Messages in this thread | | | Subject | Re: [PATCH] ftrace: fix stddev calculation | From | Steven Rostedt <> | Date | Mon, 17 Jun 2013 13:07:42 -0400 |
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On Wed, 2013-06-12 at 12:03 +0200, Juri Lelli wrote: > 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.lelli@gmail.com>
I pulled this into my 3.11 queue. Thanks!
-- Steve
> Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Ingo Molnar <mingo@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);
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