linux-kernel - Re: VolanoMark regression with 2.6.27-rc1

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Date:	Thu, 21 Aug 2008 01:10:36 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	adobriyan@...il.com, Ingo Molnar <mingo@...e.hu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ibm.com>,
	Chris Friesen <cfriesen@...tel.com>
Subject: Re: VolanoMark regression with 2.6.27-rc1

On Thursday 21 August 2008 00:33, Peter Zijlstra wrote:
> On Wed, 2008-08-20 at 18:32 +0400, adobriyan@...il.com wrote:
> > On Wed, Aug 20, 2008 at 03:32:17PM +0200, Peter Zijlstra wrote:

> > > +#define avg(x, y) ({				\
> > > +	typeof(x) _avg1 = ((x)+1)/2;		\
> > > +	typeof(x) _avg2 = ((y)+1)/2;		\
> >
> > ITYM, typeof(y)
>
> you thought right, I did mean that :-)
>
> > > +	(void) (&_avg1 == &_avg2);		\
> > > +	_avg1 + _avg2; })

I don't think this implementation of avg should go in kernel.h?

It gives an average of 1 and 1 to be 2, 3 and 3 is 4, 1 and 3 is
3 etc.

Maybe it is reasonable for very high numbers that would overflow
if added first, but it doesn't seem reasonable for a generic
averaging function.
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