Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

[Peter Zijlstra]

On Tue, 2012-04-24 at 00:21 +0100, Tommaso Cucinotta wrote:
> > Yes I can do it for x86_64, but people tend to get mighty upset if you
> > break the compile for all other arches...
> 
> rather than breaking compile, I was thinking more of using the 
> optimization for a more accurate comparison on archs that have 64-bit 
> mul and 128-bit cmp, and leaving the overflow on other archs. Though, 
> that would imply a difference in behavior on those borderline cases 
> (very big periods I guess).
> 
> However, I'm also puzzled from what would happen by compiling the 
> current code on mostly 16-bit micros which have very limited 32-bit 
> operations... 

We don't support 16bit archs, 32bit is almost useless as it is :-)

Anyway, how about something like this, I guess archs can go wild and add
asm/math128.h if they want etc..

Completely untested, hasn't even seen a compiler..

---
Subject: math128: Add {add,mult,cmp}_u128
From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date: Tue Apr 24 11:47:12 CEST 2012


Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 include/linux/math128.h |   75 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

--- /dev/null
+++ b/include/linux/math128.h
@@ -0,0 +1,75 @@
+#ifndef _LINUX_MATH128_H
+#define _LINUX_MATH128_H
+
+#include <linux/types.h>
+
+typedef struct {
+	u64 hi, lo;
+} u128;
+
+u128 add_u128(u128 a, u128 b)
+{
+	u128 res;
+
+	res.hi = a.hi + b.hi;
+	res.lo = a.lo + b.lo;
+
+	if (res.lo < a.lo || res.lo < b.lo)
+		res.hi++;
+
+	return res;
+}
+
+/*
+ * a * b = (ah * 2^32 + al) * (bh * 2^32 + bl) =
+ *   ah*bh * 2^64 + (ah*bl + bh*al) * 2^32 + al*bl
+ */
+u128 mult_u128(u64 a, u64 b)
+{
+	u128 res;
+	u64 ah, al;
+	u64 bh, bl;
+	u128 t1, t2, t3, t4;
+
+	ah = a >> 32;
+	al = a & ((1ULL << 32) - 1);
+
+	bh = b >> 32;
+	bl = b & ((1ULL << 32) - 1);
+
+	t1.lo = 0;
+	t1.hi = ah * bh;
+
+	t2.lo = ah * bl;
+	t2.hi = t2.lo >> 32;
+	t2.lo <<= 32;
+
+	t3.lo = al * bh;
+	t3.hi = t3.lo >> 32;
+	t3.lo <<= 32;
+
+	t4.lo = al * bl;
+	t4.hi = 0;
+
+	res = add_u128(t1, t2);
+	res = add_u128(res, t3);
+	res = add_u128(res, t4);
+
+	return res;
+}
+
+int cmp_u128(u128 a, u128 b)
+{
+	if (a.hi > b.hi)
+		return 1;
+	if (a.hi < b.hi)
+		return -1;
+	if (a.lo > b.lo)
+		return 1;
+	if (a.lo < b.lo)
+		return -1;
+
+	return 0;
+}
+
+#endif /* _LINUX_MATH128_H */

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