Linux kernel marker questions

[David Smith <dsmith at redhat dot com>]

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
dsmith@redhat.com
Red Hat
http://www.redhat.com
256.217.0141 (direct)
256.837.0057 (fax)