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    SubjectRe: [PATCH] net: Adjust softirq raising in __napi_schedule
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    From: Jarek Poplawski <jarkao2@gmail.com>
    Date: Wed, 21 Oct 2009 23:39:47 +0200

    > I'm not sure I can understand your question. This patch is mainly to
    > avoid using netif_rx()/netif_rx_ni() pair as a test of proper process
    > context handling; IMHO there're better tools for this (lockdep,
    > WARN_ON's).

    Semantically I think your patch is correct, but I wonder about cost.

    Something that is a simply per-cpu inline "or" operation is now a
    function call and potentially mispredicted branch inside of
    raise_softirq_irqoff().

    And netif_rx() is indeed a fast path for tunnels and other users so
    this does matter.

    I like having people call things in the correct context the function
    was built for, and thus we can avoiryd completely useless operations and
    tests as we can now in netif_rx().

    Makaing things general purpose costs something, and it costs too much
    here for this critical routine, sorry.

    I was just having a talk with Nick Piggin about these kinds of issues
    today, too few people care about these ever encrouching tiny pieces
    of bloat that slow the kernel down gradually over time, and I simply
    won't stand for it when I notice it :-)


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