Re: [RFC] SystemTap FAQ

[Eugene Teo <eugeneteo at kernel dot sg>]

Hi Srinivasa,

<quote sender="Srinivasa Ds">
> Literally, just the questions as of now :-)
>
> Please add any questions we've missed; more importantly, please attempt
> to answer a few of them too.

It's probably good to include some common error messages that one may
encounter: http://sources.redhat.com/systemtap/tutorial/node23.html

> 1) I have installed SystemTap but am getting a "semantic error: libdwfl
> failure (missing kernel debuginfo): No such file or directory", when I
> try to run a script. What am I missing? How do I fix it?

There was a problem processing the debugging information. It may simply
be missing, or may have some consistency / correctness problems. Later
compilers tend to produce better debugging information, so if you can
upgrade and recompile your kernel/application, it may help.

Ref: http://sources.redhat.com/systemtap/tutorial/node27.html

> 2) What config options need to be enabled to be able to use SystemTap on
> custom built kernels?

There are instructions at:
http://sources.redhat.com/systemtap/wiki/SystemTapWithSelfBuiltKernel

> 3) I have a kernel built with options listed in (2), but still can't get
> SystemTap to work. What am I missing?

See question 2.

> 4) Is it possible for me to view the kernel module generated by SystemTap?

You can specify -p3 to look at the kernel module code translated by
SystemTap.

stap -p3 <script> | less
stap -p3 -e '...' | less
/sbin/lsmod | grep stap

man stap
[...]
   -p NUM     stop after pass NUM 1-5, instead of 5
              (parse, elaborate, translate, compile, run)

> 5) How do I run prebuilt SystemTap modules?

You can run:

staprun /path/to/<name>.ko

Useful options:
man staprun
[...]
-L  Load module and start probes, then detach.
-A modname.  Attach to systemtap module modname.

> 6) What should I do when some tapsets functions fails to resolve like 
> "semantic error: unresolved arity-1 function: identifier 'log'"

Do ensure that the tapsets are installed at the default location:
/usr/share/systemtap/tapset/ (in RHEL or Fedora) or /usr/local/systemtap/tapset 
if you install from source.

> 7) This is way cool! I want to write my own scripts. Where do I look for
> more information?

You can sign up for our mailing list at:
http://sources.redhat.com/systemtap/getinvolved.html

Read our tutorial at:
http://sources.redhat.com/systemtap/tutorial/

Find out more at:
http://sources.redhat.com/systemtap/wiki

> 8)Is it possible to override some of the macros in SystemTap generated "C" 
> module.

Yes, it is possible:

man stap
[...]
-D NM=VAL  emit macro definition into generated C code

> 9) What are tapsets?

A tapset is just a script that designed for reuse by installation
into a special directory.

http://sources.redhat.com/systemtap/tutorial/node15.html

> 10) I see that there is no tapset for my favourite subsystem. What do I
> do?

You can contribute to the project by writing a tapset for it. Check out
the Tapset Developer's Guide at:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/~checkout~/src/tapset/DEVGUIDE?cvsroot=systemtap
or you can learn how to write a tapset by looking at the existing
collection of tapsets at /usr/share/systemtap/tapset.

> 11) I hear the latest Linux Kernels come with marker support. Does
> SystemTap support markers?

David wrote some information about using markers at:
http://sources.redhat.com/systemtap/wiki/UsingMarkers

> 12) I am a non-root user, but still want to run SystemTap scripts. Is
> that possible?

Yes, it is possible.

Add yourself as a member of 'stapusr' group. Members of stapusr group
can run SystemTap scripts/modules located at /lib/modules/`uname
-r`/systemtap directory.

> 13) This is fantastic! But how does it all work?

src/INTERNALS has some documentation on how the translator work:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/~checkout~/src/INTERNALS?cvsroot=systemtap

> 14) Do I have to be a RedHat Enterprise Linux/Fedora user to be able to
> use SystemTap?

Nope. It is possible to use SystemTap on other Linux distributions. Take
a look at our wiki for more information:
http://sources.redhat.com/systemtap/wiki

> 15) I found a bug when using the tool. How do I report it?

You can file a bug at:
http://sources.redhat.com/bugzilla/enter_bug.cgi?product=systemtap

> 16) I'd like to contribute to the project. Where to I start?

You can join our mailing list:
http://sources.redhat.com/systemtap/getinvolved.html

and/or look at our current bugs/pending features list in the bugzilla:
http://sources.redhat.com/bugzilla/buglist.cgi?query_format=&short_desc_type=allwordssubstr&short_desc=&product=systemtap&long_desc_type=substring&long_desc=&bug_file_loc_type=allwordssubstr&bug_file_loc=&gcchost_type=allwordssubstr&gcchost=&gcctarget_type=allwordssubstr&gcctarget=&gccbuild_type=allwordssubstr&gccbuild=&bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=SUSPENDED&bug_status=WAITING&bug_status=REOPENED&emailassigned_to1=1&emailtype1=substring&email1=&emailassigned_to2=1&emailreporter2=1&emailcc2=1&emailtype2=substring&email2=&bugidtype=include&bug_id=&votes=&chfieldfrom=&chfieldto=Now&chfieldvalue=&cmdtype=doit&order=Bug+Number&field0-0-0=noop&type0-0-0=noop&value0-0-0=

Thanks,
Eugene