task_finder callback (and engine?) for every uprobe

[Frank Ch. Eigler]

Hi -

On Tue, Dec 16, 2008 at 10:48:17AM -0600, David Smith wrote:
> Jim Keniston wrote:
> > Investigating PR 7082 & 7092 -- and in particular, instrumenting
> > task_finder's callbacks -- has reminded me of something we discussed
> > very briefly earlier: if we have N uprobes in a process, then every time
> > something happens in that process that requires a task_finder callback,
> > we get N (calls to) callbacks.  E.g., when I probe bash (2170 uprobes),
> > callbacks happen in blocks of about 2170.  Just starting up one instance
> > of bash yields about 101,900 task_finder callbacks on 2.6.27.  [...]
> [...]
> (5) uprobe probes on "*" on a specific bash shell
> #  stap -DDEBUG_TASK_FINDER -v -e 'probe process("bash").function("*") {
> }' -wc bash
> total attach count: 70070, attach count: 916, inuse count: 1
> callback count: 139173, vm_callback count: 0
> 
> Ouch.  According to the pass 2 output, there are 2694 probes.
> [...]
> So, based on all the above, I don't think this is a task_finder problem,
> but a stap translator problem.  The task_finder can't know when you
> really need a callback and when you don't, but the translator could
> figure that out.  Jim/Frank, got any different opinions here?

Looking at the stap -p3 output, it does request a separate
stap_task_finder_target for each uprobe.  However, all those requests
are identical (in terms of callback function pointers and
pathname/etc.).

The task finder could merge all those requests into one utrace engine
attachment.  Or the translator could put in a corresponding level of
indirection.  Either can be made to work.  (The former has the
advantage of editing C code rather than editing a C code generator,
and its greater general usefulness.)

- FChE