[tip:sched/core] sched: Fix and optimise calculation of the weight-inverse

[tip-bot for Stephan Baerwolf]

Commit-ID:  db670dac49b5423b39b5e523d28fe32045d71b10
Gitweb:     http://git.kernel.org/tip/db670dac49b5423b39b5e523d28fe32045d71b10
Author:     Stephan Baerwolf <stephan.baerwolf@xxxxxxxxxxxxx>
AuthorDate: Wed, 11 May 2011 18:03:29 +0200
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Mon, 16 May 2011 11:01:18 +0200

sched: Fix and optimise calculation of the weight-inverse

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.

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

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>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/n/tip-0pz0wnyalr4tk4ln11xwumdx@xxxxxxxxxxxxxx
[ This could explain some aritmetical issues for small shares but nothing
  concrete has been reported yet so we are not confident enough to queue
  this up in sched/urgent and for -stable backport. But if anyone finds
  this commit and sees it to fix some badness then we can certainly
  change our mind! ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/sched.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 70bec4f..c62acf4 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1330,15 +1330,15 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
 {
 	u64 tmp;
 
+	tmp = (u64)delta_exec * weight;
+
 	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:
 	 */
--
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