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    SubjectRe: [PATCH 23/32] Generic dynamic per cpu refcounting
    (cc'ing Andrew)

    On Wed, Dec 26, 2012 at 06:00:02PM -0800, Kent Overstreet wrote:
    > This implements a refcount with similar semantics to
    > atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t
    > but dynamically switches to per cpu refcounting when the rate of
    > gets/puts becomes too high.
    >
    > It also implements two stage shutdown, as we need it to tear down the
    > percpu counts. Before dropping the initial refcount, you must call
    > percpu_ref_kill(); this puts the refcount in "shutting down mode" and
    > switches back to a single atomic refcount with the appropriate barriers
    > (synchronize_rcu()).
    >
    > It's also legal to call percpu_ref_kill() multiple times - it only
    > returns true once, so callers don't have to reimplement shutdown
    > synchronization.
    >
    > For the sake of simplicity/efficiency, the heuristic is pretty simple -
    > it just switches to percpu refcounting if there are more than x gets
    > in one second (completely arbitrarily, 4096).
    >
    > It'd be more correct to count the number of cache misses or something
    > else more profile driven, but doing so would require accessing the
    > shared ref twice per get - by just counting the number of gets(), we can
    > stick that counter in the high bits of the refcount and increment both
    > with a single atomic64_add(). But I expect this'll be good enough in
    > practice.
    >
    > Signed-off-by: Kent Overstreet <koverstreet@google.com>

    What's the status of this series? The percpu-refcnt part is still
    going through review and the merge window is opening up pretty soon.
    Kent, Andrew?

    Thanks.

    --
    tejun


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