Re: Writing tapset functions for multiple versions

[Felix Lu <flu at redhat dot com>]

> > > A global integer instead of the string comparison could be set if you
> > > are worried about the overhead. In the example above, tid() could be
> > > saved in the probe alias body as a private global which would be
> > > sufficient
> > > for identifying the desired function.
> > 
> > I'm not quite sure how saving tid() in a private global would work if
> > you are probing more than one thread at a time. Or did I misunderstand
> > your idea and your global is an array? If the private global is an
> > array, now we've got 2 private global arrays (1 for version1.stp and 1
> > for version2.stp).
> 
> A global scalar should suffice in this case. All we want is for the function
> f in version1.stp to be called when version1/process is executed and likewise
> for version2. The probe alias body in the above example would update the
> saved
> tid() every time it's hit.

Using a saved global may not be more efficient than the pp() regex comparison.
Systemtap global variables are locked upon probe handler entry. By updating
the saved tid(), an exclusive lock must be obtained. The overhead from this
may be greater than the string matching.