[PATCH] sched: fix/optimise calculation of weight-inverse

From: Stephan Bärwolf
Date: Wed May 11 2011 - 12:10:02 EST


This is a multi-part message in MIME format.If the inverse loadweight should be zero, function "calc_delta_mine"
calculates the inverse of "lw->weight" (in 32bit integer ops).

This calculation is actually a little bit impure (because it is
inverting something around "lw-weight"+1), especially when
"lw->weight" becomes smaller. (This could explain some aritmetical
issues for small shares...)

The correct inverse would be 1/lw->weight multiplied by
"WMULT_CONST" for fixcomma-scaling it into integers.
(So WMULT_CONST/lw->weight ...)

For safety it is also important to check if division by zero
could happen...

The old, impure algorithm took two divisions for inverting lw->weight,
the new, more exact one only takes one and an additional unlikely-if.

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@xxxxxxxxxxxxx>
---
kernel/sched.c | 12 +++++++++---
1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 312f8b9..bb55996 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1307,15 +1307,21 @@ calc_delta_mine(unsigned long delta_exec,
unsigned long weight,
{
u64 tmp;

+ tmp = (u64)delta_exec * weight;
+
+ // actually we would have to trap - division by zero - but we stay
and pretend the limit of the operation...
+ if (unlikely(lw->weight == 0)) {
+ if (unlikely(tmp == ((u64)0))) return (unsigned long)0;
+ else return (unsigned long)LONG_MAX;
+ }
+
if (!lw->inv_weight) {
if (BITS_PER_LONG > 32 && unlikely(lw->weight >= WMULT_CONST))
lw->inv_weight = 1;
else
- lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2)
- / (lw->weight+1);
+ lw->inv_weight = WMULT_CONST / lw->weight;
}

- tmp = (u64)delta_exec * weight;
/*
* Check whether we'd overflow the 64-bit multiplication:
*/
--
1.7.3.4



--
Dipl.-Inf. Stephan Bärwolf
Ilmenau University of Technology, Integrated Communication Systems Group
Phone: +49 (0)3677 69 2821, FAX: +49 (0)3677 69 1614
Email: stephan.baerwolf@xxxxxxxxxxxxx,
Web: http://www.tu-ilmenau.de/iks