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    SubjectRe: [PATCH RFC 6/9] RCU priority boosting for preemptible RCU
    On Fri, Oct 05, 2007 at 08:24:21AM -0400, Steven Rostedt wrote:
    >
    >
    >
    > On Fri, 5 Oct 2007, Gautham R Shenoy wrote:
    >
    > > On Mon, Sep 10, 2007 at 11:39:01AM -0700, Paul E. McKenney wrote:
    > >
    > > [snip]
    > >
    > > > +
    > > > +/*
    > > > + * Return the list from which to boost target tasks.
    > > > + * May only be invoked by the booster task, so guaranteed to
    > > > + * already be initialized. Use rcu_boost_dat element least recently
    > > > + * the destination for task blocking in RCU read-side critical sections.
    > > > + */
    > > > +static inline struct rcu_boost_dat *rcu_rbd_boosting(int cpu)
    > > > +{
    > > > + int idx = (rcu_boost_idx + 1) & (RCU_BOOST_ELEMENTS - 1);
    > >
    > > Why is this masking required? When we increment
    > > the rcu_boost_idx in rcu_booster, we do perform a modulo operation
    > > to ensure that it wraps around RCU_BOOST_ELEMENTS.
    >
    > Because we are not masking rcu_boost_idx, we are masking
    > (rcu_boost_idx + 1) which may extend the bounderies of
    > RCU_BOOST_ELEMENTS.

    Thanks!

    But I'm still trying to understand why the (increment + masking)
    is required at all.

    The thread(producer) that requires boosting is added to the element
    with index rcu_boost_idx.

    The booster thread(consumer) increments the rcu_boost_idx to
    (rcu_boost_idx + 1) % RCU_BOOST_ELEMENTS, before it fetches the least
    recently used rcu_boot_dat elements and boost the eligible tasks queued
    in that element.

    So, can't we just return per_cpu(rcu_boost_dat, cpu)[rcu_boost_idx] from
    rcu_rbd_boosting(cpu) ? Isn't that already the least recently used
    element?

    >
    > -- Steve

    Thanks and Regards
    gautham.
    --
    Gautham R Shenoy
    Linux Technology Center
    IBM India.
    "Freedom comes with a price tag of responsibility, which is still a bargain,
    because Freedom is priceless!"
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