On a dual opteron box, I noticed high oprofile numbers in net/core/dst.c
, function dst_destroy(struct dst_entry * dst)
It appears the smb_rmb() done at the begining of dst_destroy() is the
killer (this is a lfence machine instruction, that apparently is doing
a *lot* of things... may be IO related...) that is responsible for 80%
of the cpu time used by the whole function.
I dont understand very much all variety of available barriers, and why
this smb_rmb() is used in dst_destroy().
I missed the corresponding wmb that should be done somewhere in the dst
code.
Do we have an alternative to smp_rmb() in the dst_destroy()/ kfree_skb()
context ?
Documentation/memory-barriers.txt mentions several 'advanced barrier
functions' but I'm really lost.
ffffffff803b5f80 <dst_destroy>: /* dst_destroy total: 237528 0.5635 */
163 3.9e-04 :ffffffff803b5f80: push %r12
3483 0.0083 :ffffffff803b5f82: push %rbp
:ffffffff803b5f83: mov %rdi,%rbp
7 1.7e-05 :ffffffff803b5f86: push %rbx
201 4.8e-04 :ffffffff803b5f87: lfence
192133 0.4558 :ffffffff803b5f8a: data16
:ffffffff803b5f8b: data16
:ffffffff803b5f8c: nop
4 9.5e-06 :ffffffff803b5f8d: data16
:ffffffff803b5f8e: data16
:ffffffff803b5f8f: nop
:ffffffff803b5f90: mov 0x90(%rbp),%rdi
ffffffff803ae8a0 <kfree_skb>: /* kfree_skb total: 145240 0.3446 */
1873 0.0044 :ffffffff803ae8a0: test %rdi,%rdi
2127 0.0050 :ffffffff803ae8a3: je ffffffff803ae8c7
<kfree_skb+0x27>
81 1.9e-04 :ffffffff803ae8a5: mov 0xbc(%rdi),%eax
1 2.4e-06 :ffffffff803ae8ab: dec %eax
2303 0.0055 :ffffffff803ae8ad: jne ffffffff803ae8b4
<kfree_skb+0x14>
221 5.2e-04 :ffffffff803ae8af: lfence
137609 0.3265 :ffffffff803ae8b2: jmp ffffffff803ae8c2
<kfree_skb+0x22>
:ffffffff803ae8b4: lock decl 0xbc(%rdi)
38 9.0e-05 :ffffffff803ae8bb: sete %al
86 2.0e-04 :ffffffff803ae8be: test %al,%al
:ffffffff803ae8c0: je ffffffff803ae8c7
<kfree_skb+0x27>
806 0.0019 :ffffffff803ae8c2: jmpq ffffffff803ae7d0
<__kfree_skb>
95 2.3e-04 :ffffffff803ae8c7: repz retq
Thank you
Eric
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