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

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Message-ID: <1395341341.9114.93.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 20 Mar 2014 11:49:01 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Patrick McHardy <kaber@...sh.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"H.K. Jerry Chu" <hkchu@...gle.com>,
	Michael Dalton <mwdalton@...gle.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: [RFC] csum experts, csum_replace2() is too expensive

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)

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 8c54870db792..86c924c16f3c 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;
+	ip_send_check(&iph);
 
 	rcu_read_lock();
 	ops = rcu_dereference(inet_offloads[proto]);


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