Re: [PATCH v4] perf stat: Include PMU name and split uncore events per PMU in metric-only JSON output

[Namhyung Kim]

Hello,

On Tue, Aug 04, 2026 at 02:06:59PM -0700, Chun-Tse Shao wrote:
> When running `perf stat` with `-A` (--no-aggr) and `--metric-only` in
> JSON output mode (`-j`), `perf stat` evaluates metric expressions
> across all matching PMUs (including uncore PMUs like `uncore_iio_0`,
> `uncore_iio_1`, etc.).
> 
> However, `perf stat` previously formatted JSON output by printing only
> "cpu" : "<id>" and grouping all metric values on a single line per CPU
> without identifying which PMU instance evaluated each metric. As a
> result, when an uncore event spans multiple PMU boxes, `perf stat`
> printed repeated, ambiguous metric keys without PMU names.
> 
> Fix this by:
> 1. Including "pmu" : "<pmu_name>" in print_aggr_id_json when evsel->pmu
>    is a non-core or hybrid PMU in AGGR_NONE mode (-A).
> 2. Starting a new JSON metric line in AGGR_NONE mode (-A) whenever the
>    underlying hardware PMU instance changes across PMU events.
> 3. Bypassing line initialization for tool events (e.g. -e user_time) in
>    metric-only mode so that JSON lines are opened only by hardware PMUs,
>    preventing tool events from corrupting PMU name labels or creating
>    spurious empty {"pmu": "tool"} JSON objects.
> 4. Updating perf_json_output_lint.py to recognize the new "pmu" key in
>    the JSON test suite.
> 
> Before the fix:
>   $ perf stat -M iio_bandwidth_read -a -A --metric-only -j -I 1000
>   {"interval" : 1.001017947, "cpu" : "0", "MB/s  iio_bandwidth_read" : "0.0", "MB/s  iio_bandwidth_read" : "0.0", "MB/s  iio_bandwidth_read" : "22.5", "MB/s  iio_bandwidth_read" : "0.0", "MB/s  iio_bandwidth_read" : "0.2", "MB/s  iio_bandwidth_read" : "0.0", "MB/s  iio_bandwidth_read" : "0.0", "MB/s  iio_bandwidth_read" : "0.0", "MB/s  iio_bandwidth_read" : "0.0", "MB/s  iio_bandwidth_read" : "0.0"}
> 
> There is no way to determine which uncore device generated the metrics.
> 
> After:
>   $ perf stat -M iio_bandwidth_read -a -A --metric-only -j -I 1000
>   {"interval" : 1.000314908, "cpu" : "0", "pmu" : "uncore_iio_0", "MB/s  iio_bandwidth_read" : "0.0"}
>   {"interval" : 1.000314908, "cpu" : "0", "pmu" : "uncore_iio_1", "MB/s  iio_bandwidth_read" : "0.1"}
>   {"interval" : 1.000314908, "cpu" : "0", "pmu" : "uncore_iio_11", "MB/s  iio_bandwidth_read" : "0.0"}
>   ...
>   {"interval" : 1.000314908, "cpu" : "56", "pmu" : "uncore_iio_0", "MB/s  iio_bandwidth_read" : "0.0"}
>   {"interval" : 1.000314908, "cpu" : "56", "pmu" : "uncore_iio_1", "MB/s  iio_bandwidth_read" : "0.0"}
>   {"interval" : 1.000314908, "cpu" : "56", "pmu" : "uncore_iio_11", "MB/s  iio_bandwidth_read" : "0.0"}

Can you please fix perf stat JSON output linter test?

  ---- start ----                                                                 
  ...                                                                             
  Checking json output: system wide no aggregation Test failed for input:         
  {"cpu" : "0", "pmu" : "software", "counter-value" : "3.000000", "unit" : "", "event" : "context-switches", "event-runtime" : 6269136, "pcnt-running" : 100.00, "metric-value" : "478.743700", "metric-unit" : "cs/sec  cs_per_second"}
  ...                                                                             
      check_json_output(expected_items)                                           
      ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^                                           
    File "linux/tools/perf/tests/shell/lib/perf_json_output_lint.py", line 89, in check_json_output
      raise RuntimeError(f'wrong number of fields. counted {count} expected {expected_items}'
                         f' in \'{item}\'')                                       
  RuntimeError: wrong number of fields. counted 9 expected [6, 8] in '{'cpu': '0', 'pmu': 'software', 'counter-value': '3.000000', 'unit': '', 'event': 'context-switches', 'event-runtime': 6269136, 'pcnt-running': 100.0, 'metric-value': '478.743700', 'metric-unit': 'cs/sec  cs_per_second'}'
  ---- end(-1) ----                                                               
  119: perf stat JSON output linter                                   : FAILED!

Thanks,
Namhyung