stap -x option

[William Cohen]

Sanket Somnath Hase wrote:
> Hi folks,
> 
> a ) I was curious to know if we can provide more than one target to stap. Say I want to run the same script for more than one processes .
> 
> b) Is there any benefit in running single script ( effectively single kernel module ) for all the processes under consideration  against running same script for each process individually ( and effectively  having many kernel modules which look for same set of events but for different targets ).
> 
> Looking forward to hear from you ,
> 
> Thanks ,
> Sanket
> 

It seems like one could have a begin probe parse a string argument with the list 
of pid interested in and enter them into a associative array using strtol and 
tokenize functions in a begin probe. Then have the probe check the associative 
array each time it fires. Maybe something like the attached script. This will 
need a newer version of systemtap that include the tokenize() function. The 
comment for syscall.read show how to check the associative array. Running the 
example something like this:

stap pids2watch.stp "124 456"
watching pids:
124
456


There are two issues with multiple probes. One is there will be additional 
overhead if there are more probes to run at a probe point. The other issue is 
that systemtap avoids recompiling the scripts when possible. It checks to see if 
a kernel module has been previously generated for a script, if so it reuses the 
module. If you attempt to use multiple instances of a script, systemtap may 
indicate that it can't load the module. The kernel only allow one instance of a 
module to be loaded into the kernel. You may need to resort to using the "-m" 
option to produce different modules to load.

-Will