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SubjectRe: [PATCH 2/2] sched: 64bit: fix overflow(was: fix divide by zero)
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On Thu, 2008-06-12 at 16:43 +0800, Lai Jiangshan wrote:
> (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 be overflow when entities
> are too many.
>
> Although, overflow is very hardly occurred, but it break fairness when
> it is occurred. And 64bits system have more memory than 32bits system
> and 64bits system can create more process usually, so overflow many be
> occurred more.
>
> This patch guarantee fairness when overflow in 64bits system.
> Thanks to the optimization of compiler, it change nothing in 32bits system.

Yeah,. bit sad for that one branch extra on 64-bit, but I get the
unlikely is as good as we can get it.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> ---
> diff --git a/kernel/sched.c b/kernel/sched.c
> index bfb8ad8..e277533 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -1337,8 +1337,13 @@ 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;
> /*
>
>
>
>



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