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    SubjectRe: [PATCH v1] sched: fix nohz idle load balancer issues
    * Suresh Siddha <suresh.b.siddha@intel.com> [2011-09-27 16:49:36]:

    > One of the reasons why we saw lib_cpu not idle is probably because that
    > info was stale.
    >
    > Consider this scenario.
    >
    > a. got a tick when the cpu was busy, so idle_at_tick was not set
    > b. cpu went idle
    > c. same cpu got the kick IPI from other busy cpu
    > d. and as it has idle_at_tick not set, it couldn't proceed with the nohz
    > idle balance.

    Good point ..we chould use idle_cpu() instead there ..

    > I think we are mostly likely seeing the above mentioned scenario.
    >
    > Also Vatsa, there is a deadlock associated by using
    > __smp_call_funciton_single() in the nohz_balancer_kick(). So I am
    > planning to remove the IPI that is used to kick the nohz balancer and
    > instead use the resched_cpu logic to kick the nohz balancer.
    >
    > I will post this patch mostly tomorrow. That patch will not use the
    > idle_at_tick check in the nohz_idle_balance(). So that should address
    > your issue in some cases if not most.

    Ok ..would be glad to test your change ..I am however doubtfull if it
    will eliminate rest of the issues I pointed out with nohz load balancer.

    - vatsa


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