netdev - Re: vxlan/veth performance issues on net.git + latest kernels

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Message-ID: <1386535824.30495.301.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sun, 08 Dec 2013 12:50:24 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mike Rapoport <mike.rapoport@...ellosystems.com>
Cc:	Or Gerlitz <ogerlitz@...lanox.com>,
	Or Gerlitz <or.gerlitz@...il.com>,
	Joseph Gasparakis <joseph.gasparakis@...el.com>,
	Pravin B Shelar <pshelar@...ira.com>,
	Jerry Chu <hkchu@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	John Fastabend <john.fastabend@...il.com>
Subject: Re: vxlan/veth performance issues on net.git + latest kernels

On Sun, 2013-12-08 at 16:30 +0200, Mike Rapoport wrote:
>  
> It was something *very* hacky:

> 		if (skb->len < 64000)
> 			skb_shinfo(skb)->gso_size = skb->len;
> 		else
> 			skb_shinfo(skb)->gso_size = 64000;

This sounds like an 16bit overflow somewhere.

This reminds the issue we fix in commit 50bceae9bd356
("tcp: Reallocate headroom if it would overflow csum_start")

You might try to reduce the 0xFFFF to something smaller.

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 993da005e087..8364bcfe1e08 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2373,7 +2373,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
 	 * beyond what csum_start can cover.
 	 */
 	if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
-		     skb_headroom(skb) >= 0xFFFF)) {
+		     skb_headroom(skb) >= 0xFF00)) {
 		struct sk_buff *nskb = __pskb_copy(skb, MAX_TCP_HEADER,
 						   GFP_ATOMIC);
 		return nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :


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