netdev - Re: [PATCH net-next] net: gro: remove obsolete code from skb_gro

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Message-Id: <20150305.160028.1421983637301031004.davem@davemloft.net>
Date:	Thu, 05 Mar 2015 16:00:28 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, herbert@...dor.apana.org.au
Subject: Re: [PATCH net-next] net: gro: remove obsolete code from
 skb_gro_receive()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 04 Mar 2015 10:26:40 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Some drivers use copybreak to copy tiny frames into smaller skb,
> and this smaller skb might not have skb->head_frag set for various
> reasons.
> 
> skb_gro_receive() currently doesn't allow to aggregate the smaller skb
> into the previous GRO packet if this GRO packet has at least 2 MSS in
> it.
> 
> Following workload easily demonstrates the problem.
> 
> netperf -t TCP_RR -H target -- -r 3000,3000 
> 
> (tcpdump shows one GRO packet with 2 MSS, plus one additional packet of
> 104 bytes that should have been appended.)
> 
> It turns out that we can remove code from skb_gro_receive(), because
> commit 8a29111c7ca6 ("net: gro: allow to build full sized skb") and its
> followups removed the assumption that a GRO packet with a frag_list had
> to have an empty head.
> 
> Removing this code allows the aggregation of the last (incomplete) frame
> in some RPC workloads. Note that tcp_gro_receive() already takes care of
> forcing a flush if necessary, including this case.
> 
> If we want to avoid using frag_list in the first place (in forwarding
> workloads for example, as the outgoing NIC is generally not able to cope
> with skbs having a frag_list), we need to address this separately.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Please get rid of the local variables that are no made unused by this
change:

net/core/skbuff.c: In function ‘skb_gro_receive’:
net/core/skbuff.c:3212:15: warning: unused variable ‘headroom’ [-Wunused-variable]
net/core/skbuff.c:3209:18: warning: unused variable ‘nskb’ [-Wunused-variable]