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Message-ID: <1395410195.6441.6.camel@edumazet-glaptop2.roam.corp.google.com>
Subject: Re: [RFC] csum experts, csum_replace2() is too expensive
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Patrick McHardy <kaber@trash.net>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "H.K. Jerry Chu" <hkchu@google.com>,
        Michael Dalton <mwdalton@google.com>, netdev <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Date: Fri, 21 Mar 2014 06:56:35 -0700
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On Fri, 2014-03-21 at 06:47 -0700, Eric Dumazet wrote:

> Another idea would be to move the ip_fast_csum() call at the end of
> inet_gro_complete()
> 
> I'll try this :
> 
> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> index 8c54870db792..0ca8f350a532 100644
> --- a/net/ipv4/af_inet.c
> +++ b/net/ipv4/af_inet.c
> @@ -1434,8 +1434,8 @@ static int inet_gro_complete(struct sk_buff *skb, int nhoff)
>         int proto = iph->protocol;
>         int err = -ENOSYS;
>  
> -       csum_replace2(&iph->check, iph->tot_len, newlen);
>         iph->tot_len = newlen;
> +       iph->check = 0;
>  
>         rcu_read_lock();
>         ops = rcu_dereference(inet_offloads[proto]);
> @@ -1447,6 +1447,7 @@ static int inet_gro_complete(struct sk_buff *skb, int nhoff)
>          * inet_gro_receive().
>          */
>         err = ops->callbacks.gro_complete(skb, nhoff + sizeof(*iph));
> +       iph->check = ip_fast_csum((u8 *)iph, 5);
>  
>  out_unlock:
>         rcu_read_unlock();
> 

This doesn't help... same stall.

Looks like the best way is to use some 16 bit loads in ip_fast_csum()
for the ihl=5 case.