Re: Controlling a systemtap script from userspace

["James Dickens" <jamesd dot wi at gmail dot com>]

On 1/9/07, David A Sperry <David_A_Sperry@raytheon.com> wrote:
> Hi,
>     I am interested in logging kernel stuff only during small
> portions of a long test. About 10 milliseconds of logging every
> 10 minutes. I have a C++ program than knows when to do the
> logging, the question is how to get that user space C++ program to set a
> global in a systemTap script?  Is there an obscure syscall
> or something else I can use as a probe to set a global?

one easy way is to have the userland app execute a signal, and your
systemtap script can watch for the signal and verify that it was
called by your application via its commandname you could use standard
signals such as

SIGUSR1   30,10,16    Term    User-defined signal 1
      SIGUSR2   31,12,17    Term    User-defined signal 2

or you could fire a non normal one such as signal 255 or something if
you are worried your signal being thought of as a normal signal.

James Dickens
uadmin.blogspot.com

> For example:
>
>
> #!/usr/bin/env stap
> #
> # This script sporatically lists the top 20 systemcalls on the system
> #
>
> global syscalls isLogging
>
> # turn on logging
> probe ### something from user space ### {isLogging = 1}
>
> # turn off logging
> probe ### something else from user space #### {isLogging = 0}
>
> # probe gated with isLogging
> probe kernel.function("sys_*") {
>         if (isLogging == 1) {
>       syscalls[probefunc()]++
>         }
> }
>
> # print top syscalls every 5 seconds
> probe timer.ms(5000) {
>       print_top ()
> }
>
> function print_top () {
>       cnt=0
>       printf ("SYSCALL\t\t\t\tCOUNT\n")
>       foreach ([name] in syscalls-) {
>             printf("%-20s\t\t%5d\n",name, syscalls[name])
>             if (cnt++ == 20)
>                   break
>       }
>       printf("--------------------------------------\n")
>       delete syscalls
> }