linux-kernel - Re: [PATCH] ftrace: print sample std dev when function profiling

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Date:	Tue, 27 Apr 2010 17:06:12 -0400
From:	Chase Douglas <chase.douglas@...onical.com>
To:	rostedt@...dmis.org
Cc:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] ftrace: print sample std dev when function profiling

On Tue, Apr 27, 2010 at 4:50 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Mon, 2010-04-26 at 14:02 -0400, Chase Douglas wrote:
>
>> +     /* Sample standard deviation (s^2) */
>> +     if (rec->counter <= 1)
>> +             stddev = 0;
>> +     else {
>> +             stddev = rec->time_squared - rec->counter * avg * avg;
>> +             do_div(stddev, (rec->counter - 1) * 1000); /* ns^2 -> us^2 */
>
> Shouldn't this be:
>
>        do_div(stddev, (rec->counter - 1) * 1000000); ?
>
> (x / 1000)^2 == x^2 / 1000^2

The trace_print_graph_duration function divides the value by 1000
again to display in us units. I could make the comment more clear, but
I didn't want to make a big fuss over a unit conversion.

I also figured this out *after* I had some really wrong looking
numbers :). I made sure this patch produced the correct value after
two events, so I assume the math is correct.

-- Chase
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