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Subjectip_queue_xmit(): 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
> ................
> 3.356152 ip_queue_xmit

hits (335615 total)
.........
ffffffff804b7045: 1001 <ip_queue_xmit>:
ffffffff804b7045: 1001 41 57 push %r15
ffffffff804b7047: 36698 41 56 push %r14
ffffffff804b7049: 0 49 89 fe mov %rdi,%r14
ffffffff804b704c: 0 41 55 push %r13
ffffffff804b704e: 447 41 54 push %r12
ffffffff804b7050: 0 55 push %rbp
ffffffff804b7051: 4 53 push %rbx
ffffffff804b7052: 465 48 83 ec 68 sub $0x68,%rsp
ffffffff804b7056: 1 89 74 24 08 mov %esi,0x8(%rsp)
ffffffff804b705a: 486 48 8b 47 28 mov 0x28(%rdi),%rax
ffffffff804b705e: 0 48 8b 6f 10 mov 0x10(%rdi),%rbp
ffffffff804b7062: 7 48 85 c0 test %rax,%rax
ffffffff804b7065: 480 48 89 44 24 58 mov %rax,0x58(%rsp)
ffffffff804b706a: 0 4c 8b bd 48 02 00 00 mov 0x248(%rbp),%r15
ffffffff804b7071: 7 0f 85 0d 01 00 00 jne ffffffff804b7184 <ip_queue_xmit+0x13f>
ffffffff804b7077: 452 31 f6 xor %esi,%esi
ffffffff804b7079: 0 48 89 ef mov %rbp,%rdi
ffffffff804b707c: 5 e8 c1 eb fc ff callq ffffffff80485c42 <__sk_dst_check>
ffffffff804b7081: 434 48 85 c0 test %rax,%rax
ffffffff804b7084: 54 48 89 44 24 58 mov %rax,0x58(%rsp)
ffffffff804b7089: 0 0f 85 e0 00 00 00 jne ffffffff804b716f <ip_queue_xmit+0x12a>
ffffffff804b708f: 0 4d 85 ff test %r15,%r15
ffffffff804b7092: 0 44 8b ad 30 02 00 00 mov 0x230(%rbp),%r13d
ffffffff804b7099: 0 74 0a je ffffffff804b70a5 <ip_queue_xmit+0x60>
ffffffff804b709b: 0 41 80 7f 05 00 cmpb $0x0,0x5(%r15)
ffffffff804b70a0: 0 74 03 je ffffffff804b70a5 <ip_queue_xmit+0x60>
ffffffff804b70a2: 0 45 8b 2f mov (%r15),%r13d
ffffffff804b70a5: 0 8b 85 3c 02 00 00 mov 0x23c(%rbp),%eax
ffffffff804b70ab: 0 48 8d b5 10 01 00 00 lea 0x110(%rbp),%rsi
ffffffff804b70b2: 0 44 8b 65 04 mov 0x4(%rbp),%r12d
ffffffff804b70b6: 0 bf 0d 00 00 00 mov $0xd,%edi
ffffffff804b70bb: 0 89 44 24 0c mov %eax,0xc(%rsp)
ffffffff804b70bf: 0 8a 9d 54 02 00 00 mov 0x254(%rbp),%bl
ffffffff804b70c5: 0 e8 9a df ff ff callq ffffffff804b5064 <constant_test_bit>
ffffffff804b70ca: 0 31 d2 xor %edx,%edx
ffffffff804b70cc: 0 48 8d 7c 24 10 lea 0x10(%rsp),%rdi
ffffffff804b70d1: 0 41 89 c3 mov %eax,%r11d
ffffffff804b70d4: 0 fc cld
ffffffff804b70d5: 0 89 d0 mov %edx,%eax
ffffffff804b70d7: 0 b9 10 00 00 00 mov $0x10,%ecx
ffffffff804b70dc: 0 44 8a 45 39 mov 0x39(%rbp),%r8b
ffffffff804b70e0: 0 40 8a b5 57 02 00 00 mov 0x257(%rbp),%sil
ffffffff804b70e7: 0 44 8b 8d 50 02 00 00 mov 0x250(%rbp),%r9d
ffffffff804b70ee: 0 83 e3 1e and $0x1e,%ebx
ffffffff804b70f1: 0 44 8b 95 38 02 00 00 mov 0x238(%rbp),%r10d
ffffffff804b70f8: 0 44 09 db or %r11d,%ebx
ffffffff804b70fb: 0 f3 ab rep stos %eax,%es:(%rdi)
ffffffff804b70fd: 0 40 c0 ee 05 shr $0x5,%sil
ffffffff804b7101: 0 88 5c 24 24 mov %bl,0x24(%rsp)
ffffffff804b7105: 0 48 8d 5c 24 10 lea 0x10(%rsp),%rbx
ffffffff804b710a: 0 83 e6 01 and $0x1,%esi
ffffffff804b710d: 0 48 89 ef mov %rbp,%rdi
ffffffff804b7110: 0 44 88 44 24 40 mov %r8b,0x40(%rsp)
ffffffff804b7115: 0 8b 44 24 0c mov 0xc(%rsp),%eax
ffffffff804b7119: 0 40 88 74 24 41 mov %sil,0x41(%rsp)
ffffffff804b711e: 0 48 89 de mov %rbx,%rsi
ffffffff804b7121: 0 66 44 89 4c 24 44 mov %r9w,0x44(%rsp)
ffffffff804b7127: 0 66 44 89 54 24 46 mov %r10w,0x46(%rsp)
ffffffff804b712d: 0 44 89 64 24 10 mov %r12d,0x10(%rsp)
ffffffff804b7132: 0 44 89 6c 24 1c mov %r13d,0x1c(%rsp)
ffffffff804b7137: 0 89 44 24 20 mov %eax,0x20(%rsp)
ffffffff804b713b: 0 e8 2d 9f e5 ff callq ffffffff8031106d <security_sk_classify_flow>
ffffffff804b7140: 0 48 8d 74 24 58 lea 0x58(%rsp),%rsi
ffffffff804b7145: 0 45 31 c0 xor %r8d,%r8d
ffffffff804b7148: 0 48 89 e9 mov %rbp,%rcx
ffffffff804b714b: 0 48 89 da mov %rbx,%rdx
ffffffff804b714e: 0 48 c7 c7 d0 15 ab 80 mov $0xffffffff80ab15d0,%rdi
ffffffff804b7155: 0 e8 1a 91 ff ff callq ffffffff804b0274 <ip_route_output_flow>
ffffffff804b715a: 0 85 c0 test %eax,%eax
ffffffff804b715c: 0 0f 85 9f 01 00 00 jne ffffffff804b7301 <ip_queue_xmit+0x2bc>
ffffffff804b7162: 0 48 8b 74 24 58 mov 0x58(%rsp),%rsi
ffffffff804b7167: 0 48 89 ef mov %rbp,%rdi
ffffffff804b716a: 0 e8 a8 eb fc ff callq ffffffff80485d17 <sk_setup_caps>
ffffffff804b716f: 441 48 8b 44 24 58 mov 0x58(%rsp),%rax
ffffffff804b7174: 1388 48 85 c0 test %rax,%rax
ffffffff804b7177: 0 74 07 je ffffffff804b7180 <ip_queue_xmit+0x13b>
ffffffff804b7179: 0 f0 ff 80 b0 00 00 00 lock incl 0xb0(%rax)
ffffffff804b7180: 556 49 89 46 28 mov %rax,0x28(%r14)
ffffffff804b7184: 8351 4d 85 ff test %r15,%r15
ffffffff804b7187: 0 be 14 00 00 00 mov $0x14,%esi
ffffffff804b718c: 461 74 26 je ffffffff804b71b4 <ip_queue_xmit+0x16f>
ffffffff804b718e: 0 41 f6 47 08 01 testb $0x1,0x8(%r15)
ffffffff804b7193: 0 74 17 je ffffffff804b71ac <ip_queue_xmit+0x167>
ffffffff804b7195: 0 48 8b 54 24 58 mov 0x58(%rsp),%rdx
ffffffff804b719a: 0 8b 82 28 01 00 00 mov 0x128(%rdx),%eax
ffffffff804b71a0: 0 39 82 1c 01 00 00 cmp %eax,0x11c(%rdx)
ffffffff804b71a6: 0 0f 85 55 01 00 00 jne ffffffff804b7301 <ip_queue_xmit+0x2bc>
ffffffff804b71ac: 0 41 0f b6 47 04 movzbl 0x4(%r15),%eax
ffffffff804b71b1: 0 8d 70 14 lea 0x14(%rax),%esi
ffffffff804b71b4: 39 4c 89 f7 mov %r14,%rdi
ffffffff804b71b7: 493 e8 f8 18 fd ff callq ffffffff80488ab4 <skb_push>
ffffffff804b71bc: 0 4c 89 f7 mov %r14,%rdi
ffffffff804b71bf: 1701 e8 99 df ff ff callq ffffffff804b515d <skb_reset_network_header>
ffffffff804b71c4: 481 0f b6 85 54 02 00 00 movzbl 0x254(%rbp),%eax
ffffffff804b71cb: 4202 41 8b 9e bc 00 00 00 mov 0xbc(%r14),%ebx
ffffffff804b71d2: 3 48 89 ef mov %rbp,%rdi
ffffffff804b71d5: 0 49 03 9e d0 00 00 00 add 0xd0(%r14),%rbx
ffffffff804b71dc: 466 80 cc 45 or $0x45,%ah
ffffffff804b71df: 7 66 c1 c0 08 rol $0x8,%ax
ffffffff804b71e3: 0 66 89 03 mov %ax,(%rbx)
ffffffff804b71e6: 492 48 8b 74 24 58 mov 0x58(%rsp),%rsi
ffffffff804b71eb: 3 e8 a0 df ff ff callq ffffffff804b5190 <ip_dont_fragment>
ffffffff804b71f0: 1405 85 c0 test %eax,%eax
ffffffff804b71f2: 4391 74 0f je ffffffff804b7203 <ip_queue_xmit+0x1be>
ffffffff804b71f4: 0 83 7c 24 08 00 cmpl $0x0,0x8(%rsp)
ffffffff804b71f9: 417 75 08 jne ffffffff804b7203 <ip_queue_xmit+0x1be>
ffffffff804b71fb: 503 66 c7 43 06 40 00 movw $0x40,0x6(%rbx)
ffffffff804b7201: 6743 eb 06 jmp ffffffff804b7209 <ip_queue_xmit+0x1c4>
ffffffff804b7203: 0 66 c7 43 06 00 00 movw $0x0,0x6(%rbx)
ffffffff804b7209: 118 0f bf 85 40 02 00 00 movswl 0x240(%rbp),%eax
ffffffff804b7210: 10867 48 8b 54 24 58 mov 0x58(%rsp),%rdx
ffffffff804b7215: 340 85 c0 test %eax,%eax
ffffffff804b7217: 0 79 06 jns ffffffff804b721f <ip_queue_xmit+0x1da>
ffffffff804b7219: 107464 8b 82 9c 00 00 00 mov 0x9c(%rdx),%eax
ffffffff804b721f: 4963 88 43 08 mov %al,0x8(%rbx)
ffffffff804b7222: 26297 8a 45 39 mov 0x39(%rbp),%al
ffffffff804b7225: 76658 4d 85 ff test %r15,%r15
ffffffff804b7228: 1712 88 43 09 mov %al,0x9(%rbx)
ffffffff804b722b: 148 48 8b 44 24 58 mov 0x58(%rsp),%rax
ffffffff804b7230: 2971 8b 80 20 01 00 00 mov 0x120(%rax),%eax
ffffffff804b7236: 14849 89 43 0c mov %eax,0xc(%rbx)
ffffffff804b7239: 84 48 8b 44 24 58 mov 0x58(%rsp),%rax
ffffffff804b723e: 360 8b 80 1c 01 00 00 mov 0x11c(%rax),%eax
ffffffff804b7244: 174 89 43 10 mov %eax,0x10(%rbx)
ffffffff804b7247: 96 74 32 je ffffffff804b727b <ip_queue_xmit+0x236>
ffffffff804b7249: 0 41 8a 57 04 mov 0x4(%r15),%dl
ffffffff804b724d: 0 84 d2 test %dl,%dl
ffffffff804b724f: 0 74 2a je ffffffff804b727b <ip_queue_xmit+0x236>
ffffffff804b7251: 0 c0 ea 02 shr $0x2,%dl
ffffffff804b7254: 0 03 13 add (%rbx),%edx
ffffffff804b7256: 0 8a 03 mov (%rbx),%al
ffffffff804b7258: 0 45 31 c0 xor %r8d,%r8d
ffffffff804b725b: 0 4c 89 fe mov %r15,%rsi
ffffffff804b725e: 0 4c 89 f7 mov %r14,%rdi
ffffffff804b7261: 0 83 e0 f0 and $0xfffffffffffffff0,%eax
ffffffff804b7264: 0 83 e2 0f and $0xf,%edx
ffffffff804b7267: 0 09 d0 or %edx,%eax
ffffffff804b7269: 0 88 03 mov %al,(%rbx)
ffffffff804b726b: 0 48 8b 4c 24 58 mov 0x58(%rsp),%rcx
ffffffff804b7270: 0 8b 95 30 02 00 00 mov 0x230(%rbp),%edx
ffffffff804b7276: 0 e8 e4 d8 ff ff callq ffffffff804b4b5f <ip_options_build>
ffffffff804b727b: 541 41 8b 86 c8 00 00 00 mov 0xc8(%r14),%eax
ffffffff804b7282: 570 31 d2 xor %edx,%edx
ffffffff804b7284: 0 49 03 86 d0 00 00 00 add 0xd0(%r14),%rax
ffffffff804b728b: 34 8b 40 08 mov 0x8(%rax),%eax
ffffffff804b728e: 496 66 85 c0 test %ax,%ax
ffffffff804b7291: 11 74 06 je ffffffff804b7299 <ip_queue_xmit+0x254>
ffffffff804b7293: 9 0f b7 c0 movzwl %ax,%eax
ffffffff804b7296: 495 8d 50 ff lea -0x1(%rax),%edx
ffffffff804b7299: 2 f6 43 06 40 testb $0x40,0x6(%rbx)
ffffffff804b729d: 9 48 8b 74 24 58 mov 0x58(%rsp),%rsi
ffffffff804b72a2: 497 74 34 je ffffffff804b72d8 <ip_queue_xmit+0x293>
ffffffff804b72a4: 8 83 bd 30 02 00 00 00 cmpl $0x0,0x230(%rbp)
ffffffff804b72ab: 10 74 23 je ffffffff804b72d0 <ip_queue_xmit+0x28b>
ffffffff804b72ad: 1044 66 8b 85 52 02 00 00 mov 0x252(%rbp),%ax
ffffffff804b72b4: 7 66 c1 c0 08 rol $0x8,%ax
ffffffff804b72b8: 8 66 89 43 04 mov %ax,0x4(%rbx)
ffffffff804b72bc: 432 66 8b 85 52 02 00 00 mov 0x252(%rbp),%ax
ffffffff804b72c3: 9 ff c0 inc %eax
ffffffff804b72c5: 14 01 d0 add %edx,%eax
ffffffff804b72c7: 1141 66 89 85 52 02 00 00 mov %ax,0x252(%rbp)
ffffffff804b72ce: 7 eb 10 jmp ffffffff804b72e0 <ip_queue_xmit+0x29b>
ffffffff804b72d0: 0 66 c7 43 04 00 00 movw $0x0,0x4(%rbx)
ffffffff804b72d6: 0 eb 08 jmp ffffffff804b72e0 <ip_queue_xmit+0x29b>
ffffffff804b72d8: 0 48 89 df mov %rbx,%rdi
ffffffff804b72db: 0 e8 b7 9d ff ff callq ffffffff804b1097 <__ip_select_ident>
ffffffff804b72e0: 6 8b 85 54 01 00 00 mov 0x154(%rbp),%eax
ffffffff804b72e6: 458 4c 89 f7 mov %r14,%rdi
ffffffff804b72e9: 2 41 89 46 78 mov %eax,0x78(%r14)
ffffffff804b72ed: 4 8b 85 f0 01 00 00 mov 0x1f0(%rbp),%eax
ffffffff804b72f3: 841 41 89 86 b0 00 00 00 mov %eax,0xb0(%r14)
ffffffff804b72fa: 11 e8 30 f2 ff ff callq ffffffff804b652f <ip_local_out>
ffffffff804b72ff: 0 eb 44 jmp ffffffff804b7345 <ip_queue_xmit+0x300>
ffffffff804b7301: 0 65 48 8b 04 25 10 00 mov %gs:0x10,%rax
ffffffff804b7308: 0 00 00
ffffffff804b730a: 0 8b 80 48 e0 ff ff mov -0x1fb8(%rax),%eax
ffffffff804b7310: 0 4c 89 f7 mov %r14,%rdi
ffffffff804b7313: 0 30 c0 xor %al,%al
ffffffff804b7315: 0 66 83 f8 01 cmp $0x1,%ax
ffffffff804b7319: 0 48 19 c0 sbb %rax,%rax
ffffffff804b731c: 0 83 e0 08 and $0x8,%eax
ffffffff804b731f: 0 48 8b 90 a8 16 ab 80 mov -0x7f54e958(%rax),%rdx
ffffffff804b7326: 0 65 8b 04 25 24 00 00 mov %gs:0x24,%eax
ffffffff804b732d: 0 00
ffffffff804b732e: 0 89 c0 mov %eax,%eax
ffffffff804b7330: 0 48 f7 d2 not %rdx
ffffffff804b7333: 0 48 8b 04 c2 mov (%rdx,%rax,8),%rax
ffffffff804b7337: 0 48 ff 40 68 incq 0x68(%rax)
ffffffff804b733b: 0 e8 b1 18 fd ff callq ffffffff80488bf1 <kfree_skb>
ffffffff804b7340: 0 b8 8f ff ff ff mov $0xffffff8f,%eax
ffffffff804b7345: 9196 48 83 c4 68 add $0x68,%rsp
ffffffff804b7349: 892 5b pop %rbx
ffffffff804b734a: 0 5d pop %rbp
ffffffff804b734b: 488 41 5c pop %r12
ffffffff804b734d: 0 41 5d pop %r13
ffffffff804b734f: 0 41 5e pop %r14
ffffffff804b7351: 513 41 5f pop %r15
ffffffff804b7353: 0 c3 retq

about 10% of this function's cost is artificial:

ffffffff804b7045: 1001 <ip_queue_xmit>:
ffffffff804b7045: 1001 41 57 push %r15
ffffffff804b7047: 36698 41 56 push %r14

there are profiler hits that leaked in via out-of-order execution from
the callsites. The callsites are hard to map unfortunately, as this
function is called via function pointers.

the most likely callsite is tcp_transmit_skb().

30% of the overhead of this function comes from:

ffffffff804b7203: 0 66 c7 43 06 00 00 movw $0x0,0x6(%rbx)
ffffffff804b7209: 118 0f bf 85 40 02 00 00 movswl 0x240(%rbp),%eax
ffffffff804b7210: 10867 48 8b 54 24 58 mov 0x58(%rsp),%rdx
ffffffff804b7215: 340 85 c0 test %eax,%eax
ffffffff804b7217: 0 79 06 jns ffffffff804b721f <ip_queue_xmit+0x1da>
ffffffff804b7219: 107464 8b 82 9c 00 00 00 mov 0x9c(%rdx),%eax
ffffffff804b721f: 4963 88 43 08 mov %al,0x8(%rbx)

the 16-bit movw looks a bit weird. It comes from line 372:

0xffffffff804b7203 is in ip_queue_xmit (net/ipv4/ip_output.c:372).
367 iph = ip_hdr(skb);
368 *((__be16 *)iph) = htons((4 << 12) | (5 << 8) | (inet->tos & 0xff));
369 if (ip_dont_fragment(sk, &rt->u.dst) && !ipfragok)
370 iph->frag_off = htons(IP_DF);
371 else
372 iph->frag_off = 0;
373 iph->ttl = ip_select_ttl(inet, &rt->u.dst);
374 iph->protocol = sk->sk_protocol;
375 iph->saddr = rt->rt_src;
376 iph->daddr = rt->rt_dst;

the ip-header fragment flag setting to zero.

16-bit ops are an on-off love/hate affair on x86 CPUs. The trend is
towards eliminating them as much as possible.

_But_, the real overhead probably comes from:

ffffffff804b7210: 10867 48 8b 54 24 58 mov 0x58(%rsp),%rdx

which is the next line, the ttl field:

373 iph->ttl = ip_select_ttl(inet, &rt->u.dst);

this shows that we are doing a hard cachemiss on the net-localhost
route dst structure cacheline. We do a plain load instruction from it
here and get a hefty cachemiss. (because 16 CPUs are banging on that
single route)

And let make sure we see this in perspective as well: that single
cachemiss is _1.0 percent_ of the total tbench cost. (!) We could make
the scheduler 10% slower straight away and it would have less of a
real-life effect than this single iph->ttl field setting.

Ingo


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