Subject: Re: [PATCH] powerpc/perf: Optimize find_alternatives_list() using binary search

From: Michael Ellerman <[email-masked]>
Date: Thu, 19 Oct 2023 12:41:45 +1100

Kuan-Wei Chiu <[email-masked]> writes:
> This patch improves the performance of event alternative lookup by
> replacing the previous linear search with a more efficient binary
> search. This change reduces the time complexity for the search process
> from O(n) to O(log(n)). A pre-sorted table of event values and their
> corresponding indices has been introduced to expedite the search
> process.

Thanks for the patch.

How did you test this? I assume you don't have a Power6 machine lying
around? :)

cheers

> diff --git a/arch/powerpc/perf/power6-pmu.c b/arch/powerpc/perf/power6-pmu.c
> index 5729b6e059de..b6030ea130eb 100644
> --- a/arch/powerpc/perf/power6-pmu.c
> +++ b/arch/powerpc/perf/power6-pmu.c
> @@ -335,25 +335,34 @@ static const unsigned int event_alternatives[][MAX_ALT] = {
>  	{ 0x3000fe, 0x400056 },			/* PM_DATA_FROM_L3MISS */
>  };
>  
> -/*
> - * This could be made more efficient with a binary search on
> - * a presorted list, if necessary
> - */
>  static int find_alternatives_list(u64 event)
>  {
> -	int i, j;
> -	unsigned int alt;
> -
> -	for (i = 0; i < ARRAY_SIZE(event_alternatives); ++i) {
> -		if (event < event_alternatives[i][0])
> -			return -1;
> -		for (j = 0; j < MAX_ALT; ++j) {
> -			alt = event_alternatives[i][j];
> -			if (!alt || event < alt)
> -				break;
> -			if (event == alt)
> -				return i;
> -		}
> +	const unsigned int presort_event_table[] = {
> +		0x0130e8, 0x080080, 0x080088, 0x10000a, 0x10000b, 0x10000d, 0x10000e,
> +		0x100010, 0x10001a, 0x100026, 0x100054, 0x100056, 0x1000f0, 0x1000f8,
> +		0x1000fc, 0x200008, 0x20000e, 0x200010, 0x200012, 0x200054, 0x2000f0,
> +		0x2000f2, 0x2000f4, 0x2000f5, 0x2000f6, 0x2000f8, 0x2000fc, 0x2000fe,
> +		0x2d0030, 0x30000a, 0x30000c, 0x300010, 0x300012, 0x30001a, 0x300056,
> +		0x3000f0, 0x3000f2, 0x3000f6, 0x3000f8, 0x3000fc, 0x3000fe, 0x400006,
> +		0x400007, 0x40000a, 0x40000e, 0x400010, 0x400018, 0x400056, 0x4000f0,
> +		0x4000f8, 0x600005};
> +	const unsigned int event_index_table[] = {
> +		0,  1,  2,  3,  4,  1, 5,  6,  7,  8,  9,  10, 11, 12, 13, 12, 14,
> +		7,  15, 2,  9,  16, 3, 4,  0,  17, 10, 18, 19, 20, 1,  17, 15, 19,
> +		18, 2,  16, 21, 8,  0, 22, 13, 14, 11, 21, 5,  20, 22, 1,  6,  3};
> +	int lo = 0;
> +	int hi = ARRAY_SIZE(presort_event_table) - 1;
> +
> +	while (lo <= hi) {
> +		int mid = lo + (hi - lo) / 2;
> +		unsigned int alt = presort_event_table[mid];
> +
> +		if (alt < event)
> +			lo = mid + 1;
> +		else if (alt > event)
> +			hi = mid - 1;
> +		else
> +			return event_index_table[mid];
>  	}
>  	return -1;
>  }
> -- 
> 2.25.1