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, 10 insertions, 0 deletions

diff --git a/tools/perf/util/pmus.c b/tools/perf/util/pmus.c
index 64e798e68a2d..ce4931461741 100644
--- a/tools/perf/util/pmus.c
+++ b/tools/perf/util/pmus.c
@@ -37,6 +37,8 @@ static LIST_HEAD(other_pmus);
 static bool read_sysfs_core_pmus;
 static bool read_sysfs_all_pmus;
 
+static void pmu_read_sysfs(bool core_only);
+
 int pmu_name_len_no_suffix(const char *str, unsigned long *num)
 {
 	int orig_len, len;
@@ -124,6 +126,14 @@ struct perf_pmu *perf_pmus__find(const char *name)
 	pmu = perf_pmu__lookup(core_pmu ? &core_pmus : &other_pmus, dirfd, name);
 	close(dirfd);
 
+	if (!pmu) {
+		/*
+		 * Looking up an inidividual PMU failed. This may mean name is
+		 * an alias, so read the PMUs from sysfs and try to find again.
+		 */
+		pmu_read_sysfs(core_pmu);
+		pmu = pmu_find(name);
+	}
 	return pmu;
 }