Re: Some newbie questions

[Avi Kivity <avi at scylladb dot com>]

On 08/07/2016 11:07 AM, Avi Kivity wrote:
> Hello,

Hi, n00b,

> A few newbie questions.
>
> 1.  Static library support
>
> I am writing a static library, and would like to ship some scripts 
> with the library for its consumers.  The library is named "seastar" 
> and one of the consumers is named "scylla".
>
> What should I write as the process name in the script?  As the writer 
> of the library, I don't know the name of the process that will be 
> consuming the library.  The consumer shouldn't have to write those 
> scripts.  I could play sed games to adjust the scripts, but it's very 
> sad to have to do this.

You can use "@1" to paste a script parameter into the script.

> 2. Inlined functions
>
> From the manual pages, it seems that inlined functions can be probed 
> (minus the .return probe), but I just get an error:
>
> semantic error: while resolving probe point: identifier 'process' at 
> script/home/avi/seastar/debug/task-latency.stap:3:7
>         source: probe 
> process("scylla").function("reactor::run_tasks()") {
>                       ^
>
> semantic error: no match (similar functions: 
> _ZN7reactor14run_exit_tasksEv, statfs, dup, mkdir, ntohs)
>
> I will note that "mkdir" does not sound very similar to 
> "reactor::run_tasks()" (but I am not a native speaker).
>
> 3. Process CPU timers
>
> (more of a feature request)
>
> I'm trying to find causes of latency in my program.  To do that, I'm 
> running a periodic timer and checking whether a function takes more 
> time than some threshold.
>
> Ideally, I'd be able to arm the timer on the function entry point and 
> disarm it on exit, rather than have it run continuously; this would 
> need to be a per-thread cpu-time timer (e.g. CLOCK_THREAD_CPUTIME_ID)/
>
> Here's my current script for reference ("running" and "start_time" 
> need to become maps for it to be thread-safe):
>
> #!/usr/bin/stap
>
> global start_time
> global running
>
> probe begin {
>     running = 0
> }
>
> probe 
> process("/home/avi/urchin/build/release/scylla").mark("reactor_run_tasks_start") 
> {
>     start_time = gettimeofday_us()
>     running = 1
> }
>
> probe 
> process("/home/avi/urchin/build/release/scylla").mark("reactor_run_tasks_end") 
> {
>     running = 0
> }
>
> probe timer.ms(10) {

You should use timer.profile instead

>     now = gettimeofday_us()
>     if (running && (now - start_time) > 30000) {

And a second parameter here.

>         printf("detected tasks running for >30ms\n")
>         print_usyms(ubacktrace())
>     }
> }
>
> I'd appreciate any tips as to whether there's a better way to do this.