Patchwork eth: Declare an optimized compare_ether_addr_64bits() function

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Submitter Eric Dumazet
Date Nov. 22, 2008, 7:19 a.m.
Message ID <4927B275.1030407@cosmosbay.com>
Download mbox | patch
Permalink /patch/10200/
State Changes Requested
Delegated to: David Miller
Headers show

Comments

Eric Dumazet - Nov. 22, 2008, 7:19 a.m.
Hello David, this is a resend of a patch previously sent in a 
"tbench regression ..." thread on lkml

We should also address the problem of skb_pull(skb, ETH_HLEN);
in eth_type_trans() :

Being not inlined, this force eth_type_trans() to be a non
leaf function, that cost precious cpu cycles on many arches.

Thank you

[PATCH] eth: Declare an optimized compare_ether_addr_64bits() function

Linus mentioned we could try to perform long word operations, even
on potentially unaligned addresses, on x86 at least.

I tried this idea and got nice assembly on 32 bits:

158:   33 82 38 01 00 00       xor    0x138(%edx),%eax
15e:   33 8a 34 01 00 00       xor    0x134(%edx),%ecx
164:   c1 e0 10                shl    $0x10,%eax
167:   09 c1                   or     %eax,%ecx
169:   74 0b                   je     176 <eth_type_trans+0x87>

And very nice assembly on 64 bits of course (one xor, one shl)

Nice oprofile improvement in eth_type_trans(), 0.17 % instead of 0.41 %,
expected since we remove 8 instructions on a fast path.

This patch implements a compare_ether_addr_64bits() function,
that handles the case of x86 cpus, but might be used on other arches as well,
if their potential misaligned long word reads are not expensive.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
 include/linux/etherdevice.h |   41 ++++++++++++++++++++++++++++++++++
 net/ethernet/eth.c          |    4 +--
 2 files changed, 43 insertions(+), 2 deletions(-)
stephen hemminger - Nov. 22, 2008, 7:22 a.m.
On Sat, 22 Nov 2008 08:19:17 +0100
Eric Dumazet <dada1@cosmosbay.com> wrote:

> Hello David, this is a resend of a patch previously sent in a 
> "tbench regression ..." thread on lkml
> 
> We should also address the problem of skb_pull(skb, ETH_HLEN);
> in eth_type_trans() :
> 
> Being not inlined, this force eth_type_trans() to be a non
> leaf function, that cost precious cpu cycles on many arches.
> 
> Thank you
> 
> [PATCH] eth: Declare an optimized compare_ether_addr_64bits() function
> 
> Linus mentioned we could try to perform long word operations, even
> on potentially unaligned addresses, on x86 at least.
> 
> I tried this idea and got nice assembly on 32 bits:
> 
> 158:   33 82 38 01 00 00       xor    0x138(%edx),%eax
> 15e:   33 8a 34 01 00 00       xor    0x134(%edx),%ecx
> 164:   c1 e0 10                shl    $0x10,%eax
> 167:   09 c1                   or     %eax,%ecx
> 169:   74 0b                   je     176 <eth_type_trans+0x87>
> 
> And very nice assembly on 64 bits of course (one xor, one shl)
> 
> Nice oprofile improvement in eth_type_trans(), 0.17 % instead of 0.41 %,
> expected since we remove 8 instructions on a fast path.
> 
> This patch implements a compare_ether_addr_64bits() function,
> that handles the case of x86 cpus, but might be used on other arches as well,
> if their potential misaligned long word reads are not expensive.
> 

Why invent another function? Why not just have compare_ether_addr() be
as optimized as possible, could even set it up to be overloadable by
asm code.
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Eric Dumazet - Nov. 22, 2008, 7:30 a.m.
Stephen Hemminger a écrit :
> On Sat, 22 Nov 2008 08:19:17 +0100
> Eric Dumazet <dada1@cosmosbay.com> wrote:
> 
>> Hello David, this is a resend of a patch previously sent in a 
>> "tbench regression ..." thread on lkml
>>
>> We should also address the problem of skb_pull(skb, ETH_HLEN);
>> in eth_type_trans() :
>>
>> Being not inlined, this force eth_type_trans() to be a non
>> leaf function, that cost precious cpu cycles on many arches.
>>
>> Thank you
>>
>> [PATCH] eth: Declare an optimized compare_ether_addr_64bits() function
>>
>> Linus mentioned we could try to perform long word operations, even
>> on potentially unaligned addresses, on x86 at least.
>>
>> I tried this idea and got nice assembly on 32 bits:
>>
>> 158:   33 82 38 01 00 00       xor    0x138(%edx),%eax
>> 15e:   33 8a 34 01 00 00       xor    0x134(%edx),%ecx
>> 164:   c1 e0 10                shl    $0x10,%eax
>> 167:   09 c1                   or     %eax,%ecx
>> 169:   74 0b                   je     176 <eth_type_trans+0x87>
>>
>> And very nice assembly on 64 bits of course (one xor, one shl)
>>
>> Nice oprofile improvement in eth_type_trans(), 0.17 % instead of 0.41 %,
>> expected since we remove 8 instructions on a fast path.
>>
>> This patch implements a compare_ether_addr_64bits() function,
>> that handles the case of x86 cpus, but might be used on other arches as well,
>> if their potential misaligned long word reads are not expensive.
>>
> 
> Why invent another function? Why not just have compare_ether_addr() be
> as optimized as possible, could even set it up to be overloadable by
> asm code.

Because I am not sure we can fetch 8 bytes from addr1 & addr2 from all
call sites. Better be safe, and convert each call sites after an audit.
Then, when fully audited, rename the function ?


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David Miller - Nov. 23, 2008, 11:45 p.m.
From: Eric Dumazet <dada1@cosmosbay.com>
Date: Sat, 22 Nov 2008 08:19:17 +0100

> This patch implements a compare_ether_addr_64bits() function, that
> handles the case of x86 cpus, but might be used on other arches as
> well, if their potential misaligned long word reads are not
> expensive.

We have a test for this, HAVE_EFFICIENT_UNALIGNED_ACCESS

Please use that instead of CONFIG_X86 and I'll apply this
to net-next-2.6

Thanks!
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Patch

diff --git a/include/linux/etherdevice.h b/include/linux/etherdevice.h
index 25d62e6..ee0df09 100644
--- a/include/linux/etherdevice.h
+++ b/include/linux/etherdevice.h
@@ -136,6 +136,47 @@  static inline unsigned compare_ether_addr(const u8 *addr1, const u8 *addr2)
 	BUILD_BUG_ON(ETH_ALEN != 6);
 	return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0;
 }
+
+static inline unsigned long zap_last_2bytes(unsigned long value)
+{
+#ifdef __BIG_ENDIAN
+	return value >> 16;
+#else
+	return value << 16;
+#endif
+}
+
+/**
+ * compare_ether_addr_64bits - Compare two Ethernet addresses
+ * @addr1: Pointer to an array of 8 bytes
+ * @addr2: Pointer to an other array of 8 bytes
+ *
+ * Compare two ethernet addresses, returns 0 if equal.
+ * Same result than "memcmp(addr1, addr2, ETH_ALEN)" but without conditional
+ * branches, and possibly long word memory accesses on CPU allowing cheap
+ * unaligned memory reads.
+ * arrays = { byte1, byte2, byte3, byte4, byte6, byte7, pad1, pad2}
+ * 
+ * Please note that alignment of addr1 & addr2 is only guaranted to be 16 bits.
+ */
+
+static inline unsigned compare_ether_addr_64bits(const u8 addr1[6+2],
+						 const u8 addr2[6+2])
+{
+#if defined(CONFIG_X86)
+	unsigned long fold = *(const unsigned long *)addr1 ^
+			     *(const unsigned long *)addr2;
+
+	if (sizeof(fold) == 8)
+		return zap_last_2bytes(fold) != 0;
+
+	fold |= zap_last_2bytes(*(const unsigned long *)(addr1 + 4) ^
+				*(const unsigned long *)(addr2 + 4));
+	return fold != 0;
+#else
+	return compare_ether_addr(addr1, addr2);
+#endif
+}
 #endif	/* __KERNEL__ */
 
 #endif	/* _LINUX_ETHERDEVICE_H */
diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index b9d85af..dcfeb9b 100644
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -166,7 +166,7 @@  __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
 	eth = eth_hdr(skb);
 
 	if (is_multicast_ether_addr(eth->h_dest)) {
-		if (!compare_ether_addr(eth->h_dest, dev->broadcast))
+		if (!compare_ether_addr_64bits(eth->h_dest, dev->broadcast))
 			skb->pkt_type = PACKET_BROADCAST;
 		else
 			skb->pkt_type = PACKET_MULTICAST;
@@ -181,7 +181,7 @@  __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
 	 */
 
 	else if (1 /*dev->flags&IFF_PROMISC */ ) {
-		if (unlikely(compare_ether_addr(eth->h_dest, dev->dev_addr)))
+		if (unlikely(compare_ether_addr_64bits(eth->h_dest, dev->dev_addr)))
 			skb->pkt_type = PACKET_OTHERHOST;
 	}