On Tue, 10 Jun 2014 09:42:06 -0400
Steven Rostedt <rost...@goodmis.org> wrote:
> On Tue, 10 Jun 2014 14:17:54 +0900
> Namhyung Kim <namhy...@gmail.com> wrote:
>
> > Hi Steve,
> >
> > On Fri, 06 Jun 2014 12:30:37 -0400, Steven Rostedt wrote:
> > > From: "Steven Rostedt (Red Hat)" <rost...@goodmis.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.
> >
>
> I noticed this too, but just to keep gcc from complaining, I left it in.
>
Oh, and you are talking about the entire if () else block. Yeah, I
thought about that as well, but I'm paranoid about doing a divide by
zero ;-)
It's code for testing purposes only. A fringe of the kernel base. Not a
hot path. Slowness isn't an issue here, but correctness is.
Thanks,
-- Steve
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