netdev - Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand

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Date:	Thu, 19 Apr 2012 14:44:50 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand_head()
 calls

On Thu, 2012-04-19 at 14:57 +0300, Ilpo Järvinen wrote:

> I'm not concerned of the output side, that seems to work because 
> of the in tcp_retransmit_skb getting rid of the extra first.
> 
> The ACK input stuff is more interesting, e.g., this one in 
> tcp_mark_head_lost:
> 
> 	err = tcp_fragment(sk, skb, (packets - oldcnt) * mss, mss);
> 
> It splits from TCP_SKB_CB(skb)->seq + (packets - oldcnt) * mss whereas
> I think the desired point would be: TCP_SKB_CB(skb)->seq + offset_ack + 
> (packets - oldcnt) * mss?
> 
> ...There is similar case in sacktag code too while it's aligning to mss 
> boundaries in tcp_match_skb_to_sack.

Hmm yes, so maybe its safer to update TCP_SKB_CB(skb)->seq in
tcp_tso_acked() (as well as offset_ack) and make needed adjustements in
tcp_fragment() if we find offset_ack being not null.



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