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authorStephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>2011-05-11 18:03:29 +0200
committerIngo Molnar <mingo@elte.hu>2011-05-16 11:01:18 +0200
commitdb670dac49b5423b39b5e523d28fe32045d71b10 (patch)
tree194220c83cb097504ddf7bcccc7ce93d6a23eac3
parentdb44fc017d5989302713ab4e7f9e922b648f4b59 (diff)
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@tu-ilmenau.de> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/n/tip-0pz0wnyalr4tk4ln11xwumdx@git.kernel.org [ 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@elte.hu>
-rw-r--r--kernel/sched.c6
1 files changed, 3 insertions, 3 deletions
diff --git a/kernel/sched.c b/kernel/sched.c
index 70bec4f1edb..c62acf45d3b 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:
*/