linux-kernel - Inlined functions in perf report

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Date:   Tue, 20 Dec 2016 12:59:54 +0100
From:   "Steinar H. Gunderson" <sgunderson@...foot.com>
To:     peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org
Subject: Inlined functions in perf report

Hi Peter,

I can't find a good point of contact for perf, so I'm contacting you based on
the MAINTAINERS file; feel free to redirect somewhere if you're not the right
person.

I'm trying to figure out how to deal with perf report when there are inlined
functions; they don't generally seem to show up in the call stack, which
sometimes can make it very hard to figure out what is going, especially in
a code base one doesn't know too well. As an example, I threw together a
minimal test program:

  #include <stdlib.h>

  inline int foo()
  {
          int k = rand();
          int sum = 1;
          for (int i = 0; i < 10000000000; ++i)
          {
                  sum ^= k;
                  sum += k;
          }
          return sum;
  }

  int main(void)
  {
          return foo();
  }

Compiling with -O2 -g, and running perf record -g yields:

  # Samples: 6K of event 'cycles:ppp'
  # Event count (approx.): 5876825543
  #
  # Children      Self  Command  Shared Object      Symbol                
  # ........  ........  .......  .................  ......................
  #
      99.98%    99.98%  inline   inline             [.] main
              |
              ---0x706258d4c544155
                 main

      99.98%     0.00%  inline   [unknown]          [.] 0x0706258d4c544155
              |
              ---0x706258d4c544155
                 main

Is there a way I can get it to show “foo” in the call graph? (I suppose also
ideally, “foo” and not “main” should show up in a non-graph run.) Of course,
this gets even more confusing if foo calls bar, since it now looks like the
call chain is main -> bar directly.

I have debug information that should be sufficient in the binary, because if
I break in gdb, I definitely get the call stack:

  Program received signal SIGINT, Interrupt.
  0x0000555555554589 in foo () at inline.c:5
  5               int k = rand();
  (gdb) bt
  #0  0x0000555555554589 in foo () at inline.c:5
  #1  main () at inline.c:17
  (gdb) 

FWIW, this is with perf from 4.10 (git as of a few days ago) and GCC 6.2.1.

/* Steinar */
-- 
Homepage: https://www.sesse.net/