netdev - Re: [RFC] csum experts, csum_replace2() is too expensive

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Date:	Fri, 21 Mar 2014 14:07:30 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	kaber@...sh.net, herbert@...dor.apana.org.au, hkchu@...gle.com,
	mwdalton@...gle.com, netdev@...r.kernel.org
Subject: Re: [RFC] csum experts, csum_replace2() is too expensive

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 20 Mar 2014 11:49:01 -0700

> csum_replace2() uses about 29 cycles, while a plain ip_send_check() is
> way faster (16 cycles)
> 
> csum_partial() is not really meant for doing checksums over 8 bytes !
> 
> Any idea how to make the thing really fast as intended ?
> 
> I saw csum_partial() consuming 1% of cpu cycles in a GRO workload, that
> is insane...
> 
> Following patch might be the fastest thing ?
> 
> (At this point we already have validated IP checksum)
 ...
> @@ -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;
> +	ip_send_check(&iph);

Yeah the csum_replace*() are extremely suboptimal.

We should be able to cons up something cheap like the trick that
ip_decrease_ttl() uses.

https://tools.ietf.org/html/rfc1624
https://tools.ietf.org/html/rfc1141
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