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    SubjectRe: [RFC][PATCH] make module refcounts use percpu_counters
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    On Mon, 2007-10-01 at 19:43 +1000, Rusty Russell wrote:
    > On Fri, 2007-09-28 at 16:00 -0700, Dave Hansen wrote:
    > > Module refcounts currently use a percpu counter stored
    > > in the 'struct module'. However, we also have a more
    > > generic implementation that does stuff like handle
    > > hotplug cpus.
    > >
    > > I'm not actually all that convinced that this refcount
    > > actually does a lot of good, with cpus racing bumping
    > > the counters at the same time that they're being
    > > summed up. But, it certainly isn't any worse than
    > > what was there before.
    >
    > That's why we look at the counters inside stop_machine_run().

    Ahhh. That makes sense. Although it wasn't apparent during my 3-second
    perusal of the code.

    > Note that (1) the module implementation handles hotplug CPUs


    You're saying it handles hotplug because of stop_machine_run()?


    > But it might be a useful cleanup (although a slight de-optimization).
    > If you want I'll queue for 2.6.24 (there are several other module
    > patches pending too).

    Might as well. It removed a very small amount of code, and opens the
    door a bit for future optimizations in a single place.

    > In an ideal world, (1) we would have percpu pointers using the same
    > percpu mechanism as percpu variables, (2) we would have a modal variant
    > of percpu counters which would collapse to a single counter when we
    > cared about the precise value (probably using stop_machine for the
    > transition). This would be useful for many other cases.

    Yeah, but before we do that, we need some kind of flag to get the
    percpu_counter_mod() fast path shoved into the slow path that takes the
    lock.

    I'm not sure the stop_machine() mechanism will work very well if we try
    to expand this much further for other users. What would the SGI guys
    think if these happened more than once in a blue moon?


    -- Dave

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