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    SubjectRe: tree rcu: call_rcu scalability problem?
    On Wed, Sep 02, 2009 at 02:27:56PM +0200, Nick Piggin wrote:
    > On Wed, Sep 02, 2009 at 11:48:35AM +0200, Nick Piggin wrote:
    > > Hi Paul,
    > >
    > > I'm testing out scalability of some vfs code paths, and I'm seeing
    > > a problem with call_rcu. This is a 2s8c opteron system, so nothing
    > > crazy.
    > >
    > > I'll show you the profile results for 1-8 threads:
    > >
    > > 1:
    > > 29768 total 0.0076
    > > 15550 default_idle 48.5938
    > > 1340 __d_lookup 3.6413
    > > 954 __link_path_walk 0.2559
    > > 816 system_call_after_swapgs 8.0792
    > > 680 kmem_cache_alloc 1.4167
    > > 669 dput 1.1946
    > > 591 __call_rcu 2.0521
    > >
    > > 2:
    > > 56733 total 0.0145
    > > 20074 default_idle 62.7313
    > > 3075 __call_rcu 10.6771
    > > 2650 __d_lookup 7.2011
    > > 2019 dput 3.6054
    > >
    > > 4:
    > > 98889 total 0.0253
    > > 21759 default_idle 67.9969
    > > 10994 __call_rcu 38.1736
    > > 5185 __d_lookup 14.0897
    > > 4475 dput 7.9911

    Four threads runs on one socket but 8 threads runs on two sockets,
    I take it?

    > > 8:
    > > 170391 total 0.0437
    > > 31815 __call_rcu 110.4688
    > > 12958 dput 23.1393
    > > 10417 __d_lookup 28.3071
    > >
    > > Of course there are other scalability factors involved too, but
    > > __call_rcu is taking 54 times more CPU to do 8 times the amount
    > > of work from 1-8 threads, or a factor of 6.7 slowdown.
    > >
    > > This is with tree RCU.
    >
    > It seems like nearly 2/3 of the cost is here:
    > /* Add the callback to our list. */
    > *rdp->nxttail[RCU_NEXT_TAIL] = head; <<<
    > rdp->nxttail[RCU_NEXT_TAIL] = &head->next;

    Hmmm... That certainly is not the first list of code in call_rcu() that
    would come to mind...

    > In loading the pointer to the next tail pointer. If I'm reading the profile
    > correctly. Can't see why that should be a probem though...

    The usual diagnosis would be false sharing.

    Hmmm... What is the workload? CPU-bound? If CONFIG_PREEMPT=n, I might
    expect interference from force_quiescent_state(), except that it should
    run only every few clock ticks. So this seems quite unlikely.

    Could you please try padding the beginning and end of struct rcu_data
    with a few hundred bytes and rerunning? Just in case there is a shared
    per-CPU variable either before or after rcu_data in your memory layout?

    Thanx, Paul

    > ffffffff8107dee0 <__call_rcu>: /* __call_rcu total: 320971 100.000 */
    > 697 0.2172 :ffffffff8107dee0: push %r12
    > 228 0.0710 :ffffffff8107dee2: push %rbp
    > 133 0.0414 :ffffffff8107dee3: mov %rdx,%rbp
    > 918 0.2860 :ffffffff8107dee6: push %rbx
    > 316 0.0985 :ffffffff8107dee7: mov %rsi,0x8(%rdi)
    > 257 0.0801 :ffffffff8107deeb: movq $0x0,(%rdi)
    > 1660 0.5172 :ffffffff8107def2: mfence
    > 27730 8.6394 :ffffffff8107def5: pushfq
    > 13153 4.0979 :ffffffff8107def6: pop %r12
    > 903 0.2813 :ffffffff8107def8: cli
    > 2562 0.7982 :ffffffff8107def9: mov %gs:0xde68,%eax
    > 1784 0.5558 :ffffffff8107df01: cltq
    > :ffffffff8107df03: mov 0x60(%rdx,%rax,8),%rbx
    > :ffffffff8107df08: pushfq
    > 3494 1.0886 :ffffffff8107df09: pop %rdx
    > 896 0.2792 :ffffffff8107df0a: cli
    > 2655 0.8272 :ffffffff8107df0b: mov 0xd0(%rbp),%rcx
    > 1800 0.5608 :ffffffff8107df12: cmp (%rbx),%rcx
    > 21 0.0065 :ffffffff8107df15: je ffffffff8107df32 <__call_rcu+0x52
    > :ffffffff8107df17: mov 0x40(%rbx),%rax
    > 81 0.0252 :ffffffff8107df1b: mov %rcx,(%rbx)
    > 3 9.3e-04 :ffffffff8107df1e: mov %rax,0x38(%rbx)
    > :ffffffff8107df22: mov 0x48(%rbx),%rax
    > :ffffffff8107df26: mov %rax,0x40(%rbx)
    > :ffffffff8107df2a: mov 0x50(%rbx),%rax
    > :ffffffff8107df2e: mov %rax,0x48(%rbx)
    > :ffffffff8107df32: push %rdx
    > 1194 0.3720 :ffffffff8107df33: popfq
    > 9518 2.9654 :ffffffff8107df34: pushfq
    > 4179 1.3020 :ffffffff8107df35: pop %rdx
    > 1277 0.3979 :ffffffff8107df36: cli
    > 2546 0.7932 :ffffffff8107df37: mov 0xc8(%rbp),%rax
    > 1748 0.5446 :ffffffff8107df3e: cmp %rax,0x8(%rbx)
    > 5 0.0016 :ffffffff8107df42: je ffffffff8107df57 <__call_rcu+0x77
    > :ffffffff8107df44: movb $0x1,0x19(%rbx)
    > 2 6.2e-04 :ffffffff8107df48: movb $0x0,0x18(%rbx)
    > :ffffffff8107df4c: mov 0xc8(%rbp),%rax
    > :ffffffff8107df53: mov %rax,0x8(%rbx)
    > 921 0.2869 :ffffffff8107df57: push %rdx
    > 151 0.0470 :ffffffff8107df58: popfq
    > 183507 57.1725 :ffffffff8107df59: mov 0x50(%rbx),%rax
    > 995 0.3100 :ffffffff8107df5d: mov %rdi,(%rax)
    > 2 6.2e-04 :ffffffff8107df60: mov %rdi,0x50(%rbx)
    > 18 0.0056 :ffffffff8107df64: mov 0xd0(%rbp),%rdx
    > 940 0.2929 :ffffffff8107df6b: mov 0xc8(%rbp),%rax
    > 15 0.0047 :ffffffff8107df72: cmp %rax,%rdx
    > 1 3.1e-04 :ffffffff8107df75: je ffffffff8107dfb0 <__call_rcu+0xd0
    > 787 0.2452 :ffffffff8107df77: mov 0x58(%rbx),%rax
    > 58 0.0181 :ffffffff8107df7b: inc %rax
    > 2 6.2e-04 :ffffffff8107df7e: mov %rax,0x58(%rbx)
    > 1679 0.5231 :ffffffff8107df82: movslq 0x4988fb(%rip),%rdx # ffff
    > 40 0.0125 :ffffffff8107df89: cmp %rdx,%rax
    > 5 0.0016 :ffffffff8107df8c: jg ffffffff8107dfd7 <__call_rcu+0xf7
    > 588 0.1832 :ffffffff8107df8e: mov 0xe0(%rbp),%rdx
    > 84 0.0262 :ffffffff8107df95: mov 0x51f924(%rip),%rax # ffff
    > 5 0.0016 :ffffffff8107df9c: cmp %rax,%rdx
    > 505 0.1573 :ffffffff8107df9f: js ffffffff8107dfc8 <__call_rcu+0xe8
    > 17580 5.4771 :ffffffff8107dfa1: push %r12
    > 1671 0.5206 :ffffffff8107dfa3: popfq
    > 24201 7.5399 :ffffffff8107dfa4: pop %rbx
    > 1367 0.4259 :ffffffff8107dfa5: pop %rbp
    > 377 0.1175 :ffffffff8107dfa6: pop %r12
    > :ffffffff8107dfa8: retq
    > :ffffffff8107dfa9: nopl 0x0(%rax)
    > :ffffffff8107dfb0: mov %rbp,%rdi
    > :ffffffff8107dfb3: callq ffffffff813be930 <_spin_lock_irqs
    > 12 0.0037 :ffffffff8107dfb8: mov %rbp,%rdi
    > :ffffffff8107dfbb: mov %rax,%rsi
    > :ffffffff8107dfbe: callq ffffffff8107d8e0 <rcu_start_gp>
    > :ffffffff8107dfc3: jmp ffffffff8107df77 <__call_rcu+0x97
    > :ffffffff8107dfc5: nopl (%rax)
    > :ffffffff8107dfc8: mov $0x1,%esi
    > 10 0.0031 :ffffffff8107dfcd: mov %rbp,%rdi
    > :ffffffff8107dfd0: callq ffffffff8107dd50 <force_quiescent
    > 1 3.1e-04 :ffffffff8107dfd5: jmp ffffffff8107dfa1 <__call_rcu+0xc1
    > 451 0.1405 :ffffffff8107dfd7: mov $0x7fffffffffffffff,%rdx
    > 411 0.1280 :ffffffff8107dfe1: xor %esi,%esi
    > :ffffffff8107dfe3: mov %rbp,%rdi
    > :ffffffff8107dfe6: mov %rdx,0x60(%rbx)
    > 317 0.0988 :ffffffff8107dfea: callq ffffffff8107dd50 <force_quiescent
    > 4510 1.4051 :ffffffff8107dfef: jmp ffffffff8107dfa1 <__call_rcu+0xc1
    > :ffffffff8107dff1: nopw %cs:0x0(%rax,%rax,1)
    >
    >


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