Re: [PATCH v1] perf stat: Additional verbose details for <not supported> events

[Namhyung Kim <namhyung@kernel.org>]

On Sun, Oct 05, 2025 at 11:14:21AM -0700, Ian Rogers wrote:
> If an event shows as "<not supported>" in perf stat output, in verbose
> mode add the strerror output to help diagnose the issue.
> 
> Consider:
> ```
> $ perf stat -e cycles,data_read,instructions true
> 
>  Performance counter stats for 'true':
> 
>            357,457      cycles:u
>    <not supported> MiB  data_read:u
>            156,182      instructions:u                   #    0.44  insn per cycle
> 
>        0.001250315 seconds time elapsed
> 
>        0.001283000 seconds user
>        0.000000000 seconds sys
> ```
> 
> To understand why the data_read uncore event failed, with this change:
> ```
> $ perf stat -v -e cycles,data_read,instructions true
> Using CPUID GenuineIntel-6-8D-1
> cycles -> cpu/cycles/
> data_read -> uncore_imc_free_running_0/data_read/
> data_read -> uncore_imc_free_running_1/data_read/
> instructions -> cpu/instructions/
> Control descriptor is not initialized
> Warning:
> kernel.perf_event_paranoid=2, trying to fall back to excluding kernel and hypervisor  samples
> Warning:
> kernel.perf_event_paranoid=2, trying to fall back to excluding kernel and hypervisor  samples
> Warning:
> kernel.perf_event_paranoid=2, trying to fall back to excluding kernel and hypervisor  samples
> Warning:
> data_read:u event is not supported by the kernel.
> Invalid event (data_read:u) in per-thread mode, enable system wide with '-a'.
> Warning:
> kernel.perf_event_paranoid=2, trying to fall back to excluding kernel and hypervisor  samples
> Warning:
> data_read:u event is not supported by the kernel.
> Invalid event (data_read:u) in per-thread mode, enable system wide with '-a'.
> cycles:u: 351621 362833 362833
> failed to read counter data_read:u
> failed to read counter data_read:u
> instructions:u: 156184 362833 362833
> 
>  Performance counter stats for 'true':
> 
>            351,621      cycles:u
>    <not supported> MiB  data_read:u
>            156,184      instructions:u                   #    0.44  insn per cycle
> 
>        0.001584472 seconds time elapsed
> 
>        0.001811000 seconds user
>        0.000000000 seconds sys
> ```
> where without this change only "data_read:u event is not supported by
> the kernel." is shown.

I think what you say is:

Before:
  data_read:u event is not supported by the kernel.

After:
  data_read:u event is not supported by the kernel.
  Invalid event (data_read:u) in per-thread mode, enable system wide with '-a'.
 

Off-topic, it'd be great if we reduce the number of the same warning
messages.  I think the data_read is from two uncore PMUs so the message
was repeated.  If we can connect the related evsels and show the
messages once, then the output is more readable.  Maybe we also want to
show the fallback message just once (globally).

Thanks,
Namhyung

> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/perf/builtin-stat.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> index 7006f848f87a..84e06ec09cc2 100644
> --- a/tools/perf/builtin-stat.c
> +++ b/tools/perf/builtin-stat.c
> @@ -624,8 +624,9 @@ static enum counter_recovery stat_handle_error(struct evsel *counter, int err)
>  	 */
>  	if (err == EINVAL || err == ENOSYS || err == ENOENT || err == ENXIO) {
>  		if (verbose > 0) {
> -			ui__warning("%s event is not supported by the kernel.\n",
> -				    evsel__name(counter));
> +			evsel__open_strerror(counter, &target, err, msg, sizeof(msg));
> +			ui__warning("%s event is not supported by the kernel.\n%s\n",
> +				    evsel__name(counter), msg);
>  		}
>  		return COUNTER_SKIP;
>  	}
> @@ -649,10 +650,11 @@ static enum counter_recovery stat_handle_error(struct evsel *counter, int err)
>  		}
>  	}
>  	if (verbose > 0) {
> +		evsel__open_strerror(counter, &target, err, msg, sizeof(msg));
>  		ui__warning(err == EOPNOTSUPP
> -			? "%s event is not supported by the kernel.\n"
> -			: "skipping event %s that kernel failed to open.\n",
> -			evsel__name(counter));
> +			? "%s event is not supported by the kernel.\n%s\n"
> +			: "skipping event %s that kernel failed to open.\n%s\n",
> +			evsel__name(counter), msg);
>  	}
>  	return COUNTER_SKIP;
>  }
> -- 
> 2.51.0.618.g983fd99d29-goog
>