[PATCH v8 17/18] perf pmu: Recognize 'default_core' as a core PMU and document matching

[Ian Rogers]

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.

Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 tools/perf/util/pmu.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

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)
-- 
2.54.0.1013.g208068f2d8-goog