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    SubjectRe: [PATCH] sched: Avoid division by zero - really
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    On Thu, 2009-08-27 at 14:19 +0200, Eric Dumazet wrote:
    > Peter Zijlstra a écrit :
    > > When re-computing the shares for each task group's cpu representation we
    > > need the ratio of weight on each cpu vs the total weight of the sched
    > > domain.
    > >
    > > Since load-balancing is loosely (read not) synchronized, the weight of
    > > individual cpus can change between doing the sum and calculating the
    > > ratio.
    > >
    > > The previous patch dealt with only one of the race scenarios, this patch
    > > side steps them all by saving a snapshot of all the individual cpu
    > > weights, thereby always working on a consistent set.
    > >
    > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > > ---
    > > kernel/sched.c | 50 +++++++++++++++++++++++++++++---------------------
    > > 1 files changed, 29 insertions(+), 21 deletions(-)
    > >
    > > diff --git a/kernel/sched.c b/kernel/sched.c
    > > index 0e76b17..4591054 100644
    > > --- a/kernel/sched.c
    > > +++ b/kernel/sched.c
    > > @@ -1515,30 +1515,29 @@ static unsigned long cpu_avg_load_per_task(int cpu)
    > >
    > > #ifdef CONFIG_FAIR_GROUP_SCHED
    > >
    > > +struct update_shares_data {
    > > + unsigned long rq_weight[NR_CPUS];
    > > +};
    > > +
    > > +static DEFINE_PER_CPU(struct update_shares_data, update_shares_data);
    >
    > ouch... thats quite large IMHO, up to 4096*8 = 32768 bytes per cpu...
    >
    > Now we have nice dynamic per cpu allocations, we could use it here,
    > and use nr_cpus instead of NR_CPUS as the array size ?

    Possibly, but I guess that should include stuff like
    static_sched_{domain,group} too, since they seem to have the same
    problem.

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