When FUNCTION_GRAPH_TRACER is enabled, ftrace can profile kernel functions
and print basic statistics about them. Unfortunately, running stddev
calculation is wrong. This patch corrects it implementing Welford’s method:
s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2) .
Signed-off-by: Juri Lelli <juri.le...@gmail.com>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Frederic Weisbecker <fweis...@gmail.com>
Cc: Ingo Molnar <mi...@redhat.com>
---
kernel/trace/ftrace.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 6c508ff..6c3fce2 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -622,12 +622,18 @@ static int function_stat_show(struct seq_file *m, void *v)
if (rec->counter <= 1)
stddev = 0;
else {
- stddev = rec->time_squared - rec->counter * avg * avg;
+ /*
+ * Apply Welford's method:
+ * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
+ */
+ stddev = rec->counter * rec->time_squared -
+ rec->time * rec->time;
+
/*
* Divide only 1000 for ns^2 -> us^2 conversion.
* trace_print_graph_duration will divide 1000 again.
*/
- do_div(stddev, (rec->counter - 1) * 1000);
+ do_div(stddev, rec->counter * (rec->counter - 1) * 1000);
}
trace_seq_init(&s);
--
1.7.9.5
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