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SubjectRe: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

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

> 100.000000 total
> ................
> 1.469183 tcp_current_mss

hits (total: 146918)
.........
ffffffff804c5237: 526 <tcp_current_mss>:
ffffffff804c5237: 526 41 54 push %r12
ffffffff804c5239: 5929 55 push %rbp
ffffffff804c523a: 32 53 push %rbx
ffffffff804c523b: 294 48 89 fb mov %rdi,%rbx
ffffffff804c523e: 539 48 83 ec 30 sub $0x30,%rsp
ffffffff804c5242: 2590 85 f6 test %esi,%esi
ffffffff804c5244: 444 48 8b 4f 78 mov 0x78(%rdi),%rcx
ffffffff804c5248: 521 8b af 4c 04 00 00 mov 0x44c(%rdi),%ebp
ffffffff804c524e: 791 74 2a je ffffffff804c527a <tcp_current_mss+0x43>
ffffffff804c5250: 433 8b 87 00 01 00 00 mov 0x100(%rdi),%eax
ffffffff804c5256: 236 c1 e0 10 shl $0x10,%eax
ffffffff804c5259: 191 89 c2 mov %eax,%edx
ffffffff804c525b: 487 23 97 fc 00 00 00 and 0xfc(%rdi),%edx
ffffffff804c5261: 362 39 c2 cmp %eax,%edx
ffffffff804c5263: 342 75 15 jne ffffffff804c527a <tcp_current_mss+0x43>
ffffffff804c5265: 473 45 31 e4 xor %r12d,%r12d
ffffffff804c5268: 221 8b 87 00 04 00 00 mov 0x400(%rdi),%eax
ffffffff804c526e: 194 3b 87 80 04 00 00 cmp 0x480(%rdi),%eax
ffffffff804c5274: 445 41 0f 94 c4 sete %r12b
ffffffff804c5278: 261 eb 03 jmp ffffffff804c527d <tcp_current_mss+0x46>
ffffffff804c527a: 0 45 31 e4 xor %r12d,%r12d
ffffffff804c527d: 185 48 85 c9 test %rcx,%rcx
ffffffff804c5280: 686 74 15 je ffffffff804c5297 <tcp_current_mss+0x60>
ffffffff804c5282: 1806 8b 71 7c mov 0x7c(%rcx),%esi
ffffffff804c5285: 1 3b b3 5c 03 00 00 cmp 0x35c(%rbx),%esi
ffffffff804c528b: 21 74 0a je ffffffff804c5297 <tcp_current_mss+0x60>
ffffffff804c528d: 0 48 89 df mov %rbx,%rdi
ffffffff804c5290: 0 e8 8b fb ff ff callq ffffffff804c4e20 <tcp_sync_mss>
ffffffff804c5295: 0 89 c5 mov %eax,%ebp
ffffffff804c5297: 864 48 8d 4c 24 28 lea 0x28(%rsp),%rcx
ffffffff804c529c: 634 48 8d 54 24 10 lea 0x10(%rsp),%rdx
ffffffff804c52a1: 995 31 f6 xor %esi,%esi
ffffffff804c52a3: 0 48 89 df mov %rbx,%rdi
ffffffff804c52a6: 2 e8 f2 fe ff ff callq ffffffff804c519d <tcp_established_options>
ffffffff804c52ab: 859 8b 8b e8 03 00 00 mov 0x3e8(%rbx),%ecx
ffffffff804c52b1: 936 83 c0 14 add $0x14,%eax
ffffffff804c52b4: 6 0f b7 d1 movzwl %cx,%edx
ffffffff804c52b7: 0 39 d0 cmp %edx,%eax
ffffffff804c52b9: 911 74 04 je ffffffff804c52bf <tcp_current_mss+0x88>
ffffffff804c52bb: 0 29 d0 sub %edx,%eax
ffffffff804c52bd: 0 29 c5 sub %eax,%ebp
ffffffff804c52bf: 0 45 85 e4 test %r12d,%r12d
ffffffff804c52c2: 6894 89 e8 mov %ebp,%eax
ffffffff804c52c4: 0 74 38 je ffffffff804c52fe <tcp_current_mss+0xc7>
ffffffff804c52c6: 990 48 8b 83 68 03 00 00 mov 0x368(%rbx),%rax
ffffffff804c52cd: 642 8b b3 04 01 00 00 mov 0x104(%rbx),%esi
ffffffff804c52d3: 3 48 89 df mov %rbx,%rdi
ffffffff804c52d6: 240 66 2b 70 30 sub 0x30(%rax),%si
ffffffff804c52da: 588 66 2b b3 7e 03 00 00 sub 0x37e(%rbx),%si
ffffffff804c52e1: 2 66 29 ce sub %cx,%si
ffffffff804c52e4: 284 ff ce dec %esi
ffffffff804c52e6: 664 0f b7 f6 movzwl %si,%esi
ffffffff804c52e9: 2 e8 0a fb ff ff callq ffffffff804c4df8 <tcp_bound_to_half_wnd>
ffffffff804c52ee: 68 0f b7 d0 movzwl %ax,%edx
ffffffff804c52f1: 1870 89 c1 mov %eax,%ecx
ffffffff804c52f3: 0 89 d0 mov %edx,%eax
ffffffff804c52f5: 0 31 d2 xor %edx,%edx
ffffffff804c52f7: 2135 f7 f5 div %ebp
ffffffff804c52f9: 107010 89 c8 mov %ecx,%eax
ffffffff804c52fb: 1670 66 29 d0 sub %dx,%ax
ffffffff804c52fe: 0 66 89 83 ea 03 00 00 mov %ax,0x3ea(%rbx)
ffffffff804c5305: 4 48 83 c4 30 add $0x30,%rsp
ffffffff804c5309: 855 89 e8 mov %ebp,%eax
ffffffff804c530b: 0 5b pop %rbx
ffffffff804c530c: 797 5d pop %rbp
ffffffff804c530d: 0 41 5c pop %r12
ffffffff804c530f: 0 c3 retq
apparently this division causes 1.0% of tbench overhead:

ffffffff804c52f5: 0 31 d2 xor %edx,%edx
ffffffff804c52f7: 2135 f7 f5 div %ebp
ffffffff804c52f9: 107010 89 c8 mov %ecx,%eax
(gdb) list *0xffffffff804c52f7
0xffffffff804c52f7 is in tcp_current_mss (net/ipv4/tcp_output.c:1078).
1073 inet_csk(sk)->icsk_af_ops->net_header_len -
1074 inet_csk(sk)->icsk_ext_hdr_len -
1075 tp->tcp_header_len);
1076
1077 xmit_size_goal = tcp_bound_to_half_wnd(tp, xmit_size_goal);
1078 xmit_size_goal -= (xmit_size_goal % mss_now);
1079 }
1080 tp->xmit_size_goal = xmit_size_goal;
1081
1082 return mss_now;
(gdb)
it's this division:

if (doing_tso) {
[...]
xmit_size_goal -= (xmit_size_goal % mss_now);
Has no-one hit this before? Perhaps this is why switching loopback
networking to TSO had a performance impact for others?

It's still a bit weird ... how can a single division cause this much
overhead? tcp_bound_to_half_wnd() [which is called straight before
this sequence] seems low-overhead.

Ingo


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