Re: Multiple handlers per marker

* Mike Mason ([email protected]) 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 ?

Mathieu

> Thanks,
> Mike

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