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sched: 64-bit: fix arithmetics overflow

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(overflow means weight >= 2^32 here, because inv_weigh = 2^32/weight)

A weight of a cfs_rq is the sum of weights of which entities
are queued on this cfs_rq, so it will overflow when there are
too many entities.

Although, overflow occurs very rarely, but it break fairness when
it occurs. 64-bits systems have more memory than 32-bit systems
and 64-bit systems can create more process usually, so overflow may
occur more frequently.

This patch guarantees fairness when overflow happens on 64-bit systems.
Thanks to the optimization of compiler, it changes nothing on 32-bit.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Lai Jiangshan 2008-06-12 16:43:07 +08:00 committed by Ingo Molnar
parent 2e084786f6
commit 7a232e0350
1 changed files with 7 additions and 2 deletions
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9
kernel/sched.c
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@ -1340,8 +1340,13 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
{
u64 tmp;
if (!lw->inv_weight)
lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2)/(lw->weight+1);
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);
}
tmp = (u64)delta_exec * weight;
/*