• Arnaldo Carvalho de Melo's avatar
    perf bpf: Add 'examples' directories · 8f12a2ff
    Arnaldo Carvalho de Melo authored
    The first one is the bare minimum that bpf infrastructure accepts before
    it expects actual events to be set up:
    
      $ cat tools/perf/examples/bpf/empty.c
      char _license[] __attribute__((section("license"), used)) = "GPL";
      int _version __attribute__((section("version"), used)) = LINUX_VERSION_CODE;
      $
    
    If you remove that "version" line, then it will be refused with:
    
      # perf trace -e tools/perf/examples/bpf/empty.c
      event syntax error: 'tools/perf/examples/bpf/empty.c'
                           \___ Failed to load tools/perf/examples/bpf/empty.c from source: 'version' section incorrect or lost
    
      (add -v to see detail)
      Run 'perf list' for a list of valid events
    
       Usage: perf trace [<options>] [<command>]
          or: perf trace [<options>] -- <command> [<options>]
          or: perf trace record [<options>] [<command>]
          or: perf trace record [<options>] -- <command> [<options>]
    
          -e, --event <event>   event/syscall selector. use 'perf list' to list available events
      #
    
    The next ones will, step by step, show simple filters, then the needs
    for headers will be made clear, it will be put in place and tested with
    new examples, rinse, repeat.
    
    Back to using this first one to test the perf+bpf infrastructure:
    
    If we run it will fail, as no functions are present connecting with,
    say, a tracepoint or a function using the kprobes or uprobes
    infrastructure:
    
      # perf trace -e tools/perf/examples/bpf/empty.c
      WARNING: event parser found nothing
      invalid or unsupported event: 'tools/perf/examples/bpf/empty.c'
      Run 'perf list' for a list of valid events
    
       Usage: perf trace [<options>] [<command>]
          or: perf trace [<options>] -- <command> [<options>]
          or: perf trace record [<options>] [<command>]
          or: perf trace record [<options>] -- <command> [<options>]
    
          -e, --event <event>   event/syscall selector. use 'perf list' to list available events
      #
    
    But, if we set things up to dump the generated object file to a file,
    and do this after having run 'make install', still on the developer's
    $HOME directory:
    
      # cat ~/.perfconfig
      [llvm]
    
    	dump-obj = true
      #
      # perf trace -e ~acme/lib/examples/perf/bpf/empty.c
      LLVM: dumping /home/acme/lib/examples/perf/bpf/empty.o
      WARNING: event parser found nothing
      invalid or unsupported event: '/home/acme/lib/examples/perf/bpf/empty.c'
      <SNIP>
      #
    
    We can look at the dumped object file:
    
      # ls -la ~acme/lib/examples/perf/bpf/empty.o
      -rw-r--r--. 1 root root 576 May  4 12:10 /home/acme/lib/examples/perf/bpf/empty.o
      # file ~acme/lib/examples/perf/bpf/empty.o
      /home/acme/lib/examples/perf/bpf/empty.o: ELF 64-bit LSB relocatable, *unknown arch 0xf7* version 1 (SYSV), not stripped
      # readelf -sw ~acme/lib/examples/perf/bpf/empty.o
    
      Symbol table '.symtab' contains 3 entries:
         Num:    Value          Size Type    Bind   Vis      Ndx Name
           0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
           1: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    3 _license
           2: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    4 _version
      #
      # tools/bpf/bpftool/bpftool --pretty ~acme/lib/examples/perf/bpf/empty.o
      null
      #
    
    Cc: Adrian Hunter <adrian.hunter@intel.com>
    Cc: David Ahern <dsahern@gmail.com>
    Cc: Jiri Olsa <jolsa@kernel.org>
    Cc: Namhyung Kim <namhyung@kernel.org>
    Cc: Wang Nan <wangnan0@huawei.com>
    Link: https://lkml.kernel.org/n/tip-y7dkhakejz3013o0w21n98xd@git.kernel.orgSigned-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
    8f12a2ff
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