SystemTap的probe定义：
probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
一个probe可以定义多个PROBEPOINT（也称为event），它们共享一个handler函数。PROBEPOINT可分为两种：
a）同步（synchronous）：
A synchronous event occurs when any process executes an instruction at a particular location in kernel code. This gives other events a reference point from which more contextual data may be available.
像syscall.system_call，kernel.function("function")都属于同步PROBEPOINT。
b）异步（asynchronous）：
Asynchronous events are not tied to a particular instruction or location in code. This family of probe points consists mainly of counters, timers, and similar constructs.
begin，end，timer等都属于异步PROBEPOINT。
参考资料：
SystemTap Scripts。