PERF_ATTACH_CHILD
if (attach_state & PERF_ATTACH_CHILD)
if (attach_state & PERF_ATTACH_CHILD) {
child_event->attach_state |= PERF_ATTACH_CHILD;
if (!(event->attach_state & PERF_ATTACH_CHILD))
event->attach_state &= ~PERF_ATTACH_CHILD;