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

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Message-ID: <1272406554.9739.78.camel@gandalf.stny.rr.com>
Date:	Tue, 27 Apr 2010 18:15:54 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Chase Douglas <chase.douglas@...onical.com>
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, 2010-04-27 at 17:06 -0400, Chase Douglas wrote:
> 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.

OK, I see that now. I'll add this patch but I'll also fix the comment.

Thanks,

-- Steve


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