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Commit 99f0b958 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

net: optimize csum_replace2() 



When changing one 16bit value by another in IP header, we can adjust
the IP checksum by doing a simple operation described in RFC 1624, as
reminded by David.

csum_partial() is a complex function on x86_64, not really suited for
small number of checksummed bytes.

I spotted csum_partial() being in the top 20 most consuming functions
(more than 1 %) in a GRO workload, which was rather unexpected.

The caller was inet_gro_complete() doing a csum_replace2() when
building the new IP header for the GRO packet.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 860b4042

include/net/checksum.h

+21 −2
Original line number Diff line number Diff line
@@ -69,6 +69,19 @@ static inline __wsum csum_sub(__wsum csum, __wsum addend) 
	return csum_add(csum, ~addend);

}



static inline __sum16 csum16_add(__sum16 csum, __be16 addend)

{

	u16 res = (__force u16)csum;



	res += (__force u16)addend;

	return (__force __sum16)(res + (res < (__force u16)addend));

}



static inline __sum16 csum16_sub(__sum16 csum, __be16 addend)

{

	return csum16_add(csum, ~addend);

}



static inline __wsum

csum_block_add(__wsum csum, __wsum csum2, int offset)

{
@@ -112,9 +125,15 @@ static inline void csum_replace4(__sum16 *sum, __be32 from, __be32 to) 
	*sum = csum_fold(csum_partial(diff, sizeof(diff), ~csum_unfold(*sum)));

}



static inline void csum_replace2(__sum16 *sum, __be16 from, __be16 to)

/* Implements RFC 1624 (Incremental Internet Checksum)

 * 3. Discussion states :

 *     HC' = ~(~HC + ~m + m')

 *  m : old value of a 16bit field

 *  m' : new value of a 16bit field

 */

static inline void csum_replace2(__sum16 *sum, __be16 old, __be16 new)

{

	csum_replace4(sum, (__force __be32)from, (__force __be32)to);

	*sum = ~csum16_add(csum16_sub(~(*sum), old), new);

}



struct sk_buff;