Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756154Ab3FLKDg (ORCPT ); Wed, 12 Jun 2013 06:03:36 -0400 Received: from mail-wi0-f180.google.com ([209.85.212.180]:51225 "EHLO mail-wi0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752409Ab3FLKDe (ORCPT ); Wed, 12 Jun 2013 06:03:34 -0400 From: Juri Lelli To: rostedt@goodmis.org, fweisbec@gmail.com, mingo@redhat.com Cc: linux-kernel@vger.kernel.org, juri.lelli@gmail.com Subject: [PATCH] ftrace: fix stddev calculation Date: Wed, 12 Jun 2013 12:03:18 +0200 Message-Id: <1371031398-24048-1-git-send-email-juri.lelli@gmail.com> X-Mailer: git-send-email 1.7.9.5 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1589 Lines: 47 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 Cc: Steven Rostedt Cc: Frederic Weisbecker Cc: Ingo Molnar --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/