Re: perf report --inline usability issues when sorting by function

[Milian Wolff <milian.wolff@kdab.com>]

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On Donnerstag, 27. April 2017 23:09:10 CEST Milian Wolff wrote:
> Hey Jin, others
> 
> I observe a usability issue with the following example code:
> 
> ~~~~~~~~~~~~~~~~~~~~
> #include <cmath>
> #include <random>
> #include <iostream>
> 
> using namespace std;
> 
> int main()
> {
>     uniform_real_distribution<double> uniform(-1E5, 1E5);
>     default_random_engine engine;
>     double s = 0;
>     for (int i = 0; i < 10000000; ++i) {
>         s += uniform(engine);
>     }
>     cout << "random sum: " << s << '\n';
>     return 0;
> }
> ~~~~~~~~~~~~~~~~~~~~
> 
> Compile it with `g++ -O2 -g` then record it with `perf record --call-graph
> dwarf` and inspect the data with inliners enabled. Here are some excerpts
> that show the issues:
> 
> ~~~~~~~~~~~~~~~~~~~~
> $ perf report --stdio --inline --no-children
> Failed to open [ext4], continuing without symbols
> # To display the perf.data header info, please use --header/--header-only
> options.
> #
> #
> # Total Lost Samples: 0
> #
> # Samples: 499  of event 'cycles'
> # Event count (approx.): 329354953
> #
> # Overhead  Command    Shared Object      Symbol
> # ........  .........  .................  ................................
> #
>     96.70%  ex_random  ex_random          [.] main
> 
>             ---main
>                __libc_start_main
>                _start
> 
>      0.96%  ex_random  ld-2.25.so         [.] do_lookup_x
> 
>             ---do_lookup_x
>                _dl_lookup_symbol_x
>                _dl_relocate_object
>                dl_main
>                _dl_sysdep_start
>                _dl_start
>                _dl_start_user
> 
>      0.30%  ex_random  ld-2.25.so         [.] _dl_lookup_symbol_x
> ...
> ~~~~~~~~~~~~~~~~~~~~~~
> 
> Note how no inlined frames are actually shown. I think the issue here is
> that when we group by function name (the default), then multiple samples
> with different IPs will get merged. Once we display the data then in e.g.
> ui/stdio/ hist.c, we will only have one node left for this function, with
> an arbitrary IP (of the first sample, I guess).
> 
> This is problematic, because depending on the IP, different inlined call
> stacks may have been used. This is observable for the above use-case by
> changing the grouping algorithm, e.g.:
> 
> ~~~~~~~~~~~~~~~~~~~~~~~
> $ perf report --stdio --inline --no-children -g srcline
> Invalid callchain mode: srcline
> Invalid callchain order: srcline
> Failed to open [ext4], continuing without symbols
> # To display the perf.data header info, please use --header/--header-only
> options.
> #
> #
> # Total Lost Samples: 0
> #
> # Samples: 499  of event 'cycles'
> # Event count (approx.): 329354953
> #
> # Overhead  Command    Shared Object      Symbol
> # ........  .........  .................  ................................
> #
>     96.70%  ex_random  ex_random          [.] main
> 
>             |--28.97%--main random.tcc:3326
>             |
>             |          /usr/include/c++/6.3.1/bits/random.h:185 (inline)
>             |          /usr/include/c++/6.3.1/bits/random.h:1818 (inline)
>             |          /usr/include/c++/6.3.1/bits/random.h:1809 (inline)
>             |          /home/milian/projects/kdab/training-material/addon/
> 
> profiling/build/ex_random/../../ex_random/main.cpp:13 (inline)
> 
>             |          __libc_start_main
>             |          _start
>             |
>             |--28.14%--main random.h:143
>             |
>             |          /usr/include/c++/6.3.1/bits/random.h:151 (inline)
>             |          /usr/include/c++/6.3.1/bits/random.h:332 (inline)
>             |          /usr/include/c++/6.3.1/bits/random.tcc:3332 (inline)
>             |          /usr/include/c++/6.3.1/bits/random.h:185 (inline)
>             |          /usr/include/c++/6.3.1/bits/random.h:1818 (inline)
>             |          /usr/include/c++/6.3.1/bits/random.h:1809 (inline)
>             |          /home/milian/projects/kdab/training-material/addon/
> 
> profiling/build/ex_random/../../ex_random/main.cpp:13 (inline)
> 
>             |          __libc_start_main
>             |          _start
> 
> ...
> ~~~~~~~~~~~~~~~~~~~~~~~
> 
> Has anyone any suggestion on how to fix this? Some ideas:
> 
> - adapt the function aggregation to also take the inlined function into
> account
> - always add inlined frames to the graph
> - keep all IPs and their cost even when aggregating functions

I have no implemented the first option from the above list and sent a patch 
for review. Feedback welcome! It seems to work quite nicely in my tests.

Thanks

-- 
Milian Wolff | milian.wolff@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts

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