perf pmus: Make PMU alias name loading lazy

PMU alias names were computed when the first perf_pmu is created,
scanning all PMUs in event sources for a file called alias that
generally doesn't exist. Switch to trying to load the file when all
PMU related files are loaded in lookup. This would cause a PMU name
lookup of an alias name to fail if no PMUs were loaded, so in that
case all PMUs are loaded and the find repeated. The overhead is
similar but in the (very) general case not all PMUs are scanned for
the alias file.

As the overhead occurs once per invocation it doesn't show in perf
bench internals pmu-scan. On a tigerlake machine, the number of openat
system calls for an event of cpu/cycles/ with perf stat reduces from
94 to 69 (ie 25 fewer openat calls).

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Link: https://lore.kernel.org/r/20230925062323.840799-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

1 files changed, 21 insertions, 18 deletions

diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
index 5b417509b8a9..6b1b7f8f00fa 100644
--- a/tools/perf/util/pmu.c
+++ b/tools/perf/util/pmu.c
@@ -960,16 +960,27 @@ perf_pmu__get_default_config(struct perf_pmu *pmu __maybe_unused)
 	return NULL;
 }
 
-const char * __weak
-pmu_find_real_name(const char *name)
+static char *pmu_find_alias_name(struct perf_pmu *pmu, int dirfd)
 {
-	return name;
-}
+	FILE *file = perf_pmu__open_file_at(pmu, dirfd, "alias");
+	char *line = NULL;
+	size_t line_len = 0;
+	ssize_t ret;
 
-const char * __weak
-pmu_find_alias_name(const char *name __maybe_unused)
-{
-	return NULL;
+	if (!file)
+		return NULL;
+
+	ret = getline(&line, &line_len, file);
+	if (ret < 0) {
+		fclose(file);
+		return NULL;
+	}
+	/* Remove trailing newline. */
+	if (ret > 0 && line[ret - 1] == '\n')
+		line[--ret] = '\0';
+
+	fclose(file);
+	return line;
 }
 
 static int pmu_max_precise(int dirfd, struct perf_pmu *pmu)
@@ -980,12 +991,10 @@ static int pmu_max_precise(int dirfd, struct perf_pmu *pmu)
 	return max_precise;
 }
 
-struct perf_pmu *perf_pmu__lookup(struct list_head *pmus, int dirfd, const char *lookup_name)
+struct perf_pmu *perf_pmu__lookup(struct list_head *pmus, int dirfd, const char *name)
 {
 	struct perf_pmu *pmu;
 	__u32 type;
-	const char *name = pmu_find_real_name(lookup_name);
-	const char *alias_name;
 
 	pmu = zalloc(sizeof(*pmu));
 	if (!pmu)
@@ -1018,18 +1027,12 @@ struct perf_pmu *perf_pmu__lookup(struct list_head *pmus, int dirfd, const char
 	pmu->is_core = is_pmu_core(name);
 	pmu->cpus = pmu_cpumask(dirfd, name, pmu->is_core);
 
-	alias_name = pmu_find_alias_name(name);
-	if (alias_name) {
-		pmu->alias_name = strdup(alias_name);
-		if (!pmu->alias_name)
-			goto err;
-	}
-
 	pmu->type = type;
 	pmu->is_uncore = pmu_is_uncore(dirfd, name);
 	if (pmu->is_uncore)
 		pmu->id = pmu_id(name);
 	pmu->max_precise = pmu_max_precise(dirfd, pmu);
+	pmu->alias_name = pmu_find_alias_name(pmu, dirfd);
 	pmu->events_table = perf_pmu__find_events_table(pmu);
 	pmu_add_sys_aliases(pmu);
 	list_add_tail(&pmu->list, pmus);