bcc/python: fix ctype warning when creating perf events

src/python/bcc/perf.py

@@ -152,9 +152,9 @@ def __setattr__(self, key, value):
  ioctl = libc.ioctl # not declaring vararg types
 
  @staticmethod
- def _open_for_cpu(cpu, attr):
+ def _open_for_cpu(cpu, attr, pid=-1):
  pfd = Perf.syscall(Perf.NR_PERF_EVENT_OPEN, ct.byref(attr),
- attr.pid, cpu, -1,
+ pid, cpu, -1,
  Perf.PERF_FLAG_FD_CLOEXEC)
  if pfd < 0:
  errno_ = ct.get_errno()
@@ -177,7 +177,6 @@ def perf_event_open(tpoint_id, pid=-1, ptype=PERF_TYPE_TRACEPOINT,
  freq=0):
  attr = Perf.perf_event_attr()
  attr.config = tpoint_id
- attr.pid = pid
  attr.type = ptype
  attr.sample_type = Perf.PERF_SAMPLE_RAW
  if freq > 0:
@@ -189,10 +188,9 @@ def perf_event_open(tpoint_id, pid=-1, ptype=PERF_TYPE_TRACEPOINT,
  attr.wakeup_events = 9999999 # don't wake up
 
  for cpu in get_online_cpus():
- Perf._open_for_cpu(cpu, attr)
+ Perf._open_for_cpu(cpu, attr, pid)
 
  @staticmethod
  def perf_custom_event_open(attr, pid=-1):
- attr.pid = pid
  for cpu in get_online_cpus():
- Perf._open_for_cpu(cpu, attr)
+ Perf._open_for_cpu(cpu, attr, pid)