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

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Date:   Fri, 8 Dec 2017 13:15:38 -0200
From:   'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Xin Long' <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        "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 03:01:31PM +0000, David Laight wrote:
> From: Marcelo Ricardo Leitner
> > Sent: 08 December 2017 14:57
> > 
> > On Fri, Dec 08, 2017 at 02:06:04PM +0000, David Laight wrote:
> > > From: Xin Long
> > > > Sent: 08 December 2017 13:04
> > > ...
> > > > @@ -264,8 +264,8 @@ struct sctp_datamsg *sctp_datamsg_from_user(struct sctp_association *asoc,
> > > >  				frag |= SCTP_DATA_SACK_IMM;
> > > >  		}
> > > >
> > > > -		chunk = sctp_make_datafrag_empty(asoc, sinfo, len, frag,
> > > > -						 0, GFP_KERNEL);
> > > > +		chunk = asoc->stream.si->make_datafrag(asoc, sinfo, len, frag,
> > > > +						       GFP_KERNEL);
> > >
> > > I know that none of the sctp code is very optimised, but that indirect
> > > call is going to be horrid.
> > 
> > Yeah.. but there is no way to avoid the double derreference
> > considering we only have the asoc pointer in there and we have to
> > reach the contents of the data chunk operations struct, and the .si
> > part is the same as 'stream' part as it's a constant offset.
> ...
> 
> It isn't only the double indirect, the indirect call itself isn't 'fun'.

I meant in this context.

The indirect call is so we don't have to flood the stack with
if (old data chunk fmt) {
	...
} else {
	...
}

So instead of this, we now have some key operations identified and
wrapped up behind this struct, allowing us to abstract whatever data
chunk format it is.

> 
> I think there are other hot paths where you've replaced a sizeof()
> with a ?: clause.
> Caching the result might be much better.

The only new ?: clause I could find this patchset is on patch 12 and
has nothing to do with sizeof().

The sizeof() results are indeed cached, as you can see in patch 4:
+static struct sctp_stream_interleave sctp_stream_interleave_0 = {
+       .data_chunk_len         = sizeof(struct sctp_data_chunk),
and the two helpers on it at the begining of the patch.

  Marcelo