SystemTap 笔记 （4）—— timer event

timer event会周期性执行handler。举个例子：

# stap -e 'probe timer.s(1) { printf("Hello world!\n");}'
Hello world!
Hello world!
Hello world!
Hello world!

上面脚本每隔1秒打印一次Hello world!。

timer event定义如下：

timer.ms(milliseconds)
timer.us(microseconds)
timer.ns(nanoseconds)
timer.hz(hertz)
timer.jiffies(jiffies)

另外，还有一种randomize表示方式（参考自这里）：

timer.jiffies(N).randomize(M)

The probe handler is run every N jiffies (a kernel-defined unit of time, typically between 1 and 60 ms). If the “randomize” component is given, a linearly distributed random value in the range [-M..+M] is added to N every time the handler is run. N is restricted to a reasonable range (1 to around a million), and M is restricted to be smaller than N.

Alternatively, intervals may be specified in units of time. There are two probe point variants similar to the jiffies timer:

timer.ms(N)

timer.ms(N).randomize(M)

Here, N and M are specified in milliseconds, but the full options for units are seconds (s/sec), milliseconds (ms/msec), microseconds (us/usec), nanoseconds (ns/nsec), and hertz (hz). Randomization is not supported for hertz timers.

最后结合一个例子看一下如何使用timer event（选自这里）：

global count_jiffies, count_ms
probe timer.jiffies(100) { count_jiffies ++ }
probe timer.ms(100) { count_ms ++ }
probe timer.ms(12345)
{
  hz=(1000*count_jiffies) / count_ms
  printf ("jiffies:ms ratio %d:%d => CONFIG_HZ=%d\n",
    count_jiffies, count_ms, hz)
  exit ()
}

首先要知道，每秒发生HZ次jiffies。

其次，每发生100次jiffies，count_jiffies计数加1，所以脚本退出时，一共发生100 * count_jiffies次HZ。一共经历了count_ms / 10秒。

最后计算CONFIG_HZ：(100 * count_jiffies) / (count_ms / 10) = (1000 * count_jiffies) / count_ms。