Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifierinterface

From: Davide Libenzi
Date: Wed Jun 17 2009 - 13:50:56 EST

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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