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Date:   Tue, 13 Mar 2018 21:31:24 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <ast@...com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        Teng Qin <qinteng@...com>
Subject: Re: [PATCH bpf-next v4 1/2] bpf: extend stackmap to save
 binary_build_id+offset instead of address



> On Mar 12, 2018, at 3:47 PM, Song Liu <songliubraving@...com> wrote:
> 
> 
> 
>> On Mar 12, 2018, at 2:31 PM, Alexei Starovoitov <ast@...com> wrote:
>> 
>> On 3/12/18 2:12 PM, Song Liu wrote:
>>> 
>>>> On Mar 12, 2018, at 2:00 PM, Alexei Starovoitov <ast@...com> wrote:
>>>> 
>>>> On 3/12/18 1:39 PM, Song Liu wrote:
>>>>> +	page = find_get_page(vma->vm_file->f_mapping, 0);
>>>> 
>>>> did you test it with config_debug_atomic_sleep ?
>>>> it should have complained...
>>> 
>>> Yeah, I have CONFIG_DEBUG_ATOMIC_SLEEP=y.
>>> 
>>> I think find_get_page() will not sleep. The variation find_get_page_flags()
>>> may sleep with flag FGP_CREAT.
>> 
>> I see. gfp_mask == 0 and no locks. should work indeed.
>> curious how perf report looks like for heavy bpf_get_stackid() usage?
> 
> I modified samples/bpf/sampleip to only call bpf_get_stackid(). The following 
> is captured with bpf_get_stackid() called at 10k Hz. stressapptest is running
> with 16 threads on a system with 56 cores. 
> 
> 
> Samples: 1M of event 'cycles:pp', Event count (approx.): 628092326243
>  Overhead  Command          Shared Object                 Symbol
> +   51.61%  stressapptest    stressapptest                 [.] AdlerMemcpyC             
> -   20.82%  stressapptest    [kernel.vmlinux]              [k] queued_spin_lock_slowpath
>   - queued_spin_lock_slowpath                                                          
>      - 20.80% pcpu_freelist_pop                                                        
>           bpf_get_stackid                                                              
>           bpf_get_stackid_tp                                                           
>         - 0x590c                                                                       
>              16.12% AdlerMemcpyC                                                       
>              4.50% OsLayer::CpuStressWorkload                                          
> +   14.36%  stressapptest    stressapptest                 [.] OsLayer::CpuStressWorkload
> -    8.74%  stressapptest    [kernel.vmlinux]              [k] _raw_spin_lock            
>   - _raw_spin_lock                                                                      
>      - 8.73% bpf_get_stackid                                                            
>           bpf_get_stackid_tp                                                            
>         + 0x590c                                                                        
> -    0.67%  stressapptest    [kernel.vmlinux]              [k] pcpu_freelist_pop         
>   - pcpu_freelist_pop                                                                   
>      - 0.67% bpf_get_stackid                                                            
>           bpf_get_stackid_tp                                                            
>         + 0x590c
> 
> Seems lock contention is the dominating overhead here. This should be the same
> for original stackmap. 
> 
> Song


Samples: 172K of event 'cycles:pp', Event count (approx.): 102311012653
Overhead  Command        Shared Object     Symbol                                        
  78.84%  stressapptest  stressapptest     [.] AdlerMemcpyC                                
   8.78%  stressapptest  stressapptest     [.] OsLayer::CpuStressWorkload                  
   3.14%  stressapptest  [kernel.vmlinux]  [k] _raw_spin_lock                              
   2.56%  stressapptest  stressapptest     [.] WorkerThread::FillPage                      
   0.45%  stressapptest  [kernel.vmlinux]  [k] perf_callchain_user                         
   0.37%  stressapptest  [kernel.vmlinux]  [.] native_irq_return_iret                      
   0.31%  stressapptest  [kernel.vmlinux]  [k] clear_page_erms                             
   0.29%  stressapptest  [kernel.vmlinux]  [k] pcpu_freelist_pop                           
   0.27%  stressapptest  stressapptest     [.] CalculateAdlerChecksum                      
   0.25%  stressapptest  [kernel.vmlinux]  [k] bpf_get_stackid                             
   0.22%  swapper        [kernel.vmlinux]  [k] poll_idle

This perf output is taken with stressapptest running on 4 cores. 
bpf_get_stackid() and pcps_free_list_pop combined about 0.54% of CPU. 

Song



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