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Date: Thu, 21 Aug 2008 10:17:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Nick Piggin <nickpiggin@...oo.com.au>
Cc: Ray Lee <ray-lk@...rabbit.org>, 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 Thu, 2008-08-21 at 16:11 +1000, Nick Piggin wrote:
> On Thursday 21 August 2008 06:56, Peter Zijlstra wrote:
> > On Wed, 2008-08-20 at 22:30 +0200, Peter Zijlstra wrote:
>
> > > works for the above example, but when I make it long long, so as to
> > > match the longest supported type, it goes boom again - for as of yet
> > > unknown reasons.
> >
> > Ok, people pointed out I got my promotion rules mixed up, I casted the
> > result of the division to signed, instead of ending up with a signed
> > division.
> >
> > #define avg(x, y) ({ \
> > typeof(x) _avg1 = (x); \
> > typeof(y) _avg2 = (y); \
> > (void) (&_avg1 == &_avg2); \
> > (typeof(x))(_avg1 + ((long long)_avg2 - _avg1)/2); })
> >
> > seems to work.
>
> Right, I guess that will work, but unfortunately the code gen on 32-bit
> is a monstrosity. If you're going to cast to 64-bit anyway, we might as
> well then just do the normal add rather than playing the game to avoid
> overflow.
>
> Secondly, this is operating on the fixed point scaled load numbers, so in
> the case of the scheduler I wouldn't worry too much about rounding... also
> in most integer operations, rounding down is less surprising than rounding
> up like the last code did.
>
> I still don't know whether it is appropriate to put it into kernel.h
> (because of rounding, and variability when it comes to what type size will
> hold the sum of parameters), but for the scheduler, I would use this:
>
> ((unsigned long long)a + b) / 2;
Right - anyway the point is moot - as Yanmin says it still sucks rocks.
But since I couldn't let it rest :-)
---
Index: linux-2.6/include/linux/kernel.h
===================================================================
--- linux-2.6.orig/include/linux/kernel.h
+++ linux-2.6/include/linux/kernel.h
@@ -367,6 +367,45 @@ static inline char *pack_hex_byte(char *
(void) (&_max1 == &_max2); \
_max1 > _max2 ? _max1 : _max2; })
+#define __avg_t(type, x, y) ({ \
+ typeof(x) __avg1 = (x); \
+ typeof(y) __avg2 = (y); \
+ __avg1 + ((type)(__avg2 - __avg1))/2; })
+
+extern void avg_unknown_size(void);
+
+#define __avg(x, y) ({ \
+ typeof(x) ret; \
+ switch (sizeof(ret)) { \
+ case 1: \
+ ret = __avg_t(s8, x, y); \
+ break; \
+ case 2: \
+ ret = __avg_t(s16, x, y); \
+ break; \
+ case 4: \
+ ret = __avg_t(s32, x, y); \
+ break; \
+ case 8: \
+ ret = __avg_t(s64, x, y); \
+ break; \
+ default: \
+ avg_unknown_size(); \
+ break; \
+ } \
+ ret; })
+
+#define avg(x, y) ({ \
+ typeof(x) _avg1 = (x); \
+ typeof(y) _avg2 = (y); \
+ (void) (&_avg1 == &_avg2); \
+ __avg(_avg1, _avg2); })
+
+#define avg_t(type, x, y) ({ \
+ type _avg1 = (x); \
+ type _avg2 = (y); \
+ __avg(_avg1, _avg2); })
+
/**
* clamp - return a value clamped to a given range with strict typechecking
* @val: current value
--
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