linux-kernel - Re: [PATCH v1] perf cpumap: Reduce cpu size from int to int16

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Message-ID: <86ed629f736bcad8ffcb636182442b541b2f8daa.camel@linux.intel.com>
Date: Mon, 09 Dec 2024 10:16:15 -0800
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Ian Rogers <irogers@...gle.com>, Peter Zijlstra <peterz@...radead.org>, 
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa
 <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>, Kan Liang
 <kan.liang@...ux.intel.com>, James Clark <james.clark@...aro.org>, Yicong
 Yang <yangyicong@...ilicon.com>, Ravi Bangoria <ravi.bangoria@....com>,
 linux-perf-users@...r.kernel.org,  linux-kernel@...r.kernel.org, Leo Yan
 <leo.yan@....com>, Kyle Meyer <kyle.meyer@....com>
Subject: Re: [PATCH v1] perf cpumap: Reduce cpu size from int to int16_t

On Fri, 2024-12-06 at 21:21 -0800, Ian Rogers wrote:
> Fewer than 32k CPUs are currently supported by perf. 
> 
Minor suggestions for the commit log. Perhaps it is clearer to say

Perf does not support more than 32k CPUs.

> A cpumap stores
> an int per CPU, so its size is 4 times the number of CPUs in the
> cpumap. 
> 
Perf cpumap currently uses a 4 byte int to index CPU, where
a 2 byte int is sufficient to cover the CPU range needed.

Otherwise

Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>

Tim
> We can reduce the size of the int to an int16_t, saving 2
> bytes per CPU in the map.
> 
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
> This change is on top of:
> https://lore.kernel.org/lkml/20241206044035.1062032-1-irogers@google.com/
> ---
>  tools/lib/perf/include/perf/cpumap.h |  3 ++-
>  tools/perf/util/cpumap.c             | 13 ++++++++-----
>  tools/perf/util/env.c                |  2 +-
>  3 files changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/lib/perf/include/perf/cpumap.h b/tools/lib/perf/include/perf/cpumap.h
> index cbb65e55fc67..760a9aae9884 100644
> --- a/tools/lib/perf/include/perf/cpumap.h
> +++ b/tools/lib/perf/include/perf/cpumap.h
> @@ -4,10 +4,11 @@
>  
>  #include <perf/core.h>
>  #include <stdbool.h>
> +#include <stdint.h>
>  
>  /** A wrapper around a CPU to avoid confusion with the perf_cpu_map's map's indices. */
>  struct perf_cpu {
> -	int cpu;
> +	int16_t cpu;
>  };
>  
>  struct perf_cache {
> diff --git a/tools/perf/util/cpumap.c b/tools/perf/util/cpumap.c
> index 27094211edd8..85e224d8631b 100644
> --- a/tools/perf/util/cpumap.c
> +++ b/tools/perf/util/cpumap.c
> @@ -427,7 +427,7 @@ static void set_max_cpu_num(void)
>  {
>  	const char *mnt;
>  	char path[PATH_MAX];
> -	int ret = -1;
> +	int max, ret = -1;
>  
>  	/* set up default */
>  	max_cpu_num.cpu = 4096;
> @@ -444,10 +444,12 @@ static void set_max_cpu_num(void)
>  		goto out;
>  	}
>  
> -	ret = get_max_num(path, &max_cpu_num.cpu);
> +	ret = get_max_num(path, &max);
>  	if (ret)
>  		goto out;
>  
> +	max_cpu_num.cpu = max;
> +
>  	/* get the highest present cpu number for a sparse allocation */
>  	ret = snprintf(path, PATH_MAX, "%s/devices/system/cpu/present", mnt);
>  	if (ret >= PATH_MAX) {
> @@ -455,8 +457,9 @@ static void set_max_cpu_num(void)
>  		goto out;
>  	}
>  
> -	ret = get_max_num(path, &max_present_cpu_num.cpu);
> -
> +	ret = get_max_num(path, &max);
> +	if (!ret)
> +		max_present_cpu_num.cpu = max;
>  out:
>  	if (ret)
>  		pr_err("Failed to read max cpus, using default of %d\n", max_cpu_num.cpu);
> @@ -606,7 +609,7 @@ size_t cpu_map__snprint(struct perf_cpu_map *map, char *buf, size_t size)
>  #define COMMA first ? "" : ","
>  
>  	for (i = 0; i < perf_cpu_map__nr(map) + 1; i++) {
> -		struct perf_cpu cpu = { .cpu = INT_MAX };
> +		struct perf_cpu cpu = { .cpu = INT16_MAX };
>  		bool last = i == perf_cpu_map__nr(map);
>  
>  		if (!last)
> diff --git a/tools/perf/util/env.c b/tools/perf/util/env.c
> index e2843ca2edd9..f1d7d22e7e98 100644
> --- a/tools/perf/util/env.c
> +++ b/tools/perf/util/env.c
> @@ -531,7 +531,7 @@ int perf_env__numa_node(struct perf_env *env, struct perf_cpu cpu)
>  
>  		for (i = 0; i < env->nr_numa_nodes; i++) {
>  			nn = &env->numa_nodes[i];
> -			nr = max(nr, perf_cpu_map__max(nn->map).cpu);
> +			nr = max(nr, (int)perf_cpu_map__max(nn->map).cpu);
>  		}
>  
>  		nr++;