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    SubjectRe: [patch] sched: add locking when update the task_group's cfs_rq[] array.
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    On Wed, Nov 19, 2008 at 8:54 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
    > On Tue, 2008-11-18 at 22:48 -0800, Ken Chen wrote:
    >> add locking when update the task_group's cfs_rq[] array. tg_shares_up()
    >> can be potentially executed concurrently on multiple CPUs with overlaping
    >> cpu mask depending on where task_cpu() was when a task got woken up. Lack
    >> of any locking while redistribute tg->shares over cfs_rq[] array opens up
    >> a large window for conflict updates and utimately cause corruptions to the
    >> integrity of per cpu cfs_rq shares. Add a tg_lock to protect the operations.
    >
    > I see why you want to do this, but introducing a global lock makes me
    > sad :/

    I wholly agree on the scalability. The bigger the system, the more it
    needs to protect the integrity of cfs_rq[]->shares that the sum still
    adds up to tg->shares. Otherwise, the share distributed on each CPU's
    cfs_rq might go wildly and indirectly leads to fluctuation of
    effective total tg->shares. However, I have the same doubt that this
    will scale on large CPU system. Does CFS really have to iterate the
    whole task_group tree?

    - Ken


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