Re: tree rcu: call_rcu scalability problem?

[Nick Piggin]

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
> 
> 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;

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...

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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