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

[Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>]

* Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org> wrote:

> 100.000000 total
> ................
>   2.118525 skb_release_head_state

                      hits (total: 211852)
                 .........
ffffffff8048938e:      967 <skb_release_head_state>:
ffffffff8048938e:      967 	53                   	push   %rbx
ffffffff8048938f:     3975 	48 89 fb             	mov    %rdi,%rbx
ffffffff80489392:       17 	48 8b 7f 28          	mov    0x28(%rdi),%rdi
ffffffff80489396:        0 	e8 9c 93 00 00       	callq  ffffffff80492737 <dst_release>
ffffffff8048939b:        6 	48 8b 7b 30          	mov    0x30(%rbx),%rdi
ffffffff8048939f:     2887 	48 85 ff             	test   %rdi,%rdi
ffffffff804893a2:      859 	74 0f                	je     ffffffff804893b3 <skb_release_head_state+0x25>
ffffffff804893a4:        0 	f0 ff 0f             	lock decl (%rdi)
ffffffff804893a7:        0 	0f 94 c0             	sete   %al
ffffffff804893aa:        0 	84 c0                	test   %al,%al
ffffffff804893ac:        0 	74 05                	je     ffffffff804893b3 <skb_release_head_state+0x25>
ffffffff804893ae:        0 	e8 7a 14 06 00       	callq  ffffffff804ea82d <__secpath_destroy>
ffffffff804893b3:       16 	48 83 bb 80 00 00 00 	cmpq   $0x0,0x80(%rbx)
ffffffff804893ba:        0 	00 
ffffffff804893bb:     4294 	74 31                	je     ffffffff804893ee <skb_release_head_state+0x60>
ffffffff804893bd:        0 	65 48 8b 04 25 10 00 	mov    %gs:0x10,%rax
ffffffff804893c4:        0 	00 00 
ffffffff804893c6:     6540 	48 63 80 48 e0 ff ff 	movslq -0x1fb8(%rax),%rax
ffffffff804893cd:       14 	a9 00 00 ff 0f       	test   $0xfff0000,%eax
ffffffff804893d2:      471 	74 11                	je     ffffffff804893e5 <skb_release_head_state+0x57>
ffffffff804893d4:        0 	be 89 01 00 00       	mov    $0x189,%esi
ffffffff804893d9:        0 	48 c7 c7 cc b1 6a 80 	mov    $0xffffffff806ab1cc,%rdi
ffffffff804893e0:        0 	e8 d0 cd da ff       	callq  ffffffff802361b5 <warn_on_slowpath>
ffffffff804893e5:        0 	48 89 df             	mov    %rbx,%rdi
ffffffff804893e8:     1733 	ff 93 80 00 00 00    	callq  *0x80(%rbx)
ffffffff804893ee:      888 	48 8b bb 88 00 00 00 	mov    0x88(%rbx),%rdi
ffffffff804893f5:     3959 	48 85 ff             	test   %rdi,%rdi
ffffffff804893f8:        0 	74 0f                	je     ffffffff80489409 <skb_release_head_state+0x7b>
ffffffff804893fa:        0 	f0 ff 0f             	lock decl (%rdi)
ffffffff804893fd:        0 	0f 94 c0             	sete   %al
ffffffff80489400:        0 	84 c0                	test   %al,%al
ffffffff80489402:        0 	74 05                	je     ffffffff80489409 <skb_release_head_state+0x7b>
ffffffff80489404:        0 	e8 48 f2 01 00       	callq  ffffffff804a8651 <nf_conntrack_destroy>
ffffffff80489409:        0 	48 8b bb 90 00 00 00 	mov    0x90(%rbx),%rdi
ffffffff80489410:     3132 	48 85 ff             	test   %rdi,%rdi
ffffffff80489413:        1 	74 05                	je     ffffffff8048941a <skb_release_head_state+0x8c>
ffffffff80489415:        0 	e8 d7 f7 ff ff       	callq  ffffffff80488bf1 <kfree_skb>
ffffffff8048941a:      958 	48 8b bb 98 00 00 00 	mov    0x98(%rbx),%rdi
ffffffff80489421:     1999 	48 85 ff             	test   %rdi,%rdi
ffffffff80489424:        0 	74 0f                	je     ffffffff80489435 <skb_release_head_state+0xa7>
ffffffff80489426:        0 	f0 ff 0f             	lock decl (%rdi)
ffffffff80489429:        0 	0f 94 c0             	sete   %al
ffffffff8048942c:        0 	84 c0                	test   %al,%al
ffffffff8048942e:        0 	74 05                	je     ffffffff80489435 <skb_release_head_state+0xa7>
ffffffff80489430:        0 	e8 a7 5f e0 ff       	callq  ffffffff8028f3dc <kfree>
ffffffff80489435:        0 	66 c7 83 a6 00 00 00 	movw   $0x0,0xa6(%rbx)
ffffffff8048943c:        0 	00 00 
ffffffff8048943e:     6503 	66 c7 83 a8 00 00 00 	movw   $0x0,0xa8(%rbx)
ffffffff80489445:        0 	00 00 
ffffffff80489447:   174101 	5b                   	pop    %rbx
ffffffff80489448:        0 	c3                   	retq   

this function _really_ hurts from a 16-bit op:

ffffffff8048943e:     6503 	66 c7 83 a8 00 00 00 	movw   $0x0,0xa8(%rbx)
ffffffff80489445:        0 	00 00 
ffffffff80489447:   174101 	5b                   	pop    %rbx

(gdb) list *0xffffffff8048943e
0xffffffff8048943e is in skb_release_head_state 
(net/core/skbuff.c:407).
402	#endif
403	/* XXX: IS this still necessary? - JHS */
404	#ifdef CONFIG_NET_SCHED
405		skb->tc_index = 0;
406	#ifdef CONFIG_NET_CLS_ACT
407		skb->tc_verd = 0;
408	#endif
409	#endif
410	}
411	

dirtying skb->tc_verd. I do have:

CONFIG_NET_CLS_ACT=y

BUT, on a second look, i dont think it's really this 16-bit op that 
hurts us. The wider context is:

ffffffff80489426:        0 	f0 ff 0f             	lock decl (%rdi)
ffffffff80489429:        0 	0f 94 c0             	sete   %al
ffffffff8048942c:        0 	84 c0                	test   %al,%al
ffffffff8048942e:        0 	74 05                	je     ffffffff80489435 <skb_release_head_state+0xa7>
ffffffff80489430:        0 	e8 a7 5f e0 ff       	callq  ffffffff8028f3dc <kfree>
ffffffff80489435:        0 	66 c7 83 a6 00 00 00 	movw   $0x0,0xa6(%rbx)
ffffffff8048943c:        0 	00 00 
ffffffff8048943e:     6503 	66 c7 83 a8 00 00 00 	movw   $0x0,0xa8(%rbx)
ffffffff80489445:        0 	00 00 
ffffffff80489447:   174101 	5b                   	pop    %rbx
ffffffff80489448:        0 	c3                   	retq   

look how we jump over the callq most of the time - so what we are 
seeing here i believe is the cost of the atomic op at 
ffffffff80489426. That comes from:

(gdb) list *0xffffffff8048942e
0xffffffff8048942e is in skb_release_head_state (include/linux/skbuff.h:1783).
1778	}
1779	#endif
1780	#ifdef CONFIG_BRIDGE_NETFILTER
1781	static inline void nf_bridge_put(struct nf_bridge_info *nf_bridge)
1782	{
1783		if (nf_bridge && atomic_dec_and_test(&nf_bridge->use))
1784			kfree(nf_bridge);
1785	}
1786	static inline void nf_bridge_get(struct nf_bridge_info *nf_bridge)
1787	{

and ouch does that global dec on &nf_bridge->use hurt!

i do have:

  CONFIG_BRIDGE_NETFILTER=y

(this is a Fedora distro kernel derived .config)

	Ingo