Re: [patch 2.6.15] i386: Optimize local APIC timer interrupt code

[Willy Tarreau <willy@w.ods.org>]

On Wed, Jan 04, 2006 at 01:48:09PM -0500, Chuck Ebbert wrote:
> Local APIC timer interrupt happens HZ times per second on each CPU.
> 
> Optimize it for the case where profile multiplier equals one and does
> not change (99+% of cases); this saves about 20 CPU cycles on Pentium II.
> 
> Also update the old multiplier immediately after noticing it changed,
> while values are register-hot, saving eight bytes of stack depth.
> 
> Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
> 
> diff -up 2.6.15a.orig/arch/i386/kernel/apic.c 2.6.15a/arch/i386/kernel/apic.c
> --- 2.6.15a.orig/arch/i386/kernel/apic.c
> +++ 2.6.15a/arch/i386/kernel/apic.c
> @@ -1137,7 +1137,7 @@ inline void smp_local_timer_interrupt(st
>  	int cpu = smp_processor_id();
>  
>  	profile_tick(CPU_PROFILING, regs);
> -	if (--per_cpu(prof_counter, cpu) <= 0) {
> +	if (likely(--per_cpu(prof_counter, cpu)) <= 0) {
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
are you sure about this ? it looks suspicious to me. I would have
expected something like  this instead :

+	if (likely(--per_cpu(prof_counter, cpu) <= 0)) {

Willy

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