ftrace: Fix stddev calculation in function profiler

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) .
Link: http://lkml.kernel.org/r/1371031398-24048-1-git-send-email-juri.lelli@gmail.com
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 800a8a2fbddb34ec06381a0cd4cf881af965f7e1..26e19105cdcc4116a5cf22c882491df176f8cb32 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -641,12 +641,18 @@ 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);