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Subjecteth_type_trans(): Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

* Ingo Molnar <mingo@elte.hu> wrote:

> 100.000000 total
> ................
> 1.717771 eth_type_trans

hits (total: 171777)
.........
ffffffff8049e215: 457 <eth_type_trans>:
ffffffff8049e215: 457 41 54 push %r12
ffffffff8049e217: 6514 55 push %rbp
ffffffff8049e218: 0 48 89 f5 mov %rsi,%rbp
ffffffff8049e21b: 0 53 push %rbx
ffffffff8049e21c: 441 48 8b 87 d8 00 00 00 mov 0xd8(%rdi),%rax
ffffffff8049e223: 5 48 89 fb mov %rdi,%rbx
ffffffff8049e226: 0 2b 87 d0 00 00 00 sub 0xd0(%rdi),%eax
ffffffff8049e22c: 493 48 89 73 20 mov %rsi,0x20(%rbx)
ffffffff8049e230: 2 be 0e 00 00 00 mov $0xe,%esi
ffffffff8049e235: 0 89 87 c0 00 00 00 mov %eax,0xc0(%rdi)
ffffffff8049e23b: 472 e8 2c 98 fe ff callq ffffffff80487a6c <skb_pull>
ffffffff8049e240: 501 44 8b a3 c0 00 00 00 mov 0xc0(%rbx),%r12d
ffffffff8049e247: 763 4c 03 a3 d0 00 00 00 add 0xd0(%rbx),%r12
ffffffff8049e24e: 0 41 f6 04 24 01 testb $0x1,(%r12)
ffffffff8049e253: 497 74 26 je ffffffff8049e27b <eth_type_trans+0x66>
ffffffff8049e255: 0 48 8d b5 38 02 00 00 lea 0x238(%rbp),%rsi
ffffffff8049e25c: 0 4c 89 e7 mov %r12,%rdi
ffffffff8049e25f: 0 e8 49 fc ff ff callq ffffffff8049dead <compare_ether_addr>
ffffffff8049e264: 0 85 c0 test %eax,%eax
ffffffff8049e266: 0 8a 43 7d mov 0x7d(%rbx),%al
ffffffff8049e269: 0 75 08 jne ffffffff8049e273 <eth_type_trans+0x5e>
ffffffff8049e26b: 0 83 e0 f8 and $0xfffffffffffffff8,%eax
ffffffff8049e26e: 0 83 c8 01 or $0x1,%eax
ffffffff8049e271: 0 eb 24 jmp ffffffff8049e297 <eth_type_trans+0x82>
ffffffff8049e273: 0 83 e0 f8 and $0xfffffffffffffff8,%eax
ffffffff8049e276: 0 83 c8 02 or $0x2,%eax
ffffffff8049e279: 0 eb 1c jmp ffffffff8049e297 <eth_type_trans+0x82>
ffffffff8049e27b: 82 48 8d b5 18 02 00 00 lea 0x218(%rbp),%rsi
ffffffff8049e282: 8782 4c 89 e7 mov %r12,%rdi
ffffffff8049e285: 1752 e8 23 fc ff ff callq ffffffff8049dead <compare_ether_addr>
ffffffff8049e28a: 0 85 c0 test %eax,%eax
ffffffff8049e28c: 757 74 0c je ffffffff8049e29a <eth_type_trans+0x85>
ffffffff8049e28e: 0 8a 43 7d mov 0x7d(%rbx),%al
ffffffff8049e291: 0 83 e0 f8 and $0xfffffffffffffff8,%eax
ffffffff8049e294: 0 83 c8 03 or $0x3,%eax
ffffffff8049e297: 0 88 43 7d mov %al,0x7d(%rbx)
ffffffff8049e29a: 107 66 41 8b 44 24 0c mov 0xc(%r12),%ax
ffffffff8049e2a0: 1031 0f b7 c8 movzwl %ax,%ecx
ffffffff8049e2a3: 518 66 c1 e8 08 shr $0x8,%ax
ffffffff8049e2a7: 0 89 ca mov %ecx,%edx
ffffffff8049e2a9: 0 c1 e2 08 shl $0x8,%edx
ffffffff8049e2ac: 484 09 d0 or %edx,%eax
ffffffff8049e2ae: 0 0f b7 c0 movzwl %ax,%eax
ffffffff8049e2b1: 0 3d ff 05 00 00 cmp $0x5ff,%eax
ffffffff8049e2b6: 468 7f 18 jg ffffffff8049e2d0 <eth_type_trans+0xbb>
ffffffff8049e2b8: 0 48 8b 83 d8 00 00 00 mov 0xd8(%rbx),%rax
ffffffff8049e2bf: 0 b9 00 01 00 00 mov $0x100,%ecx
ffffffff8049e2c4: 0 66 83 38 ff cmpw $0xffffffffffffffff,(%rax)
ffffffff8049e2c8: 0 b8 00 04 00 00 mov $0x400,%eax
ffffffff8049e2cd: 0 0f 45 c8 cmovne %eax,%ecx
ffffffff8049e2d0: 0 5b pop %rbx
ffffffff8049e2d1: 85064 5d pop %rbp
ffffffff8049e2d2: 63776 41 5c pop %r12
ffffffff8049e2d4: 1 89 c8 mov %ecx,%eax
ffffffff8049e2d6: 474 c3 retq

small function, big bang - 1.7% of the total overhead.

90% of this function's cost is in the closing sequence. My guess would
be that it originates from ffffffff8049e2ae (the branch after that is
not taken), which corresponds to this source code context:

(gdb) list *0xffffffff8049e2ae
0xffffffff8049e2ae is in eth_type_trans (net/ethernet/eth.c:199).
194 if (netdev_uses_dsa_tags(dev))
195 return htons(ETH_P_DSA);
196 if (netdev_uses_trailer_tags(dev))
197 return htons(ETH_P_TRAILER);
198
199 if (ntohs(eth->h_proto) >= 1536)
200 return eth->h_proto;
201
202 rawp = skb->data;
203

eth->h_proto access.

Given that this workload does localhost networking, my guess would be
that eth->h_proto is bouncing around between 16 CPUs? At minimum this
read-mostly field should be separated from the bouncing bits.

Ingo


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