Re: [to-be-posted-soon] Multiple handlers per marker

[Mike Mason <mmlnx@us.ibm.com>]

Mathieu Desnoyers wrote:
> * Mathieu Desnoyers (compudj@krystal.dyndns.org) 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 ?
> > >
> > > Mathieu
> > I'm working on an implementation.
>
> It's ready for testing. Please grab
> http://ltt.polymtl.ca/lttng/patch-2.6.24-rc1-git13-lttng-0.10-pre18.tar.bz2
> patch name :
>
> markers-support-multiple-probes.patch

This patch alone doesn't apply cleanly at all on 2.6.24-rc1-git14.  Are there other patches in this series I should apply first?

Mike

> It still need to go through patchcheck.pl and some polishing, but it
> seems to work fine for me with multiple probes (the sample marker,
> sample probe and multiple instances of my lttng probes can
> connect/disconnect without problem).
>
> Currently, the "connect/disconnect" and "arm/disarm" operations are
> separate. However, they could be merged. Any comment/preference on this?
> Being separate, a probe provider can wait until the very last moment
> before it activates its markers, with a minimalistic impact on the
> system, but it is not such a strong argument.
>
> Mathieu
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