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

[Steven Rostedt]

On Tue, 10 Jun 2014 14:17:54 +0900
Namhyung Kim <namhyung@xxxxxxxxx> wrote:

> Hi Steve,
> 
> On Fri, 06 Jun 2014 12:30:37 -0400, Steven Rostedt wrote:
> > From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
> >
> > 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.

-- Steve
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