linux-kernel - Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

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Date:	Wed, 17 Jun 2009 12:50:10 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com, paulmck@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier
 interface

On Wed, 17 Jun 2009, Gregory Haskins wrote:

> Davide Libenzi wrote:
> 
> > How much the (possible, but not certain) kernel thread context switch time 
> > weighs in the overall KVM IRQ service time?
> >   
> 
> Generally each one is costing me about 7us on average.  For something
> like high-speed networking, we have a path that has about 30us of
> base-line overhead.  So one additional ctx-switch puts me at base+7 ( =
> ~37us), two puts me in base+2*7 (= ~44us).  So in that context (no pun
> intended ;), it hurts quite a bit.  I'll be the first to admit that not
> everyone (most?) will care about latency, though.  But FWIW, I do.

And how a frame reception is handled in Linux nowadays?



> True, but thats the notifiee's burden, not eventfd's.  And its always
> going to be opt-in.  Even today, someone is free to either try to sleep
> (which will oops on the might_sleep()), ...

No, today you just can't sleep. As you can't sleep in any 
callback-registered wakeups, like epoll, for example.



- Davide


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