Subject: Re: [PATCH 2/5] perf cpumap: More cpu map reuse by merge.

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Mon, 28 Mar 2022 17:26:24 -0300

Em Sun, Mar 27, 2022 at 11:24:11PM -0700, Ian Rogers escreveu:
> perf_cpu_map__merge will reuse one of its arguments if they are equal or
> the other argument is NULL. The arguments could be reused if it is known
> one set of values is a subset of the other. For example, a map of 0-1
> and a map of just 0 when merged yields the map of 0-1. Currently a new
> map is created rather than adding a reference count to the original 0-1
> map.
> 
> Signed-off-by: Ian Rogers <[email-masked]>
> ---
>  tools/lib/perf/cpumap.c | 38 ++++++++++++++++++++++++++++----------
>  1 file changed, 28 insertions(+), 10 deletions(-)
> 
> diff --git a/tools/lib/perf/cpumap.c b/tools/lib/perf/cpumap.c
> index ee66760f1e63..953bc50b0e41 100644
> --- a/tools/lib/perf/cpumap.c
> +++ b/tools/lib/perf/cpumap.c
> @@ -319,6 +319,29 @@ struct perf_cpu perf_cpu_map__max(struct perf_cpu_map *map)
>  	return map->nr > 0 ? map->map[map->nr - 1] : result;
>  }
>  
> +/** Is 'b' a subset of 'a'. */
> +static bool perf_cpu_map__is_subset(const struct perf_cpu_map *a,
> +				    const struct perf_cpu_map *b)
> +{
> +	int i, j;
> +
> +	if (a == b || !b)
> +		return true;
> +	if (!a || b->nr > a->nr)
> +		return false;
> +	j = 0;
> +	for (i = 0; i < a->nr; i++) {

Since the kernel bumped the minimum gcc version to one that supports
declaring loop variables locally and that perf has been using this since
forever:

⬢[acme@toolbox perf]$ grep -r '(int [[:alpha:]] = 0;' tools/perf
tools/perf/util/block-info.c:	for (int i = 0; i < nr_hpps; i++)
tools/perf/util/block-info.c:	for (int i = 0; i < nr_hpps; i++) {
tools/perf/util/block-info.c:	for (int i = 0; i < nr_reps; i++)
tools/perf/util/stream.c:	for (int i = 0; i < nr_evsel; i++)
tools/perf/util/stream.c:	for (int i = 0; i < nr_evsel; i++) {
tools/perf/util/stream.c:	for (int i = 0; i < els->nr_evsel; i++) {
tools/perf/util/stream.c:	for (int i = 0; i < es_pair->nr_streams; i++) {
tools/perf/util/stream.c:	for (int i = 0; i < es_base->nr_streams; i++) {
tools/perf/util/cpumap.c:		for (int j = 0; j < c->nr; j++) {
tools/perf/util/mem-events.c:	for (int j = 0; j < PERF_MEM_EVENTS__MAX; j++) {
tools/perf/util/header.c:	for (int i = 0; i < ff->ph->env.nr_hybrid_cpc_nodes; i++) {
tools/perf/builtin-diff.c:	for (int i = 0; i < num; i++)
tools/perf/builtin-diff.c:		for (int i = 0; i < pair->block_info->num; i++) {
tools/perf/builtin-stat.c:	for (int i = 0; i < perf_cpu_map__nr(a->core.cpus); i++) {
⬢[acme@toolbox perf]$

And this builds on all my test containers, please use:

	for (int i = 0, j = 0; i < a->nr; i++)

In this case to make the source code more compact.

> +		if (a->map[i].cpu > b->map[j].cpu)
> +			return false;
> +		if (a->map[i].cpu == b->map[j].cpu) {
> +			j++;
> +			if (j == b->nr)
> +				return true;

Ok, as its guaranteed that cpu_maps are ordered.

> +		}
> +	}
> +	return false;
> +}
> +
>  /*
>   * Merge two cpumaps
>   *
> @@ -335,17 +358,12 @@ struct perf_cpu_map *perf_cpu_map__merge(struct perf_cpu_map *orig,
>  	int i, j, k;
>  	struct perf_cpu_map *merged;
>  
> -	if (!orig && !other)
> -		return NULL;
> -	if (!orig) {
> -		perf_cpu_map__get(other);
> -		return other;
> -	}
> -	if (!other)
> -		return orig;
> -	if (orig->nr == other->nr &&
> -	    !memcmp(orig->map, other->map, orig->nr * sizeof(struct perf_cpu)))
> +	if (perf_cpu_map__is_subset(orig, other))
>  		return orig;

Can't we have first the introduction of perf_cpu_map__is_subset() and
then another patch that gets the refcount, i.e. the four lines below?

> +	if (perf_cpu_map__is_subset(other, orig)) {
> +		perf_cpu_map__put(orig);
> +		return perf_cpu_map__get(other);
> +	}
>  
>  	tmp_len = orig->nr + other->nr;
>  	tmp_cpus = malloc(tmp_len * sizeof(struct perf_cpu));
> -- 
> 2.35.1.1021.g381101b075-goog

-- 

- Arnaldo