PERF_ATTACH_GROUP
if (event->attach_state & PERF_ATTACH_GROUP)
event->attach_state |= PERF_ATTACH_GROUP;
if (!(event->attach_state & PERF_ATTACH_GROUP))
event->attach_state &= ~PERF_ATTACH_GROUP;
(PERF_ATTACH_CONTEXT|PERF_ATTACH_GROUP));