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    SubjectRe: [RFC, PATCH] kernel/rcu: add kfree_rcu
    Lai Jiangshan wrote:
    > I have not posted it. -:)
    Could you post it?

    Paul: What would break if we stop processing rcu entries in (cpu) order?

    The head->func(head) in rcu_do_batch() is probably a nightmare for the
    branch target predictor.

    What about:
    - shrinking struct rcu_head to just a pointer (let's start with the goodie)
    - Adding a register_rcu_callback() function.
    It allocates the per-cpu storage for the rcu grace period lists.
    Seperate lists for each registered callback - thus no need to copy the
    callback target into each rcu_head structure.
    It returns a pointer/handle to these lists.
    - call_rcu gets that handle instead of the plain function pointer.
    - rcu_do_batch enumerates all registered callbacks. Thus first all
    callback_struct->func(head) calls for the first registered callback,
    then the calls for the 2nd callback, etc.
    Better for the icache, better for the branch predictor.

    Paul: Do you have a test case that is suitable for benchmarking rcu?
    Any workloads were rcu appears significantly in oprofile?
    And: Do you know how many rcu entries are typically alive? How much
    memory is used for the function pointers?


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