How can a SystemTap script determine the current thread count?

Question

I want to write a SystemTap script that can determine the actual number of threads for the current PID inside a probe call. The number should be the same as shown in the output of /proc/4711/status in this moment.

My first approach was to count kprocess.create and kprocess.exit event occurrences, but this obviously gives you only the relative increase / decrease of the thread count.

How could a SystemTap script use one of the given API functions to determine this number ? Maybe the script could somehow read the same kernel information as being used for the proc file system output ?

Answer 1

You will be subject to race conditions in either case - a stap probe cannot take locks on kernel structures, which would be required to guarantee that the task list does not change while it's being counted. This is especially true for general systemtap probe context, like in the middle of a kprobe.

For the first approach, you could add a "probe begin {}"-time iteration of the task list to prime the initial thread counts from a bit of embedded-C code. One challenge would be to set systemtap script globals from the embedded-C code (there's no documented API for that), but if you look at what the translator generates (stap -p3), it should be doable.

The second approach would be to do the same iteration, but for locking reasons above, this is not generally safe.