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perf ftrace: Fix latency stats with BPF

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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>
This commit is contained in:
Namhyung Kim
2025-03-13 00:18:10 -07:00
parent 5b562763d7
commit 9c33441418
2 changed files with 15 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files
tools/perf/util/bpf_ftrace.c
+7 -1
View File
@@ -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);
tools/perf/util/bpf_skel/func_latency.bpf.c
+8 -12
View File
@@ -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)