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    SubjectRe: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq to kthread
    On Tue, Feb 22, 2011 at 05:39:40PM -0800, Paul E. McKenney wrote:
    > +/*
    > + * Wake up the current CPU's kthread. This replaces raise_softirq()
    > + * in earlier versions of RCU.
    > + */
    > +static void invoke_rcu_kthread(void)
    > +{
    > + unsigned long flags;
    > + wait_queue_head_t *q;
    > + int cpu;
    > +
    > + local_irq_save(flags);
    > + cpu = smp_processor_id();
    > + if (per_cpu(rcu_cpu_kthread_task, cpu) == NULL) {
    > + local_irq_restore(flags);
    > + return;
    > + }
    > + per_cpu(rcu_cpu_has_work, cpu) = 1;
    > + q = &per_cpu(rcu_cpu_wq, cpu);

    I see you make extensive use of per_cpu() accessors even for
    local variables.

    I tend to think it's better to use __get_cpu_var() for local
    accesses and keep per_cpu() for remote accesses.

    There are several reasons for that:

    * __get_cpu_var() checks we are in a non-preemptible section,
    per_cpu() doesn't. That may sound of a limited interest for code like the
    above, but by the time code can move, and then we might lose track of some
    things, etc...

    * local accesses can be optimized by architectures. per_cpu() implies
    finding the local cpu number, and dereference an array of cpu offsets with
    that number to find the local cpu offset.
    __get_cpu_var() does a direct access to __my_cpu_offset which is a nice
    shortcut if the arch implements it.

    * It makes code easier to review: we know that __get_cpu_var() is
    for local accesses and per_cpu() for remote.

    > + wake_up(q);
    > + local_irq_restore(flags);
    > +}


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