Re: [RFC PATCH v1 01/37] libperf cpumap: improve idx function

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Riccardo Mancini <rickyman7@gmail.com>
Cc: Ian Rogers <irogers@google.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>, Jiri Olsa <jolsa@redhat.com>,
	linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org
Subject: Re: [RFC PATCH v1 01/37] libperf cpumap: improve idx function
Date: Tue, 31 Aug 2021 15:46:32 -0300	[thread overview]
Message-ID: <YS55COgKH5CRsYE0@kernel.org> (raw)
In-Reply-To: <f1543c15797169c21e8b205a4a6751159180580d.1629490974.git.rickyman7@gmail.com>

Em Sat, Aug 21, 2021 at 11:19:07AM +0200, Riccardo Mancini escreveu:
> >From commit 7074674e7338863e ("perf cpumap: Maintain cpumaps ordered
> and without dups"), perf_cpu_map elements are sorted in ascending order.
> 
> This patch improves the perf_cpu_map__idx function by using a binary
> search.

Why not use bsearch()?  See 'man bsearch' and look at the example.

- Arnaldo

> Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
> ---
>  tools/lib/perf/cpumap.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/lib/perf/cpumap.c b/tools/lib/perf/cpumap.c
> index ca0215047c326af4..fb633272be3aaed9 100644
> --- a/tools/lib/perf/cpumap.c
> +++ b/tools/lib/perf/cpumap.c
> @@ -265,11 +265,18 @@ bool perf_cpu_map__empty(const struct perf_cpu_map *map)
>  
>  int perf_cpu_map__idx(struct perf_cpu_map *cpus, int cpu)
>  {
> -	int i;
> +	int low = 0, high = cpus->nr, idx, cpu_at_idx;
>  
> -	for (i = 0; i < cpus->nr; ++i) {
> -		if (cpus->map[i] == cpu)
> -			return i;
> +	while (low < high) {
> +		idx = (low + high) / 2;
> +		cpu_at_idx = cpus->map[idx];
> +
> +		if (cpu_at_idx == cpu)
> +			return idx;
> +		else if (cpu_at_idx > cpu)
> +			high = idx;
> +		else
> +			low = idx+1;
>  	}
>  
>  	return -1;
> -- 
> 2.31.1

-- 

- Arnaldo