linux-kernel - [PATCH v2] perf cpumap: Reduce cpu size from int to int16

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Message-Id: <20241220185207.106161-1-irogers@google.com>
Date: Fri, 20 Dec 2024 10:52:07 -0800
From: Ian Rogers <irogers@...gle.com>
To: 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>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, James Clark <james.clark@...aro.org>, 
	Tim Chen <tim.c.chen@...ux.intel.com>, 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: [PATCH v2] perf cpumap: Reduce cpu size from int to int16_t

Fewer than 32k logical CPUs are currently supported by perf. A cpumap
is indexed by an integer (see perf_cpu_map__cpu) yielding a perf_cpu
that wraps a 4-byte int for the logical CPU - the wrapping is done
deliberately to avoid confusing a logical CPU with an index into a
cpumap. Using a 4-byte int within the perf_cpu is larger than required
so this patch reduces it to the 2-byte int16_t. For a cpumap
containing 16 entries this will reduce the array size from 64 to 32
bytes. For very large servers with lots of logical CPUs the size
savings will be greater.

Signed-off-by: Ian Rogers <irogers@...gle.com>
Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
v2. Rebase and tweak commit message.
---
 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 188a667babc6..8c1ab0f9194e 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 610c57da5b37..961a92545039 100644
--- a/tools/perf/util/env.c
+++ b/tools/perf/util/env.c
@@ -543,7 +543,7 @@ 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++;
-- 
2.47.1.613.gc27f4b7a9f-goog