perf ftrace: Fix latency stats with BPF

When BPF collects the stats for the latency in usec, it first divides
the time by 1000.  But that means it would have 0 if the delta is small
and won't update the total time properly.

Let's keep the stats in nsec always and adjust to usec before printing.

Before:

  $ sudo ./perf ftrace latency -ab -T mutex_lock --hide-empty -- sleep 0.1
  #   DURATION     |      COUNT | GRAPH                                          |
       0 -    1 us |        765 | #############################################  |
       1 -    2 us |         10 |                                                |
       2 -    4 us |          2 |                                                |
       4 -    8 us |          5 |                                                |

  # statistics  (in usec)
    total time:                    0    <<<--- (here)
      avg time:                    0
      max time:                    6
      min time:                    0
         count:                  782

After:

  $ sudo ./perf ftrace latency -ab -T mutex_lock --hide-empty -- sleep 0.1
  #   DURATION     |      COUNT | GRAPH                                          |
       0 -    1 us |        880 | ############################################   |
       1 -    2 us |         13 |                                                |
       2 -    4 us |          8 |                                                |
       4 -    8 us |          3 |                                                |

  # statistics  (in usec)
    total time:                  268    <<<--- (here)
      avg time:                    0
      max time:                    6
      min time:                    0
         count:                  904

Tested-by: Athira Rajeev <atrajeev@linux.ibm.com>
Cc: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20250227191223.1288473-1-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

2 files changed, 15 insertions, 13 deletions

diff --git a/tools/perf/util/bpf_ftrace.c b/tools/perf/util/bpf_ftrace.c
index 51f407a782d6..7324668cc83e 100644
--- a/tools/perf/util/bpf_ftrace.c
+++ b/tools/perf/util/bpf_ftrace.c
@@ -128,7 +128,7 @@ int perf_ftrace__latency_stop_bpf(struct perf_ftrace *ftrace __maybe_unused)
 	return 0;
 }
 
-int perf_ftrace__latency_read_bpf(struct perf_ftrace *ftrace __maybe_unused,
+int perf_ftrace__latency_read_bpf(struct perf_ftrace *ftrace,
 				  int buckets[], struct stats *stats)
 {
 	int i, fd, err;
@@ -158,6 +158,12 @@ int perf_ftrace__latency_read_bpf(struct perf_ftrace *ftrace __maybe_unused,
 		stats->n = skel->bss->count;
 		stats->max = skel->bss->max;
 		stats->min = skel->bss->min;
+
+		if (!ftrace->use_nsec) {
+			stats->mean /= 1000;
+			stats->max /= 1000;
+			stats->min /= 1000;
+		}
 	}
 
 	free(hist);
diff --git a/tools/perf/util/bpf_skel/func_latency.bpf.c b/tools/perf/util/bpf_skel/func_latency.bpf.c
index 09e70d40a0f4..3d3d9f427c20 100644
--- a/tools/perf/util/bpf_skel/func_latency.bpf.c
+++ b/tools/perf/util/bpf_skel/func_latency.bpf.c
@@ -102,6 +102,7 @@ int BPF_PROG(func_end)
 	start = bpf_map_lookup_elem(&functime, &tid);
 	if (start) {
 		__s64 delta = bpf_ktime_get_ns() - *start;
+		__u64 val = delta;
 		__u32 key = 0;
 		__u64 *hist;
 
@@ -111,26 +112,24 @@ int BPF_PROG(func_end)
 			return 0;
 
 		if (bucket_range != 0) {
-			delta /= cmp_base;
+			val = delta / cmp_base;
 
 			if (min_latency > 0) {
-				if (delta > min_latency)
-					delta -= min_latency;
+				if (val > min_latency)
+					val -= min_latency;
 				else
 					goto do_lookup;
 			}
 
 			// Less than 1 unit (ms or ns), or, in the future,
 			// than the min latency desired.
-			if (delta > 0) { // 1st entry: [ 1 unit .. bucket_range units )
-				// clang 12 doesn't like s64 / u32 division
-				key = (__u64)delta / bucket_range + 1;
+			if (val > 0) { // 1st entry: [ 1 unit .. bucket_range units )
+				key = val / bucket_range + 1;
 				if (key >= bucket_num ||
-					delta >= max_latency - min_latency)
+					val >= max_latency - min_latency)
 					key = bucket_num - 1;
 			}
 
-			delta += min_latency;
 			goto do_lookup;
 		}
 		// calculate index using delta
@@ -146,10 +145,7 @@ do_lookup:
 
 		*hist += 1;
 
-		if (bucket_range == 0)
-			delta /= cmp_base;
-
-		__sync_fetch_and_add(&total, delta);
+		__sync_fetch_and_add(&total, delta); // always in nsec
 		__sync_fetch_and_add(&count, 1);
 
 		if (delta > max)