ftrace: fix stddev calculation

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 <[email protected]>

Cc Steven Rostedt <[email protected]>

Cc Frederic Weisbecker <[email protected]>

Cc Ingo Molnar <[email protected]>

---
 kernel/trace/ftrace.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

kernel/trace/ftrace.c +8 -2

index 6c508ff..6c3fce2 100644
@@ -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);