPE_EXTINT:
> +	if (regs->fred_ss.type == EVENT_TYPE_EXTINT)
>   		return fred_extint(regs);
> +	else if (regs->fred_ss.type == EVENT_TYPE_OTHER)
> +		return fred_other(regs);
> +
> +	/*
> +	 * Dispatch EVENT_TYPE_EXTINT and EVENT_TYPE_OTHER(syscall) type events
> +	 * first due to their high probability and let the compiler create binary search
> +	 * dispatching for the remaining events
> +	 */

nit: At least to me it makes sense to have the comment above the 'if' so 
that the flow is linear.

> +
> +	switch (regs->fred_ss.type) {
>   	case EVENT_TYPE_NMI:
>   		if (likely(regs->fred_ss.vector == X86_TRAP_NMI))
>   			return fred_exc_nmi(regs);
> @@ -245,8 +254,6 @@ __visible noinstr void fred_entry_from_user(struct pt_regs *regs)
>   		break;
>   	case EVENT_TYPE_SWEXC:
>   		return fred_swexc(regs, error_code);
> -	case EVENT_TYPE_OTHER:
> -		return fred_other(regs);
>   	default: break;
>   	}
>   
> @@ -260,9 +267,15 @@ __visible noinstr void fred_entry_from_kernel(struct pt_regs *regs)
>   	/* Invalidate orig_ax so that syscall_get_nr() works correctly */
>   	regs->orig_ax = -1;
>   
> -	switch (regs->fred_ss.type) {
> -	case EVENT_TYPE_EXTINT:
> +	if (regs->fred_ss.type == EVENT_TYPE_EXTINT)
>   		return fred_extint(regs);
> +
> +	/*
> +	 * Dispatch EVENT_TYPE_EXTINT type event first due to its high probability
> +	 * and let the compiler do binary search dispatching for the other events
> +	 */
> +
> +	switch (regs->fred_ss.type) {
>   	case EVENT_TYPE_NMI:
>   		if (likely(regs->fred_ss.vector == X86_TRAP_NMI))
>   			return fred_exc_nmi(regs);