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    SubjectRe: [patch 6/8] pull RT tasks
    Hi Steven,

    agreed with your comments in the previous message. Indeed, I missed some points.

    > On wakeup, we can wake up several RT tasks (as my test case does) and if
    > we only push one task, then the other tasks may not migrate over to the
    > other run queues. I logged this happening in my tests.

    I guess, what may happen is that while we are running push_rt_tasks()
    on CPU-k (say, as a result on try_to_wake_up(task_1))
    and as this_rq->lock may get released (in double_lock_balance()) , we
    may get in a 'race'
    with try_to_wake_up(task_2) from (another) CPU-m.
    It places a woken up task on the same run-queue (for which
    push_rt_task() is already running on CPU-k) and, actually, run
    push_rt_task() for the same rq again!

    So it may happen that both task_1 and task_2 will be pushed from the same CPU...

    Do you see an error in my description? (it's a late hour so I can miss
    something again ... sure, otherwise I'm almost perfect :-/ ) Can it
    correlate with what you have observed in your tests?

    Otherwise, there is 1:1 relation : push_rt_task() is called for every
    new (single) task activated by try_to_wake_up() and for a preempted
    task... so it's not like a few tasks are placed on the run-queue and
    then push_rt_tasks() is called once.

    btw., if this scenario may take place... maybe it would make sense to
    have something like RTLB_INPROGRESS/PENDING and to avoid competing
    push_rt_tasks() calls for the same 'rq' from different CPUs?
    (although, there can be some disadvantages here as well. e.g. we would
    likely need to remove 'max 3 tasks at once' limit and get,
    theoretically, unbounded time spent in push_rt_tasks() on a single

    > -- Steve

    Best regards,
    Dmitry Adamushko
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      Last update: 2007-10-23 00:37    [W:0.021 / U:0.640 seconds]
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