netdev - Re: tbench wrt. loopback TSO

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Date:	Wed, 05 Nov 2008 04:09:43 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	zbr@...emap.net, netdev@...r.kernel.org, efault@....de,
	mingo@...e.hu, a.p.zijlstra@...llo.nl, herbert@...dor.apana.org.au
Subject: Re: tbench wrt. loopback TSO

From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Wed, 5 Nov 2008 14:04:14 +0200 (EET)

> On Wed, 5 Nov 2008, David Miller wrote:
> 
> > From: Evgeniy Polyakov <zbr@...emap.net>
> > Date: Wed, 5 Nov 2008 14:49:00 +0300
> > 
> > > But what if we just remove that trimming at all? This may result in
> > > a not fully filled tso frame, but who cares if we copy data from
> > > userspace anyway, will add another potentially small copy at sending
> > > time?
> > 
> > Ok, the only part I care about is whether taking away this modulus
> > will result in a sub-MSS sized packet on the wire, which is
> > undesirable.
> >
> > The send logic will prevent this, right?
> 
> I don't see what currently would do that. We'd need to resegment skbs 
> there then...

Look at tcp_mss_split_point() in tcp_output.c

I think it ends up doing exactly this.

'needed' is set to min(skb->len, window)

and the return value is "needed - (needed % mss_now)"

The caller splits up the TSO segment as needed to satisfy this
returned limit.

That should effectively chop off the sub-mss part.
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