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Re: Multiple handlers per marker

Mathieu Desnoyers wrote:
* Mathieu Desnoyers ( wrote:
* Mike Mason ( wrote:
Hi Mathieu,

Are you aware of any working being done to allow multiple handlers to be attached to a marker? Something like what kprobes allows. I've started to look into this and don't want to duplicate efforts.

Nope, but I know we will have to address this.

Something along the lines of walking an RCU list of function pointers,
calling them.

The only downside I see is that we will have to pass a va_list * instead
of real va args. The could make the marker site a little bit bigger and
will change the probe callback arguments.

What do you think about these ideas ?

If we can find a way to make the common case (only one probe connected)
_ultra_ fast, and yet architecture independent, that would be awesome. A
simple call is kind of hard to beat though.. So we may have to think
about a design with :

- One call at the marker site
- if 1 probe is installed :
  - If the format string is empty, connect a probe without va args.
  - If the format string is not empty, connect a "stage 1" probe that takes
    the va args, starts/ends the va_list and calls _one_ function (let's
    call it "stage 2" probe), that takes va_list as parameter.
- if more than 1 probe is installed :
  - The stage 1 probe creates the va_list and passes it to each function
    connected, iterated with an RCU list.

What do you think ?

Your proposal sounds reasonable to me. How would marker_arm(), marker_disarm() and marker_probe_unregister() change? They'll need to work on a per probe handler basis, rather than per marker. They'll need to know the marker name *and* the specific handler, or perhaps just the handler if you keep a master list of all handlers. In any case, the interface will need to change.


I'm working on an implementation.

Great! When you need reviewers/testers let me know.

I'm copying this to the SystemTap list. Multiple markers is a "must have" feature for SystemTap.


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

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