netdev - Re: [PATCHv2 net-next 04/12] sctp: implement make_datafrag for sctp_stream

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Date:   Fri, 8 Dec 2017 18:37:43 -0200
From:   'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>
To:     Neil Horman <nhorman@...driver.com>
Cc:     David Laight <David.Laight@...LAB.COM>,
        'Xin Long' <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCHv2 net-next 04/12] sctp: implement make_datafrag for
 sctp_stream_interleave

On Fri, Dec 08, 2017 at 11:08:56AM -0500, Neil Horman wrote:
> On Fri, Dec 08, 2017 at 04:04:58PM +0000, David Laight wrote:
> > From: Marcelo Ricardo Leitner
> > > Sent: 08 December 2017 16:00
> > ...
> > > > Is it worth replacing the si struct with an index/enum value, and indexing an
> > > > array of method pointer structs?  That would save you at least one dereference.
> > > 
> > > Hmmm, maybe, yes. It would be like
> > > sctp_stream_interleave[asoc->stream.si].make_datafrag(...)
> > 
> > If you only expect 2 choices then an if () is likely
> > to produce better code that the above.
> > 
> > The actual implementation can be hidden inside a #define
> > or static inline function.
> > 
> Thats the real question though, will we expect more than two interleaving
> strategies?  Currently its a boolean operation so the answer seems like yes, but
> is there a possiblity of a biased interleaving, or other worthwhile algorithm?

For the chunk format, I don't think so. It would require another RFC
update.
For other possibilities on having a 3rd choice in there, I also don't
think so. Stream scheduling is handled apart from it and rx buffer
stuff is being covered by it now, don't see how we could have a 3rd
option without another chunk format.

But that said, I don't think the macro or even inline wrappers are as
clear as the struct with function pointers here and in some cases it
even won't avoid the second deref. I wouldn't like to compromise code
readability and OO because of 1 fetch, specially considering that this
will be barely noticeable.

Neil's idea on using the array of structs and indexing on it is nice.
Saves a deref and keeps the abstractions without inserting too much
noise on it. Plus, it also allows replacing the struct pointer in
sctp_stream with a bit. If then placed before the union in there, we
can save up to the whole pointer. Looks like a good compromise to me.

  Marcelo