Re: Linux kernel marker questions

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* David Smith (dsmith@redhat.com) wrote:
> 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?
> 

Yes. A marker can be declared more than once, but bears the same
semantics and should have the same parameter types.

> _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.
> 

See answer below. Also, if a marker is defined in an inline function or
an unrolled loop (this last optimization is disabled in the kernel build
though), it will result in multiple occurences of the marker.

> 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?
> 

Correct. It could be fixed by adapting _marker_set_probe, making it take
an optinional struct module* argument. It will have more effects on the
forthcoming markers version though, since I keep a data structure of the
active markers throrough the kernel, so that if a marker is set before a
module is loaded, the marker will be activated at module load time.

> 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.
> 

format string and flags are only there to make sure nobody writes
foolishly a "bad" probe without considering the reentrancy impact and
by expecting the wrong arguments. Note that passing a NULL format string
is valid and means that the probe will parse the string dynamicall,
therefore getting around the verification.

> 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?
> 

Hrm.. well, isn't this information is a __markers section, which points
to a __markers data section ? It seems to me that it could be extracted
from the binary too, just like the __markers_strings. Please keep in
mind that these sections and data structures will change in the
forthcoming markers version.


> 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?
> 

It will change in the new version. I guess we should rediscuss it once I
submit the new patches.

> Thanks for the help.
> 

Thanks for your collaboration, I hope I answered to your questions
appropriately.

Mathieu

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

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68