Re: [dpdk-dev] [PATCH RFC 0/3] DPDK ethdev callback support

[Neil Horman <nhorman@tuxdriver.com>]

On Mon, Dec 22, 2014 at 05:33:07PM +0000, Bruce Richardson wrote:
> On Mon, Dec 22, 2014 at 06:02:53PM +0100, Thomas Monjalon wrote:
> > Hi Bruce,
> > 
> > Callbacks, as hooks for applications, give more flexibility and are
> > generally a good idea.
> > In DPDK the main issue will be to avoid performance degradation.
> > I see you use "unlikely" for callback branching.
> > Could we reduce more the impact of this test by removing the queue array,
> > i.e. having port-wide callbacks instead of per-queue callbacks?
> 
> I can give that a try, but I don't see it making much difference if any. The
> main thing to avoid with branching is branch mis-prediction, which should not
> be a problem here, as the user is not going to be adding or removing callbacks
> between each RX and TX call, making the branches highly predictable - i.e. always
> go the same way.
I was going to ask about exactly that.  You say no one will be adding/removing
callbacks between RX/TX calls, but you don't know that, people will try to do so
at some point.  You should add a check so that callbacks can only be
registered/unregistered on stopped queues, otherwise this is extreemely racy.
it won't impact performance to do so, and will save a good deal of debugging
down the road at some point.

Neil

>