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    Subject__inet_lookup_established(): 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
    > ................
    > 1.673249 __inet_lookup_established

    hits (total: 167324)
    .........
    ffffffff804b9b12: 446 <__inet_lookup_established>:
    ffffffff804b9b12: 446 41 57 push %r15
    ffffffff804b9b14: 4810 89 d0 mov %edx,%eax
    ffffffff804b9b16: 0 0f b7 c9 movzwl %cx,%ecx
    ffffffff804b9b19: 0 41 56 push %r14
    ffffffff804b9b1b: 456 41 55 push %r13
    ffffffff804b9b1d: 0 41 54 push %r12
    ffffffff804b9b1f: 0 55 push %rbp
    ffffffff804b9b20: 427 53 push %rbx
    ffffffff804b9b21: 4 48 89 f3 mov %rsi,%rbx
    ffffffff804b9b24: 2 44 89 c6 mov %r8d,%esi
    ffffffff804b9b27: 504 41 89 c8 mov %ecx,%r8d
    ffffffff804b9b2a: 1 49 89 f7 mov %rsi,%r15
    ffffffff804b9b2d: 1 48 83 ec 08 sub $0x8,%rsp
    ffffffff804b9b31: 462 49 c1 e7 20 shl $0x20,%r15
    ffffffff804b9b35: 0 48 89 3c 24 mov %rdi,(%rsp)
    ffffffff804b9b39: 507 89 d7 mov %edx,%edi
    ffffffff804b9b3b: 38 41 0f b7 d1 movzwl %r9w,%edx
    ffffffff804b9b3f: 0 41 89 d6 mov %edx,%r14d
    ffffffff804b9b42: 863 49 09 c7 or %rax,%r15
    ffffffff804b9b45: 24 41 c1 e6 10 shl $0x10,%r14d
    ffffffff804b9b49: 0 41 09 ce or %ecx,%r14d
    ffffffff804b9b4c: 479 89 f9 mov %edi,%ecx
    ffffffff804b9b4e: 8 48 8b 3c 24 mov (%rsp),%rdi
    ffffffff804b9b52: 0 e8 cc f4 ff ff callq ffffffff804b9023 <inet_ehashfn>
    ffffffff804b9b57: 413 48 89 df mov %rbx,%rdi
    ffffffff804b9b5a: 122 41 89 c5 mov %eax,%r13d
    ffffffff804b9b5d: 0 89 c6 mov %eax,%esi
    ffffffff804b9b5f: 635 e8 3e f5 ff ff callq ffffffff804b90a2 <inet_ehash_bucket>
    ffffffff804b9b64: 511 48 89 c5 mov %rax,%rbp
    ffffffff804b9b67: 6 44 89 e8 mov %r13d,%eax
    ffffffff804b9b6a: 0 23 43 14 and 0x14(%rbx),%eax
    ffffffff804b9b6d: 497 4c 8d 24 85 00 00 00 lea 0x0(,%rax,4),%r12
    ffffffff804b9b74: 0 00
    ffffffff804b9b75: 1 4c 03 63 08 add 0x8(%rbx),%r12
    ffffffff804b9b79: 0 48 8b 45 00 mov 0x0(%rbp),%rax
    ffffffff804b9b7d: 470 0f 18 08 prefetcht0 (%rax)
    ffffffff804b9b80: 0 4c 89 e7 mov %r12,%rdi
    ffffffff804b9b83: 1089 e8 32 cd 05 00 callq ffffffff805168ba <_read_lock>
    ffffffff804b9b88: 6752 48 8b 55 00 mov 0x0(%rbp),%rdx
    ffffffff804b9b8c: 598 eb 2c jmp ffffffff804b9bba <__inet_lookup_established+0xa8>
    ffffffff804b9b8e: 447 48 81 3c 24 d0 15 ab cmpq $0xffffffff80ab15d0,(%rsp)
    ffffffff804b9b95: 0 80
    ffffffff804b9b96: 1119 75 1f jne ffffffff804b9bb7 <__inet_lookup_established+0xa5>
    ffffffff804b9b98: 21 4c 39 b8 30 02 00 00 cmp %r15,0x230(%rax)
    ffffffff804b9b9f: 0 75 16 jne ffffffff804b9bb7 <__inet_lookup_established+0xa5>
    ffffffff804b9ba1: 492 44 39 b0 38 02 00 00 cmp %r14d,0x238(%rax)
    ffffffff804b9ba8: 0 75 0d jne ffffffff804b9bb7 <__inet_lookup_established+0xa5>
    ffffffff804b9baa: 0 8b 52 fc mov -0x4(%rdx),%edx
    ffffffff804b9bad: 451 85 d2 test %edx,%edx
    ffffffff804b9baf: 0 74 67 je ffffffff804b9c18 <__inet_lookup_established+0x106>
    ffffffff804b9bb1: 0 3b 54 24 40 cmp 0x40(%rsp),%edx
    ffffffff804b9bb5: 0 74 61 je ffffffff804b9c18 <__inet_lookup_established+0x106>
    ffffffff804b9bb7: 0 48 89 ca mov %rcx,%rdx
    ffffffff804b9bba: 402 48 85 d2 test %rdx,%rdx
    ffffffff804b9bbd: 1006 74 12 je ffffffff804b9bd1 <__inet_lookup_established+0xbf>
    ffffffff804b9bbf: 0 48 8d 42 f8 lea -0x8(%rdx),%rax
    ffffffff804b9bc3: 821 48 8b 0a mov (%rdx),%rcx
    ffffffff804b9bc6: 78 44 39 68 2c cmp %r13d,0x2c(%rax)
    ffffffff804b9bca: 4 0f 18 09 prefetcht0 (%rcx)
    ffffffff804b9bcd: 685 75 e8 jne ffffffff804b9bb7 <__inet_lookup_established+0xa5>
    ffffffff804b9bcf: 139502 eb bd jmp ffffffff804b9b8e <__inet_lookup_established+0x7c>
    ffffffff804b9bd1: 0 48 8b 55 08 mov 0x8(%rbp),%rdx
    ffffffff804b9bd5: 0 eb 26 jmp ffffffff804b9bfd <__inet_lookup_established+0xeb>
    ffffffff804b9bd7: 0 48 81 3c 24 d0 15 ab cmpq $0xffffffff80ab15d0,(%rsp)
    ffffffff804b9bde: 0 80
    ffffffff804b9bdf: 0 75 19 jne ffffffff804b9bfa <__inet_lookup_established+0xe8>
    ffffffff804b9be1: 0 4c 39 78 40 cmp %r15,0x40(%rax)
    ffffffff804b9be5: 0 75 13 jne ffffffff804b9bfa <__inet_lookup_established+0xe8>
    ffffffff804b9be7: 0 44 39 70 48 cmp %r14d,0x48(%rax)
    ffffffff804b9beb: 0 75 0d jne ffffffff804b9bfa <__inet_lookup_established+0xe8>
    ffffffff804b9bed: 0 8b 52 fc mov -0x4(%rdx),%edx
    ffffffff804b9bf0: 0 85 d2 test %edx,%edx
    ffffffff804b9bf2: 0 74 24 je ffffffff804b9c18 <__inet_lookup_established+0x106>
    ffffffff804b9bf4: 0 3b 54 24 40 cmp 0x40(%rsp),%edx
    ffffffff804b9bf8: 0 74 1e je ffffffff804b9c18 <__inet_lookup_established+0x106>
    ffffffff804b9bfa: 0 48 89 ca mov %rcx,%rdx
    ffffffff804b9bfd: 0 48 85 d2 test %rdx,%rdx
    ffffffff804b9c00: 0 74 12 je ffffffff804b9c14 <__inet_lookup_established+0x102>
    ffffffff804b9c02: 0 48 8d 42 f8 lea -0x8(%rdx),%rax
    ffffffff804b9c06: 0 48 8b 0a mov (%rdx),%rcx
    ffffffff804b9c09: 0 44 39 68 2c cmp %r13d,0x2c(%rax)
    ffffffff804b9c0d: 0 0f 18 09 prefetcht0 (%rcx)
    ffffffff804b9c10: 0 75 e8 jne ffffffff804b9bfa <__inet_lookup_established+0xe8>
    ffffffff804b9c12: 0 eb c3 jmp ffffffff804b9bd7 <__inet_lookup_established+0xc5>
    ffffffff804b9c14: 0 31 c0 xor %eax,%eax
    ffffffff804b9c16: 0 eb 04 jmp ffffffff804b9c1c <__inet_lookup_established+0x10a>
    ffffffff804b9c18: 441 f0 ff 40 28 lock incl 0x28(%rax)
    ffffffff804b9c1c: 1442 f0 41 ff 04 24 lock incl (%r12)
    ffffffff804b9c21: 476 41 5b pop %r11
    ffffffff804b9c23: 1 5b pop %rbx
    ffffffff804b9c24: 0 5d pop %rbp
    ffffffff804b9c25: 475 41 5c pop %r12
    ffffffff804b9c27: 0 41 5d pop %r13
    ffffffff804b9c29: 1 41 5e pop %r14
    ffffffff804b9c2b: 494 41 5f pop %r15
    ffffffff804b9c2d: 0 c3 retq
    ffffffff804b9c2e: 0 90 nop
    ffffffff804b9c2f: 0 90 nop

    80% of the overhead comes from cachemisses here:

    ffffffff804b9bc6: 78 44 39 68 2c cmp %r13d,0x2c(%rax)
    ffffffff804b9bca: 4 0f 18 09 prefetcht0 (%rcx)
    ffffffff804b9bcd: 685 75 e8 jne ffffffff804b9bb7 <__inet_lookup_established+0xa5>
    ffffffff804b9bcf: 139502 eb bd jmp ffffffff804b9b8e <__inet_lookup_established+0x7c>

    corresponding to:

    (gdb) list *0xffffffff804b9bc6
    0xffffffff804b9bc6 is in __inet_lookup_established (net/ipv4/inet_hashtables.c:237).
    232 rwlock_t *lock = inet_ehash_lockp(hashinfo, hash);
    233
    234 prefetch(head->chain.first);
    235 read_lock(lock);
    236 sk_for_each(sk, node, &head->chain) {
    237 if (INET_MATCH(sk, net, hash, acookie,
    238 saddr, daddr, ports, dif))
    239 goto hit; /* You sunk my battleship! */
    240 }
    241

    Seeing the first hard cachemiss on hash lookups is a familiar and
    partly expected pattern - it is the first thing that touches
    cache-cold data structures.

    Seeing 1.4% of the totaly tbench overhead go into this single
    cachemiss is a bit surprising to me though: tbench works via
    long-lived connections (TCP establish costs and nowhere to be seen in
    the profiles) so the socket hash should be relatively stable and
    read-mostly on most CPUs in theory. The CPUs here have 2MB of L2 cache
    per socket.

    Could we be somehow dirtying these cachelines perhaps, causing
    unnecessary cachemisses in hash lookups? Is the hash linkage portion
    of the socket data structure frequently dirtied? Padding that to 64
    bytes (or next to 64 bytes worth of read-mostly fields) could perhaps
    give us a +1.7% tbench speedup.

    Ingo


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