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    SubjectRe: [patch 3/3] sched: move sched_avg_update() to update_cpu_load()
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    On Mon, 2010-08-16 at 10:46 -0700, Suresh Siddha wrote:

    > There is no guarantee that the original cpu won't be doing this in
    > parallel with nohz idle load balancing cpu.

    Hmm, true.. bugger.

    > > > Fix it by moving the sched_avg_update() to more appropriate update_cpu_load()
    > > > where the CFS load gets updated aswell.
    > >
    > > Right, except it breaks things a bit, at the very least you really need
    > > that update right before reading it, otherwise you can end up with >100%
    > > fractions, which are odd indeed ;-)
    >
    > with the patch, the update always happens before reading it. isn't it?
    >
    > update now happens during the scheduler tick (or during nohz load
    > balancing tick). And the load balancer gets triggered with the tick.
    > So the update (at the tick) should happen before reading it (used by
    > load balancing triggered by the tick). Am I missing something?

    We run the load-balancer in softirq context, on -rt that's a task, and
    we could have ran other (more important) RT tasks between the hardirq
    and the softirq running, which would increase the rt_avg and could thus
    result in >100%.

    But I think we can simply retain the sched_avg_update(rq) in
    sched_rt_avg_update(), that is ran with rq->lock held and should be
    enough to avoid that case.

    We can retain the other bit of you patch, moving sched_avg_update() from
    scale_rt_power() to update_cpu_load(), since that is only concerned with
    lowering the average when there is no actual activity.




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