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

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Frank Ch. Eigler (fche@redhat.com) wrote:
> 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
> 
> 
Hi Frank,

Yes, I think there is much to gain to switch from the 5 nops "jumpprobe" to
this scheme. In its parametrized variant, the jump will probably jump over a
stack setup and function call. Do you think I should simply switch from the
5 nops marker to this technique ? I guess the performance impact of a
predicted branch will be similar to 5 nops anyway...

The clear advantage I see in the parametrized variant is that the parameters
will be ready for the called function : it makes it trivial to access any
variable from the traced function.

Mathieu


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