linux-kernel - [PATCH v1] perf stat display: Make %f precision consistent

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Message-ID: <20251209173610.2033973-1-irogers@google.com>
Date: Tue,  9 Dec 2025 09:36:10 -0800
From: Ian Rogers <irogers@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	James Clark <james.clark@...aro.org>, Chun-Tse Shao <ctshao@...gle.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...nel.org>
Subject: [PATCH v1] perf stat display: Make %f precision consistent

Commit bc22de9bcdb2 ("perf stat: Display time in precision based on
std deviation") added multirun workload elapsed time. There was an
effort to make the precision in the output most useful for the user,
however, when gathering over runs it means the formatting varies. This
change just makes the output format fixed.

Before:
```
$ while :; do perf stat --null --repeat 3 sleep 0.1 2>&1 | grep elapsed; done
          0.101140 +- 0.000149 seconds time elapsed  ( +-  0.15% )
         0.1011396 +- 0.0000218 seconds time elapsed  ( +-  0.02% )
          0.101331 +- 0.000124 seconds time elapsed  ( +-  0.12% )
^C
$ while :; do perf stat --null --repeat 3 sleep 1 2>&1 | grep elapsed; done
          1.001317 +- 0.000146 seconds time elapsed  ( +-  0.01% )
          1.001377 +- 0.000172 seconds time elapsed  ( +-  0.02% )
           1.00253 +- 0.00131 seconds time elapsed  ( +-  0.13% )
```

After:
```
$ while :; do perf stat --null --repeat 3 sleep 0.1 2>&1 | grep elapsed; done
       0.101406408 +- 0.000064778 seconds time elapsed  ( +-  0.06% )
       0.101367315 +- 0.000027253 seconds time elapsed  ( +-  0.03% )
       0.101434164 +- 0.000084750 seconds time elapsed  ( +-  0.08% )
^C
$ while :; do perf stat --null --repeat 3 sleep 1 2>&1 | grep elapsed; done
       1.001525467 +- 0.000051703 seconds time elapsed  ( +-  0.01% )
       1.001375093 +- 0.000116200 seconds time elapsed  ( +-  0.01% )
       1.001141025 +- 0.000046361 seconds time elapsed  ( +-  0.00% )
```

Suggested-by: Ingo Molnar <mingo@...nel.org>
Closes: https://lore.kernel.org/lkml/aTQRgAOpKyI53TEq@gmail.com/
Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 tools/perf/util/stat-display.c | 26 +++++---------------------
 1 file changed, 5 insertions(+), 21 deletions(-)

diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
index 6d02f84c5691..2ce0602974a1 100644
--- a/tools/perf/util/stat-display.c
+++ b/tools/perf/util/stat-display.c
@@ -1397,21 +1397,12 @@ static void print_header(struct perf_stat_config *config,
 		num_print_iv = 0;
 }
 
-static int get_precision(double num)
-{
-	if (num > 1)
-		return 0;
-
-	return lround(ceil(-log10(num)));
-}
-
-static void print_table(struct perf_stat_config *config,
-			FILE *output, int precision, double avg)
+static void print_table(struct perf_stat_config *config, FILE *output, double avg)
 {
 	char tmp[64];
 	int idx, indent = 0;
 
-	scnprintf(tmp, 64, " %17.*f", precision, avg);
+	scnprintf(tmp, 64, " %17.9f", avg);
 	while (tmp[indent] == ' ')
 		indent++;
 
@@ -1421,8 +1412,7 @@ static void print_table(struct perf_stat_config *config,
 		double run = (double) config->walltime_run[idx] / NSEC_PER_SEC;
 		int h, n = 1 + abs((int) (100.0 * (run - avg)/run) / 5);
 
-		fprintf(output, " %17.*f (%+.*f) ",
-			precision, run, precision, run - avg);
+		fprintf(output, " %17.9f (%+.9f) ", run, run - avg);
 
 		for (h = 0; h < n; h++)
 			fprintf(output, "#");
@@ -1462,17 +1452,11 @@ static void print_footer(struct perf_stat_config *config)
 		}
 	} else {
 		double sd = stddev_stats(config->walltime_nsecs_stats) / NSEC_PER_SEC;
-		/*
-		 * Display at most 2 more significant
-		 * digits than the stddev inaccuracy.
-		 */
-		int precision = get_precision(sd) + 2;
 
 		if (config->walltime_run_table)
-			print_table(config, output, precision, avg);
+			print_table(config, output, avg);
 
-		fprintf(output, " %17.*f +- %.*f seconds time elapsed",
-			precision, avg, precision, sd);
+		fprintf(output, " %17.9f +- %.9f seconds time elapsed", avg, sd);
 
 		print_noise_pct(config, NULL, sd, avg, /*before_metric=*/false);
 	}
-- 
2.52.0.223.gf5cc29aaa4-goog