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    SubjectRe: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface
    On Wed, 17 Jun 2009, Gregory Haskins wrote:

    > Can you elaborate? I currently do not see how I could do the proposed
    > concept inside of irqfd while still using eventfd. Of course, that
    > would be possible if we fork irqfd from eventfd, and perhaps this is
    > what you are proposing. As previously stated I don't want to give up on
    > the prospect of re-using it quite yet, so bear with me. :)
    >
    > The issue with eventfd, as I see it, is that eventfd uses a
    > spin_lock_irqsave (by virtue of the wait-queue stuff) across the
    > "signal" callback (which today is implemented as a wake-up). This
    > spin_lock implicitly creates a non-preemptible critical section that
    > occurs independently of whether eventfd_signal() itself is invoked from
    > a sleepable context or not.
    >
    > What I strive to achieve is to remove the creation of this internal
    > critical section. If eventfd_signal() is called from atomic context, so
    > be it. We will detect this in the callback and be forced to take the
    > slow-path, and I am ok with that. *But*, if eventfd_signal() (or
    > f_ops->write(), for that matter) are called from a sleepable context
    > *and* eventfd doesn't introduce its own critical section (such as with
    > my srcu patch), we can potentially optimize within the callback by
    > executing serially instead of deferring (e.g. via a workqueue).

    Since when the scheduling (assuming it's not permanently running on
    another core due to high frequency work post) of a kernel thread is such
    a big impact that interfaces need to be redesigned for that?
    How much the (possible, but not certain) kernel thread context switch time
    weighs in the overall KVM IRQ service time?



    > It can! :) This is not changing from whats in mainline today (covered
    > above).

    It can/could, if the signal() function takes very accurate care of doing
    the magic atomic check.



    - Davide




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