Re: wiki page on using stap without dwarf

[Jim Keniston <jkenisto at us dot ibm dot com>]

On Mon, 2008-05-19 at 18:50 -0400, Frank Ch. Eigler wrote:
> Jim Keniston <jkenisto@us.ibm.com> writes:
> 
> > I just created this [...]
> > http://sourceware.org/systemtap/wiki/MakeDoWithoutDebugInfo
> 
> Thanks.  One question re. usage: when would one want to use the
> "--kelf" option?

--kelf has the following implications:
1. It affects whether/how we report the absence of dwarf info.
2. It brings into consideration functions that are in the symbol table
but not the dwarf info -- typically assembly-language functions.

> Could "--kmap=[DEFAULT]" be automagically
> activated if dwarf is not found?

Well, we consult the elf if it's there, because the elf enables us to
verify that we're probing the right architecture and such.  So the
default would be --kelf first, then --kmap[=DEFAULT] if we can't find
vmlinux.

That said, your suggestion of ~1 month ago -- that we consult the dwarf
only if the elf doesn't provide what we need, and thereby save some
cycles in stap -- has merit.  I'll open an enhancement request for that,
unless you've changed your mind about it.

My hunch is that we'll save a lot MORE cycles if we bail out of
dwfl_linux_kernel_report_offline() as soon as we've "reported" all the
modules we're probing (typically just the kernel).  I thought I saw a PR
about that, but I can't find it at the moment.

> 
> - FChE

Jim