netdev - Re: [PATCH] [TCP]: Force TSO splits to MSS boundaries

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Date:	Mon, 24 Dec 2007 21:35:00 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	herbert@...dor.apana.org.au, netdev@...r.kernel.org
Subject: Re: [PATCH] [TCP]: Force TSO splits to MSS boundaries

From: "Ilpo_Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Fri, 21 Dec 2007 20:55:28 +0200 (EET)

> [PATCH] [TCP]: Force TSO splits to MSS boundaries
> 
> If snd_wnd - snd_nxt wasn't multiple of MSS, skb was split on
> odd boundary by the callers of tcp_window_allows.
> 
> We try really hard to avoid unnecessary modulos. Therefore the
> old caller side check "if (skb->len < limit)" was too wide as
> well because limit is not bound in any way to skb->len and can
> cause spurious testing for trimming in the middle of the queue
> while we only wanted that to happen at the tail of the queue.
> A simple additional caller side check for tcp_write_queue_tail
> would likely have resulted 2 x modulos because the limit would
> have to be first calculated from window, however, doing that
> unnecessary modulo is not mandatory. After a minor change to
> the algorithm, simply determine first if the modulo is needed
> at all and at that point immediately decide also from which
> value it should be calculated from.
> 
> This approach also kills some duplicated code.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>

Looks good, patch applied, thanks.

With respect to code duplicateion, tcp_push_one() is essentially
the inner loop of tcp_write_xmit() :-)
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