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Re: Multiple handlers per marker
- From: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- To: Mike Mason <mmlnx at us dot ibm dot com>
- Cc: ltt-dev at shafik dot org, SystemTAP <systemtap at sources dot redhat dot com>
- Date: Fri, 2 Nov 2007 12:42:35 -0400
- Subject: Re: Multiple handlers per marker
- References: <472A345C.email@example.com> <20071101221530.GC19700@Krystal> <20071102033654.GA1301@Krystal> <472B3001.firstname.lastname@example.org>
* Mike Mason (email@example.com) wrote:
> Mathieu Desnoyers wrote:
> >* Mathieu Desnoyers (firstname.lastname@example.org) wrote:
> >>* Mike Mason (email@example.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.
marker_arm/disarm: They already use a refcount, so no change is needed
I just want to make sure that we can register/unregister probes when a
marker is armed.. I'll have to be careful about this.
My constraints are kind of difficult, but I think I can manage to
implement a good solution :
- Minimal memory footprint when disabled.
- Fast standard case (1 probe). Small memory footprint in that case.
- Use RCU-style updates to the markers structures (this one is hard).
Requires atomic updates, quiescent states...
- Don't create va_list when a marker has no arguments.
I'm half-way there.. I'll keep you posted when I get to an interesting
> >I'm working on an implementation.
> Great! When you need reviewers/testers let me know.
This is always appreciated.
> 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
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68