rtla osnoise hist: average duration is always zero

From: Andreas Ziegler
Date: Sat Dec 24 2022 - 07:48:10 EST


-- Observed in, but not limited to, Linux 6.1.1

Dear all,

rtla osnoise hist always outputs '0' as average duration value. Example:

# rtla osnoise hist -P F:1 -c 0-1 -r 900000 -d 1M -b 1 -E 5000 -T 1
# RTLA osnoise histogram
# Time unit is microseconds (us)
# Duration: 0 00:01:00
...
count: 5629 1364
min: 1 1
avg: 0 0
max: 2955 56

This is due to sum_sample in osnoise_hist_update_multiple() being calculated as the sum (duration), not as sum (duration * count).

Rounding, instead of truncating, of the average value would be cool.

The following patch would solve the issue described above:


Sampled duration must be weighted by observed quantity, to arrive at a
correct average duration value.

Fix calculation of total duration by summing (duration * count).
Introduce rounding for calculation of final value.

Signed-off-by: Andreas Ziegler <br015@xxxxxxxxxx>

--- a/tools/tracing/rtla/src/osnoise_hist.c
+++ b/tools/tracing/rtla/src/osnoise_hist.c
@@ -121,6 +121,7 @@
{
struct osnoise_hist_params *params = tool->params;
struct osnoise_hist_data *data = tool->data;
+ unsigned long long total_duration;
int entries = data->entries;
int bucket;
int *hist;
@@ -131,10 +132,12 @@
if (data->bucket_size)
bucket = duration / data->bucket_size;

+ total_duration = duration * count;
+
hist = data->hist[cpu].samples;
data->hist[cpu].count += count;
update_min(&data->hist[cpu].min_sample, &duration);
- update_sum(&data->hist[cpu].sum_sample, &duration);
+ update_sum(&data->hist[cpu].sum_sample, &total_duration);
update_max(&data->hist[cpu].max_sample, &duration);

if (bucket < entries)
@@ -333,7 +336,7 @@

if (data->hist[cpu].count)
trace_seq_printf(trace->seq, "%9llu ",
- data->hist[cpu].sum_sample / data->hist[cpu].count);
+ (data->hist[cpu].sum_sample + data->hist[cpu].count / 2) / data->hist[cpu].count);
else
trace_seq_printf(trace->seq, " - ");
}


Kind regards,
Andreas