• Andi Kleen's avatar
    perf script: Minimize "not reaching sample" for '-F +brstackinsn' · bf0db8c7
    Andi Kleen authored
    In some situations 'perf script -F +brstackinsn' sees a lot of "not
    reaching sample" messages.
    
    This happens when the last LBR block before the sample contains a branch
    that is not in the LBR, and the instruction dumping stops.
    
      $ perf record -b  emacs -Q --batch '()'
      [ perf record: Woken up 1 times to write data ]
      [ perf record: Captured and wrote 0.396 MB perf.data (443 samples) ]
      $ perf script -F +brstackinsn
      ...
              00007f0ab2d171a4        insn: 41 0f 94 c0
              00007f0ab2d171a8        insn: 83 fa 01
              00007f0ab2d171ab        insn: 74 d3                     # PRED 6 cycles [313] 1.00 IPC
              00007f0ab2d17180        insn: 45 84 c0
              00007f0ab2d17183        insn: 74 28
              ... not reaching sample ...
    
      $ perf script -F +brstackinsn | grep -c reach
      136
      $
    
    This is a problem for further analysis that wants to see the full code
    upto the sample.
    
    There are two common cases where the message is bogus:
    
    - The LBR only logs taken branches, but the branch might be a
      conditional branch that is not taken (that is the most common case
      actually)
    
    - The LBR sampling uses a filter ignoring some branches, but the perf
      script check checks for all branches.
    
    This patch fixes these two conditions, by only checking for conditional
    branches, as well as checking the perf_event_attr's branch filter
    attributes.
    
    For the test case above it fixes all the messages:
    
      $ ./perf script -F +brstackinsn | grep -c reach
      0
    
    Note that there are still conditions when the message is hit --
    sometimes there can be a unconditional branch that misses the LBR update
    before the sample -- but they are much more rare now.
    Signed-off-by: default avatarAndi Kleen <ak@linux.intel.com>
    Reviewed-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
    Link: https://lore.kernel.org/r/20240229161828.386397-1-ak@linux.intel.comSigned-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
    bf0db8c7
dump-insn.h 535 Bytes