Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753313Ab3FQRHr (ORCPT ); Mon, 17 Jun 2013 13:07:47 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:25341 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753244Ab3FQRHn (ORCPT ); Mon, 17 Jun 2013 13:07:43 -0400 X-Authority-Analysis: v=2.0 cv=Du3UCRD+ c=1 sm=0 a=rXTBtCOcEpjy1lPqhTCpEQ==:17 a=mNMOxpOpBa8A:10 a=9-gx877wJvsA:10 a=5SG0PmZfjMsA:10 a=IkcTkHD0fZMA:10 a=meVymXHHAAAA:8 a=KGjhK52YXX0A:10 a=sAMK0X7KwfYA:10 a=pGLkceISAAAA:8 a=20KFwNOVAAAA:8 a=m6v0A_qGWvULShiTw24A:9 a=QEXdDO2ut3YA:10 a=MSl-tDqOz04A:10 a=jeBq3FmKZ4MA:10 a=jEp0ucaQiEUA:10 a=SJowZ604iqnVpbTL:21 a=o85v4qbudk0yLlV9:21 a=rXTBtCOcEpjy1lPqhTCpEQ==:117 X-Cloudmark-Score: 0 X-Authenticated-User: X-Originating-IP: 74.67.115.198 Message-ID: <1371488862.18733.21.camel@gandalf.local.home> Subject: Re: [PATCH] ftrace: fix stddev calculation From: Steven Rostedt To: Juri Lelli Cc: fweisbec@gmail.com, mingo@redhat.com, linux-kernel@vger.kernel.org Date: Mon, 17 Jun 2013 13:07:42 -0400 In-Reply-To: <1371031398-24048-1-git-send-email-juri.lelli@gmail.com> References: <1371031398-24048-1-git-send-email-juri.lelli@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4-3 Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1763 Lines: 52 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 I pulled this into my 3.11 queue. Thanks! -- Steve > 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); -- 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/