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

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Date:	Wed, 20 Aug 2008 18:51:43 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ray Lee <ray-lk@...rabbit.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, 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 Wed, 2008-08-20 at 09:29 -0700, Ray Lee wrote:
> On Wed, Aug 20, 2008 at 8:10 AM, Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > 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.
> 
> The usual way of averaging numbers that may be large is
> 
> #define avg(x, y) ({            \
>        typeof(x) _x = (x);      \
>        typeof(x) _y = (y);      \
>        (void) (&_x == &_y);     \
>        _x + (_y - _x)/2; })
> 
> ....which also works for small and negative numbers.

D'oh, why didn't I think of that..

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