netdev - Re: [PATCH] tcp: allow splice() to build full TSO packets

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Message-Id: <20120403.172126.672236532461758456.davem@davemloft.net>
Date:	Tue, 03 Apr 2012 17:21:26 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ncardwell@...gle.com, therbert@...gle.com,
	ycheng@...gle.com, hkchu@...gle.com, maze@...gle.com,
	maheshb@...gle.com, ilpo.jarvinen@...sinki.fi, nanditad@...gle.com
Subject: Re: [PATCH] tcp: allow splice() to build full TSO packets

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 03 Apr 2012 21:37:01 +0200

> vmsplice()/splice(pipe, socket) call do_tcp_sendpages() one page at a
> time, adding at most 4096 bytes to an skb. (assuming PAGE_SIZE=4096)
> 
> The call to tcp_push() at the end of do_tcp_sendpages() forces an
> immediate xmit when pipe is not already filled, and tso_fragment() try
> to split these skb to MSS multiples.
> 
> 4096 bytes are usually split in a skb with 2 MSS, and a remaining
> sub-mss skb (assuming MTU=1500)

Interesting.

But why doesn't TCP_NAGLE_CORK save us?  That gets passed down into
the push pending frames logic when MSG_MORE is specified.

As far as I can tell, the combination of TCP_NAGLE_CORK and the TSO
deferral logic should do the right thing here.

Obviously you see different behavior, but why?

Also, by eliding the tcp_push() call you are introducing other side
effects:

1) we won't do the tcp_mark_push logic

2) we don't set the URG seq

I think #2 can never happen in the vmsplice/splice path, but #1 might
matter.

That's why I want to concentrate on why the tcp_push() path doesn't
behave properly when MSG_MORE is set.
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