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

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Message-ID: <2c0942db0808200929r640b3a1cj33efc56cfd6db9b3@mail.gmail.com>
Date:	Wed, 20 Aug 2008 09:29:28 -0700
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>, 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, 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.
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