Re: kernel summit session on systemtap

["R. J. Moore" <rasman at uk dot ibm dot com>]

I don't know whether it is possible with the latest code, but for 
debugging purposes, I would be happy if SystemTap could operate on 
external names and relative addresses - i.e. without the need to have 
any symbolic information. This is the way I used ancestors to systemtap 
for shooting very difficult kernel problems in the field. Generally I 
started with a crashdump. Determined that I needed some extra info in a 
particular code path. Looked at the underlying assembler and plugged in 
a probepoint at the required location as a relative address to the 
beginning of the load module. I used this technique 100s and 100s of 
times to shoot those bugs that would only show in live environment with 
a complex work load patterns. I admit that this is extreme debugging, 
but if system tap won't even operate without a ton of extra junk present 
then I see its application as being very limited indeed.   Not everyone 
will want to work at the assembler level, but if System Tap can, then 
tools can be built that do code analysis to help generate and divine 
probepoints. Much can be done knowing that nearly 100% of the code we 
probe is generated by the one tool - gcc. In theory, what is generated 
is deterministic, or the reverse engineering of it is.

We should not ignore System Tap efficacy when being used to complement 
core dumps, crash dumps, kernel and application debuggers.

Richard

--
Richard J Moore
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