Linux kernel marker questions

Mathieu,

I've just finished checking in initial support for your new kernel 
markers into systemtap.  I've got some rough edges to work on, but in 
general it works.

As I implemented the marker support, I come up with several questions 
I'd like some help/clarification with.

1)  According to Documentation/marker.txt, the marker name 
(subsystem_event) is "is an identifier unique to your event".

Am I correct that unique marker names are not strictly enforced but 
"uniqueness" is more of a convention?

_marker_set_probe() seems to support the view that marker names aren't 
unique since it doesn't stop looking for more marker matches once it 
finds a match.

One problem this creates for systemtap is that systemtap's probe syntax 
looks like 'kernel.mark("marker_name")' and 
'module("module_name").mark('marker_name').  I believe with the current 
code if a marker exists with the same name, format string, and flags in 
the kernel and a loadable module there isn't any way to only enable the 
marker in one place or the other - you can only enable both markers 
(assuming the module is loaded).  Am I correct?

2) _marker_set_probe() expects the marker name, format string, and flags 
that were specified when the marker was inserted.  If marker names were 
truly unique, I would really only need the marker name to enable a marker.

Since systemtap can compile modules for a kernel that isn't running, I 
can't really use marker_list_probe() for getting a list of markers 
present (even if the output of marker_list_probe() was more than just 
printks).  So, currently to get a list of markers for a particular 
kernel, the code reads and parses the kernel/module '__markers_strings' 
elf section, which gets systemtap the marker names and format strings. 
(Getting the marker flags out of a kernel/module elf file is possible, 
but won't be easy.)

Currently systemtap can only enable markers that used the MF_DEFAULT set 
of flags.

Have you got any ideas on how systemtap (or any other program) can get a 
list of all the data _marker_set_probe needs?

3) From systemtap's point of view, _marker_set_probe() doesn't return 
enough error information.  Currently it just returns the number of 
probes enabled.  If _marker_set_probe returns a 0 (meaning no markers 
were enabled), I don't know which of the following that means:

- the marker name wasn't found
- the marker name was found, but format string didn't match the compiled 
format string
- the marker name was found and the format strings matched, but the 
marker flags didn't match the compiled marker flags
- the marker is already enabled (since markers can only have one 
function attached to them at a time)

Would there be any way of getting more detailed error information?

Thanks for the help.

-- 
David Smith
[email protected]
Red Hat
http://www.redhat.com
256.217.0141 (direct)
256.837.0057 (fax)