Subject: Re: [PATCH 2/2] IB/ipoib: fix GRO merge failure for IPoIB originated TCP streams

From: Or Gerlitz <ogerlitz-VPRAkNaXOzVWk0Htik3J/[email-masked]>
Date: Thu, 2 Feb 2012 16:01:47 +0200

On 2/1/2012 4:07 PM, Eric Dumazet wrote:
>>> If we just turn it into a memcmp with a variable length would
>>> that work for you?
>>
>> I think yes, FWIW we will compare the IPoIB header, Roland is that okay for you?
>
> A memcmp(xxx, yyy, variable_len) will be out of line and slow, its a bit sad ... Are skb_mac_header(p) / skb_gro_mac_header(skb) going to point to IPoIB header ?

yes, both skb_mac_header(p) / skb_gro_mac_header(skb) point to IPoIB 
header, however (see next)

> Maybe we can keep a fastpath for ethernet case... (the "if (hlen == ETH_HLEN) being always predicted)
> Maybe need to introduce gro_hard_header_len as well)

today, IPoIB advertizes hard_header_len which is bigger than the IPoIB 
header len, this is done such that skbs sent by the network stack have 
enough headroom for a "pseudoheader" which for few flows (e.g unicast 
arp replies and multicast) is placed there by the ipoib hard_header 
function and later used by the xmit function.

So we can either try and change that, such that hard_header_len will be 
equal to the ipoib header len or add gro_hard_header_len as you 
suggested, any preferences?

Or.

Or.

>
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 115dee1..62abee4 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3500,14 +3500,20 @@ static inline gro_result_t
>   __napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
>   {
>   	struct sk_buff *p;
> +	unsigned int hlen = skb->dev->hard_header_len;
>
>   	for (p = napi->gro_list; p; p = p->next) {
>   		unsigned long diffs;
>
>   		diffs = (unsigned long)p->dev ^ (unsigned long)skb->dev;
>   		diffs |= p->vlan_tci ^ skb->vlan_tci;
> -		diffs |= compare_ether_header(skb_mac_header(p),
> -					      skb_gro_mac_header(skb));
> +		if (hlen == ETH_HLEN)
> +			diffs |= compare_ether_header(skb_mac_header(p),
> +						      skb_gro_mac_header(skb));
> +		else if (!diffs)
> +			diffs = memcmp(skb_mac_header(p),
> +					skb_gro_mac_header(skb),
> +					skb->dev->hard_header_len);
>   		NAPI_GRO_CB(p)->same_flow = !diffs;
>   		NAPI_GRO_CB(p)->flush = 0;
>   	}
>
>

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