netdev - RE: [PATCH net-next] tcp: remove a bogus TSO split

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Date:	Fri, 13 Dec 2013 09:56:04 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Van Jacobson <vanj@...gle.com>
Subject: RE: [PATCH net-next] tcp: remove a bogus TSO split

On Fri, 2013-12-13 at 16:58 +0000, David Laight wrote:
> > From: Neal Cardwell
> ...
> > Seems like a nice improvement, but if we apply this patch then AFAICT
> > to get the Nagle-enabled case right we also have to update
> > tcp_minshall_update() to notice these new non-MSS-aligned segments
> > going out, and count those as non-full-size segments for the
> > minshall-nagle check (to ensure we have no more than one outstanding
> > un-ACKed sub-MSS packet). Maybe something like (please excuse the
> > formatting):
> 
> This sort of begs the question about how Nagle should work.
> IIRC Nagle just suppresses short segments when there is unacked data? [1]
> 
> If you have sent a TSO packet then nagle will always be 'waiting for an ack',
> so should only send full segments. What is questionable is whether you should
> send the final short segment, or buffer it waiting for further data from
> the application to fill the segment (or an ack from the remote system).

Point is the current code does a grouped send of the two skbs, without
any wait or even release of socket lock, allowing a tcp_sendmsg() to
aggregate/coalesce more data to the last (partial) segment.

Nagle test is properly done _before_ the call to tcp_mss_split_point()

I think the current behavior of tcp_mss_split_point() is a leftover of
the old days, when David Miller (and others) added TSO support to the
stack.



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