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    SubjectRe: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq to kthread
    On Wed, Feb 23, 2011 at 12:03:56PM -0500, Mathieu Desnoyers wrote:
    > * Steven Rostedt (rostedt@goodmis.org) wrote:
    > > On Wed, 2011-02-23 at 17:16 +0100, Frederic Weisbecker wrote:
    > > > 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...
    > >
    > > Ah, but so does smp_processor_id() ;-)
    > >
    > > >
    > > > * 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.
    >
    > [Adding Christoph Lameter to CC list]
    >
    > This is not quite true on x86_64 and s390 anymore. __get_cpu_var() now
    > uses a segment selector override to get the local CPU variable on x86.
    > See x86's percpu.h for details.
    >
    > So even performance-wise, using __get_cpu_var() over per_cpu() should be
    > a win on widely used architectures nowadays,

    Looking at x86_64, it indeed optimizes further by overriding this_cpu_ptr().
    It does the same than the generic this_cpu_ptr() on an
    overriden my_cpu_offset, but it also economizes a temporary store.

    >
    > >
    > > True, but we could also argue that the multiple checks for being preempt
    > > can also be an issue.
    >
    > At least on x86 preemption don't actually need to be disabled: selection
    > of the right per-cpu memory location is done atomically with the rest of
    > the instruction by the segment selector.

    It depends on the case, you may still need to disable preemption if you use
    your variable further than just a quick op, which is often the case.

    That's up to this_cpu_add() op things, depending on what the arch is capable
    of wrt. local atomicity.


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