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    SubjectRe: [PATCH tip/core/rcu 3/4] rcu: add expedited grace-period support for preemptible RCU
    Paul E. McKenney wrote:
    > From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    >
    > Implement an synchronize_rcu_expedited() for preemptible RCU that actually
    > is expedited. This uses synchronize_sched_expedited() to force all
    > threads currently running in a preemptible-RCU read-side critical section
    > onto the appropriate ->blocked_tasks[] list, then takes a snapshot of
    > all of these lists and waits for them to drain.
    >

    > 3. Add an implementation of synchronize_rcu_expedited() that
    > actually expedites preemptible-RCU grace periods.

    It's very nice.

    But I don't understand all things.

    1) Why it can be speeded up (in theory)?
    synchronize_sched_expedited() does speed up, it is due to
    migration_threads are the most highest priority threads.

    But for synchronize_rcu_expedited(), some preempted tasks in ->blocked_tasks[]
    may be waiting at runqueue for long long time because some other
    higher priority threads comes.

    simply comparison:
    synchronize_sched_expedited()
    ==> wake_up_process(rq->migration_thread) to force schedule on cpus.
    which forces read-sides notify the end earlier,
    or we can say "it forces read-sides run to end faster"

    synchronize_rcu_expedited()
    ==> Nothing to force preempted read-site run to end faster.

    2) Why we introduce a API which no one use it.
    I remember that Net guys request a expedited synchronize_rcu().
    but currently there is still no one use it.


    Beware my thinking may be wrong!
    Thanks, Lai


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