linux-kernel - Re: [PATCH] ftrace: fix stddev calculation in function profiler (again)

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Date:   Mon, 26 Mar 2018 12:51:31 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Matthias Schiffer <mschiffer@...verse-factory.net>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: fix stddev calculation in function profiler
 (again)

On Sat, 24 Mar 2018 17:26:38 +0100
Matthias Schiffer <mschiffer@...verse-factory.net> wrote:

> @@ -905,8 +898,20 @@ static void profile_graph_return(struct ftrace_graph_ret *trace)
>  
>  	rec = ftrace_find_profiled_func(stat, trace->func);
>  	if (rec) {
> +		unsigned long long avg, delta1, delta2;
> +
>  		rec->time += calltime;
> -		rec->time_squared += calltime * calltime;
> +
> +		/* Apply Welford's method */
> +		delta1 = calltime - rec->avg;
> +
> +		avg = rec->time;
> +		do_div(avg, rec->counter);

Can you find a way not to do a divide in every function call?

-- Steve

> +		rec->avg = avg;
> +
> +		delta2 = calltime - rec->avg;
> +
> +		rec->stddev += delta1 * delta2;
>  	}
>  
>   out: