linux-kernel - Re: [PATCH] ftrace: fix stddev calculation

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Date:	Mon, 17 Jun 2013 13:07:42 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Juri Lelli <juri.lelli@...il.com>
Cc:	fweisbec@...il.com, mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: fix stddev calculation

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@...il.com>

I pulled this into my 3.11 queue. Thanks!

-- Steve

> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...hat.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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