perf ftrace latency: allow to hide empty buckets

Especially while using several buckets, it isn't uncommon to have some
of them empty and reading the histogram may be a bit more complex:

  # perf ftrace latency -a -T mutex_lock --bucket-range 5 --max-latency 200
  #   DURATION     |  COUNT | GRAPH                                  |
       0 -    5 us |  14816 | ###################################### |
       5 -   10 us |   1228 | ###                                    |
      10 -   15 us |    438 | #                                      |
      15 -   20 us |    106 |                                        |
      20 -   25 us |     21 |                                        |
      25 -   30 us |     11 |                                        |
      30 -   35 us |      1 |                                        |
      35 -   40 us |      2 |                                        |
      40 -   45 us |      4 |                                        |
      45 -   50 us |      0 |                                        |
      50 -   55 us |      1 |                                        |
      55 -   60 us |      0 |                                        |
      60 -   65 us |      1 |                                        |
      65 -   70 us |      1 |                                        |
      70 -   75 us |      1 |                                        |
      75 -   80 us |      2 |                                        |
      80 -   85 us |      0 |                                        |
      85 -   90 us |      1 |                                        |
      90 -   95 us |      0 |                                        |
      95 -  100 us |      1 |                                        |
     100 -  105 us |      0 |                                        |
     105 -  110 us |      0 |                                        |
     110 -  115 us |      0 |                                        |
     115 -  120 us |      0 |                                        |
     120 -  125 us |      1 |                                        |
     125 -  130 us |      0 |                                        |
     130 -  135 us |      0 |                                        |
     135 -  140 us |      1 |                                        |
     140 -  145 us |      0 |                                        |
     145 -  150 us |      0 |                                        |
     150 -  155 us |      0 |                                        |
     155 -  160 us |      0 |                                        |
     160 -  165 us |      0 |                                        |
     165 -  170 us |      0 |                                        |
     170 -  175 us |      0 |                                        |
     175 -  180 us |      0 |                                        |
     180 -  185 us |      0 |                                        |
     185 -  190 us |      0 |                                        |
     190 -  195 us |      0 |                                        |
     195 -  200 us |      0 |                                        |
     200 -  ... us |      2 |                                        |

Allow the optional flag --hide-empty to remove buckets with no element
and produce a more compact graph. This feature could be misleading since
there is no clear indication for missing buckets, for this reason it's
disabled by default.

  # perf ftrace latency -a -T mutex_lock --bucket-range 5 --max-latency --hide-empty 200
  #   DURATION     |  COUNT | GRAPH                                  |
       0 -    5 us |  14816 | ###################################### |
       5 -   10 us |   1228 | ###                                    |
      10 -   15 us |    438 | #                                      |
      15 -   20 us |    106 |                                        |
      20 -   25 us |     21 |                                        |
      25 -   30 us |     11 |                                        |
      30 -   35 us |      1 |                                        |
      35 -   40 us |      2 |                                        |
      40 -   45 us |      4 |                                        |
      50 -   55 us |      1 |                                        |
      60 -   65 us |      1 |                                        |
      65 -   70 us |      1 |                                        |
      70 -   75 us |      1 |                                        |
      75 -   80 us |      2 |                                        |
      85 -   90 us |      1 |                                        |
      95 -  100 us |      1 |                                        |
     120 -  125 us |      1 |                                        |
     135 -  140 us |      1 |                                        |
     200 -  ... us |      2 |                                        |

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20250207080446.77630-2-gmonaco@redhat.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

2 files changed, 12 insertions, 3 deletions

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index 4f76094ea06d..7caa18d5ffc3 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -841,14 +841,17 @@ static void display_histogram(struct perf_ftrace *ftrace, int buckets[])
 
 	bar_len = buckets[0] * bar_total / total;
 
-	printf("  %4d - %4d %s | %10d | %.*s%*s |\n",
-	       0, min_latency ?: 1, use_nsec ? "ns" : "us",
-	       buckets[0], bar_len, bar, bar_total - bar_len, "");
+	if (!ftrace->hide_empty || buckets[0])
+		printf("  %4d - %4d %s | %10d | %.*s%*s |\n",
+		       0, min_latency ?: 1, use_nsec ? "ns" : "us",
+		       buckets[0], bar_len, bar, bar_total - bar_len, "");
 
 	for (i = 1; i < bucket_num - 1; i++) {
 		unsigned int start, stop;
 		const char *unit = use_nsec ? "ns" : "us";
 
+		if (ftrace->hide_empty && !buckets[i])
+			continue;
 		if (!ftrace->bucket_range) {
 			start = (1 << (i - 1));
 			stop  = 1 << i;
@@ -884,6 +887,8 @@ print_bucket_info:
 	}
 
 	bar_len = buckets[bucket_num - 1] * bar_total / total;
+	if (ftrace->hide_empty && !buckets[bucket_num - 1])
+		goto print_stats;
 	if (!ftrace->bucket_range) {
 		printf("  %4d - %-4s %s", 1, "...", use_nsec ? "ms" : "s ");
 	} else {
@@ -902,6 +907,7 @@ print_bucket_info:
 	printf(" | %10d | %.*s%*s |\n", buckets[bucket_num - 1],
 	       bar_len, bar, bar_total - bar_len, "");
 
+print_stats:
 	printf("\n# statistics  (in %s)\n", ftrace->use_nsec ? "nsec" : "usec");
 	printf("  total time: %20.0f\n", latency_stats.mean * latency_stats.n);
 	printf("    avg time: %20.0f\n", latency_stats.mean);
@@ -1645,6 +1651,8 @@ int cmd_ftrace(int argc, const char **argv)
 		    "Minimum latency (1st bucket). Works only with --bucket-range."),
 	OPT_UINTEGER(0, "max-latency", &ftrace.max_latency,
 		    "Maximum latency (last bucket). Works only with --bucket-range."),
+	OPT_BOOLEAN(0, "hide-empty", &ftrace.hide_empty,
+		    "Hide empty buckets in the histogram"),
 	OPT_PARENT(common_options),
 	};
 	const struct option profile_options[] = {
diff --git a/tools/perf/util/ftrace.h b/tools/perf/util/ftrace.h
index 395f97b203ea..a9bc47da83a5 100644
--- a/tools/perf/util/ftrace.h
+++ b/tools/perf/util/ftrace.h
@@ -25,6 +25,7 @@ struct perf_ftrace {
 	unsigned int		min_latency;
 	unsigned int		max_latency;
 	unsigned int		bucket_num;
+	bool			hide_empty;
 	int			graph_depth;
 	int			func_stack_trace;
 	int			func_irq_info;