linux-kernel - Re: [for-next][PATCH 3/6] tracing: Only calculate stats of tracepoint benchmarks for 2^32 times

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Date:	Tue, 10 Jun 2014 14:17:54 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [for-next][PATCH 3/6] tracing: Only calculate stats of tracepoint benchmarks for 2^32 times

Hi Steve,

On Fri, 06 Jun 2014 12:30:37 -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
>
> When calculating the average and standard deviation, it is required that
> the count be less than UINT_MAX, otherwise the do_div() will get
> undefined results. After 2^32 counts of data, the average and standard
> deviation should pretty much be set anyway.

[SNIP]
>  	if (bm_cnt > 1) {
>  		/*
>  		 * Apply Welford's method to calculate standard deviation:
>  		 * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
>  		 */
>  		stddev = (u64)bm_cnt * bm_totalsq - bm_total * bm_total;
> -		do_div(stddev, bm_cnt);
> -		do_div(stddev, bm_cnt - 1);
> +		do_div(stddev, (u32)bm_cnt);
> +		do_div(stddev, (u32)bm_cnt - 1);
>  	} else
>  		stddev = 0;

I also think that this if-else can go as it checks bm_cnt == 1 above.

Thanks,
Namhyung
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