netdev - Re: [PATCH net-next v3 2/2] net: Check skb->rxhash in gro

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Date:	Wed, 15 Jan 2014 09:25:08 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Jerry Chu <hkchu@...gle.com>
Subject: Re: [PATCH net-next v3 2/2] net:  Check skb->rxhash in gro_receive

On Wed, 2014-01-15 at 08:58 -0800, Tom Herbert wrote:
> When initializing a gro_list for a packet, first check the rxhash of
> the incoming skb against that of the skb's in the list. This should be
> a very strong inidicator of whether the flow is going to be matched,
> and potentially allows a lot of other checks to be short circuited.
> Use skb_hash_raw so that we don't force the hash to be calculated.
> 
> Tested by running netperf 200 TCP_STREAMs between two machines with
> GRO, HW rxhash, and 1G. Saw no performance degration, slight reduction
> of time in dev_gro_receive.
> 
> Signed-off-by: Tom Herbert <therbert@...gle.com>
> ---
>  net/core/dev.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 20c834e..c063c7c 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3818,10 +3818,18 @@ static void gro_list_prepare(struct napi_struct *napi, struct sk_buff *skb)
>  {
>  	struct sk_buff *p;
>  	unsigned int maclen = skb->dev->hard_header_len;
> +	u32 hash = skb_get_hash_raw(skb);
>  
>  	for (p = napi->gro_list; p; p = p->next) {
>  		unsigned long diffs;
>  
> +		NAPI_GRO_CB(p)->flush = 0;
> +
> +		if (hash != skb_get_hash_raw(p)) {
> +			NAPI_GRO_CB(p)->same_flow = 0;
> +			continue;
> +		}
> +
>  		diffs = (unsigned long)p->dev ^ (unsigned long)skb->dev;
>  		diffs |= p->vlan_tci ^ skb->vlan_tci;
>  		if (maclen == ETH_HLEN)
> @@ -3832,7 +3840,6 @@ static void gro_list_prepare(struct napi_struct *napi, struct sk_buff *skb)
>  				       skb_gro_mac_header(skb),
>  				       maclen);
>  		NAPI_GRO_CB(p)->same_flow = !diffs;
> -		NAPI_GRO_CB(p)->flush = 0;
>  	}
>  }
>  

Acked-by: Eric Dumazet <edumazet@...gle.com>

Hmm, this looks like we should clear flush_id in ipv6 handler,
otherwise we might reuse a flush_id set from a prior gro invocation in
ipv4 (skb can be reused in napi_reuse_skb())

Jerry, what do you think of following fix ?

diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index 1e8683b135bb..598acd76ca4a 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -256,6 +256,7 @@ static struct sk_buff **ipv6_gro_receive(struct sk_buff **head,
                /* flush if Traffic Class fields are different */
                NAPI_GRO_CB(p)->flush |= !!(first_word & htonl(0x0FF00000));
                NAPI_GRO_CB(p)->flush |= flush;
+               NAPI_GRO_CB(p)->flush_id = 0;
        }
 
        NAPI_GRO_CB(skb)->flush |= flush;



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