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

[Ingo Molnar <mingo@elte.hu>]

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

> 100.000000 total
> ................
>   3.038025 skb_release_data

                      hits (303802 total)
                 .........
ffffffff80488c7e:      780 <skb_release_data>:
ffffffff80488c7e:      780 	55                   	push   %rbp
ffffffff80488c7f:   267141 	53                   	push   %rbx
ffffffff80488c80:        0 	48 89 fb             	mov    %rdi,%rbx
ffffffff80488c83:     3552 	48 83 ec 08          	sub    $0x8,%rsp
ffffffff80488c87:      604 	8a 47 7c             	mov    0x7c(%rdi),%al
ffffffff80488c8a:     2644 	a8 02                	test   $0x2,%al
ffffffff80488c8c:       49 	74 2a                	je     ffffffff80488cb8 <skb_release_data+0x3a>
ffffffff80488c8e:        0 	83 e0 10             	and    $0x10,%eax
ffffffff80488c91:     2079 	8b 97 c8 00 00 00    	mov    0xc8(%rdi),%edx
ffffffff80488c97:       53 	3c 01                	cmp    $0x1,%al
ffffffff80488c99:        0 	19 c0                	sbb    %eax,%eax
ffffffff80488c9b:      870 	48 03 97 d0 00 00 00 	add    0xd0(%rdi),%rdx
ffffffff80488ca2:       65 	66 31 c0             	xor    %ax,%ax
ffffffff80488ca5:        0 	05 01 00 01 00       	add    $0x10001,%eax
ffffffff80488caa:      888 	f7 d8                	neg    %eax
ffffffff80488cac:       49 	89 c1                	mov    %eax,%ecx
ffffffff80488cae:        0 	f0 0f c1 0a          	lock xadd %ecx,(%rdx)
ffffffff80488cb2:     1909 	01 c8                	add    %ecx,%eax
ffffffff80488cb4:     1040 	85 c0                	test   %eax,%eax
ffffffff80488cb6:        0 	75 6d                	jne    ffffffff80488d25 <skb_release_data+0xa7>
ffffffff80488cb8:        0 	8b 93 c8 00 00 00    	mov    0xc8(%rbx),%edx
ffffffff80488cbe:     4199 	48 8b 83 d0 00 00 00 	mov    0xd0(%rbx),%rax
ffffffff80488cc5:     4995 	31 ed                	xor    %ebp,%ebp
ffffffff80488cc7:        0 	66 83 7c 10 04 00    	cmpw   $0x0,0x4(%rax,%rdx,1)
ffffffff80488ccd:      983 	75 15                	jne    ffffffff80488ce4 <skb_release_data+0x66>
ffffffff80488ccf:       15 	eb 28                	jmp    ffffffff80488cf9 <skb_release_data+0x7b>
ffffffff80488cd1:      665 	48 63 c5             	movslq %ebp,%rax
ffffffff80488cd4:      546 	ff c5                	inc    %ebp
ffffffff80488cd6:      328 	48 c1 e0 04          	shl    $0x4,%rax
ffffffff80488cda:      356 	48 8b 7c 02 20       	mov    0x20(%rdx,%rax,1),%rdi
ffffffff80488cdf:       95 	e8 be 87 de ff       	callq  ffffffff802714a2 <put_page>
ffffffff80488ce4:       66 	8b 93 c8 00 00 00    	mov    0xc8(%rbx),%edx
ffffffff80488cea:     1321 	48 03 93 d0 00 00 00 	add    0xd0(%rbx),%rdx
ffffffff80488cf1:      439 	0f b7 42 04          	movzwl 0x4(%rdx),%eax
ffffffff80488cf5:        0 	39 c5                	cmp    %eax,%ebp
ffffffff80488cf7:     1887 	7c d8                	jl     ffffffff80488cd1 <skb_release_data+0x53>
ffffffff80488cf9:     2187 	8b 93 c8 00 00 00    	mov    0xc8(%rbx),%edx
ffffffff80488cff:     1784 	48 8b 83 d0 00 00 00 	mov    0xd0(%rbx),%rax
ffffffff80488d06:      422 	48 83 7c 10 18 00    	cmpq   $0x0,0x18(%rax,%rdx,1)
ffffffff80488d0c:      110 	74 08                	je     ffffffff80488d16 <skb_release_data+0x98>
ffffffff80488d0e:        0 	48 89 df             	mov    %rbx,%rdi
ffffffff80488d11:        0 	e8 52 ff ff ff       	callq  ffffffff80488c68 <skb_drop_fraglist>
ffffffff80488d16:       14 	48 8b bb d0 00 00 00 	mov    0xd0(%rbx),%rdi
ffffffff80488d1d:      715 	5e                   	pop    %rsi
ffffffff80488d1e:      109 	5b                   	pop    %rbx
ffffffff80488d1f:       20 	5d                   	pop    %rbp
ffffffff80488d20:      980 	e9 b7 66 e0 ff       	jmpq   ffffffff8028f3dc <kfree>
ffffffff80488d25:        0 	59                   	pop    %rcx
ffffffff80488d26:     1948 	5b                   	pop    %rbx
ffffffff80488d27:        0 	5d                   	pop    %rbp
ffffffff80488d28:        0 	c3                   	retq   

this is a short function, and 90% of the overhead is false leaked-in 
overhead from callsites:

ffffffff80488c7f:   267141 	53                   	push   %rbx

unfortunately i have a hard time mapping its callsites. 
pskb_expand_head() is the only static callsite, but it's not active in 
the profile.

The _usual_ callsite is normally skb_release_all(), which does have 
overhead:

ffffffff80489449:      925 <skb_release_all>:
ffffffff80489449:      925 	53                   	push   %rbx
ffffffff8048944a:     5249 	48 89 fb             	mov    %rdi,%rbx
ffffffff8048944d:        4 	e8 3c ff ff ff       	callq  ffffffff8048938e <skb_release_head_state>
ffffffff80489452:     1149 	48 89 df             	mov    %rbx,%rdi
ffffffff80489455:    13163 	5b                   	pop    %rbx
ffffffff80489456:        0 	e9 23 f8 ff ff       	jmpq   ffffffff80488c7e <skb_release_data>

it is also tail-optimized, which explains why i found little 
callsites. The main callsite of skb_release_all() is:

ffffffff80488b86:       26 	e8 be 08 00 00       	callq  ffffffff80489449 <skb_release_all>

which is __kfree_skb(). That is a frequently referenced function, and 
in my profile there's a single callsite active:

ffffffff804c1027:      432 	e8 56 7b fc ff       	callq  ffffffff80488b82 <__kfree_skb>

which is tcp_ack() - subject of a later email. The wider context is:

ffffffff804c0ffc:      433 	41 2b 85 e0 00 00 00 	sub    0xe0(%r13),%eax
ffffffff804c1003:     4843 	89 85 f0 00 00 00    	mov    %eax,0xf0(%rbp)
ffffffff804c1009:     1730 	48 8b 45 30          	mov    0x30(%rbp),%rax
ffffffff804c100d:      311 	41 8b 95 e0 00 00 00 	mov    0xe0(%r13),%edx
ffffffff804c1014:        0 	48 83 b8 b0 00 00 00 	cmpq   $0x0,0xb0(%rax)
ffffffff804c101b:        0 	00 
ffffffff804c101c:      418 	74 06                	je     ffffffff804c1024 <tcp_ack+0x50d>
ffffffff804c101e:       37 	01 95 f4 00 00 00    	add    %edx,0xf4(%rbp)
ffffffff804c1024:        2 	4c 89 ef             	mov    %r13,%rdi
ffffffff804c1027:      432 	e8 56 7b fc ff       	callq  ffffffff80488b82 <__kfree_skb>

this is a good, top-of-the-line x86 CPU with a really good BTB 
implementation that seems to be able to fall through calls and tail 
optimizations as if they werent there.

some guesses are:

(gdb) list *0xffffffff804c1003
0xffffffff804c1003 is in tcp_ack (include/net/sock.h:789).
784	
785	static inline void sk_wmem_free_skb(struct sock *sk, struct sk_buff *skb)
786	{
787		skb_truesize_check(skb);
788		sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
789		sk->sk_wmem_queued -= skb->truesize;
790		sk_mem_uncharge(sk, skb->truesize);
791		__kfree_skb(skb);
792	}
793	

both sk and skb should be cache-hot here so this seems unlikely.

(gdb) list *0xffffffff804c10090xffffffff804c1009 is in tcp_ack (include/net/sock.h:736).
731	}
732	
733	static inline int sk_has_account(struct sock *sk)
734	{
735		/* return true if protocol supports memory accounting */
736		return !!sk->sk_prot->memory_allocated;
737	}
738	
739	static inline int sk_wmem_schedule(struct sock *sk, int size)
740	{

this cannot be it - unless sk_prot somehow ends up being dirtied or 
false-shared?

Still, my guess would be on ffffffff804c1009 and a 
sk_prot->memory_allocated cachemiss: look at how this instruction uses 
%ebp, and the one that shows the many hits in skb_release_data() 
pushes %ebp to the stack - that's where the CPU's OOO trick ends: it 
has to compute the result and serialize on the cachemiss.

	Ingo