[PATCH v9 1/2] perf pmu: Recognize 'default_core' as a core PMU and document matching

[Ian Rogers <irogers@google.com>]

The is_pmu_core function checks if a PMU name corresponds to a core
CPU PMU. However, it currently fails to recognize "default_core" as
a core PMU.

When "default_core" is used, the PMU scanning fallback in pmus.c
scans the "other_pmus" list. This scan is slow and always misses because
"default_core" is a core PMU, leading to unnecessary overhead.

Update is_pmu_core to recognize "default_core" directly. Additionally,
document the different matching approaches (exact name for x86/s390,
sysfs-based cpus file check for ARM/hybrid) to clarify how core PMUs are
classified.

Also, explicitly treat "default_core" as `all_pmus` in `setup_metric_events()`
to preserve the original metric resolution behavior for this pseudo-PMU.

Assisted-by: Gemini-CLI:Google Gemini 3.1 Pro
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/metricgroup.c |  3 ++-
 tools/perf/util/pmu.c         | 19 ++++++++++++++++++-
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/metricgroup.c b/tools/perf/util/metricgroup.c
index 5a489e97c413..c2ce3e53aaee 100644
--- a/tools/perf/util/metricgroup.c
+++ b/tools/perf/util/metricgroup.c
@@ -295,7 +295,8 @@ static int setup_metric_events(const char *pmu, struct hashmap *ids,
 	const char *metric_id;
 	struct evsel *ev;
 	size_t ids_size, matched_events, i;
-	bool all_pmus = !strcmp(pmu, "all") || perf_pmus__num_core_pmus() == 1 || !is_pmu_core(pmu);
+	bool all_pmus = !strcmp(pmu, "all") || !strcmp(pmu, "default_core") ||
+			perf_pmus__num_core_pmus() == 1 || !is_pmu_core(pmu);
 
 	*out_metric_events = NULL;
 	ids_size = hashmap__size(ids);
diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
index 23337d2fa281..9994709ef12b 100644
--- a/tools/perf/util/pmu.c
+++ b/tools/perf/util/pmu.c
@@ -2029,9 +2029,26 @@ int perf_pmu__for_each_format(struct perf_pmu *pmu, void *state, pmu_format_call
 	return 0;
 }
 
+/**
+ * is_pmu_core() - Check if the given PMU name corresponds to a core CPU PMU.
+ * @name: The PMU name to check.
+ *
+ * Core PMUs can be identified by:
+ * 1. Exact name match:
+ *    - "cpu": Typically used on x86 architectures.
+ *    - "cpum_cf": Typically used on s390 architectures (CPU Measurement Counter Facility).
+ *    - "default_core": A generic name used to refer to the default core PMU.
+ * 2. Sysfs file existence check (is_sysfs_pmu_core):
+ *    - Typically used on ARM systems or Intel hybrid architectures (e.g., "cpu_atom",
+ *      "cpu_core"). This approach checks if the sysfs directory for the PMU
+ *      contains a "cpus" file.
+ */
 bool is_pmu_core(const char *name)
 {
-	return !strcmp(name, "cpu") || !strcmp(name, "cpum_cf") || is_sysfs_pmu_core(name);
+	return !strcmp(name, "cpu") ||
+	       !strcmp(name, "cpum_cf") ||
+	       !strcmp(name, "default_core") ||
+	       is_sysfs_pmu_core(name);
 }
 
 bool perf_pmu__supports_legacy_cache(const struct perf_pmu *pmu)
-- 
2.54.0.1032.g2f8565e1d1-goog