Multiple handlers per marker

[Mike Mason]

Mathieu Desnoyers wrote:
> * Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote:
>> * Mike Mason (mmlnx@us.ibm.com) 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.

>>
>> Mathieu
>>
> 
> 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.

Thanks,
Mike

> 
>>> 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
>