Re: [PATCH] Linux Kernel Markers 0.2 for Linux 2.6.17

["Frank Ch. Eigler" <fche@redhat.com>]

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Hi -

> > [...]  For the static part of the instrumentation, a
> > marker that could be hooked up to either type of probing system was
> > desirable, which implies some sort of run-time changeability.
> 
> Ok. So if I get what you're saying here, you'd like to be able to
> overload a marker? 

Sort of.  Remember, we discussed markers as *marking* places and
things, with the intent that they be decoupled from the actual
*action* that is taken when the marker is hit.

> Can you suggest a macro that can do what you'd like. [...]

Compare the kind of marker I showed at OLS and presently supported by
systemtap.  Its unparametrized version looks like this:

#define STAP_MARK(name) do { \
   static void (*__mark_##name##_)(); \
   if (unlikely (__mark_##name##_)) \
   (void) (__mark_##name##_()); \
} while (0)

A tracing/probing tool would hook up to a particular and specific
marker at run time by locating the __mark_NAME static variable (a
function pointer) in the data segment, for example using the ordinary
symbol table, and swapping into it the address of a compatible
back-end handler function.  When a particular tracing/probing session
ends, the function pointer is reset to null.

Note that this technique:

- operates at run time
- is portable
- in its parametrized variants, is type-safe
- does not require any future technology
- does impose some overhead even when a marker is not active


- FChE

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