Re: Dynamically enabling/disabling probe points from within stap script

[fche@redhat.com (Frank Ch. Eigler)]

James Y Knight <foom@fuhm.net> writes:

> Is there any plan to allow the ability to dynamically and
> efficiently enable and disable probes from within the systemtap
> script?

Yup.

> I know you can do:
>   probe process(...).mark("foo") if (whatever) {}
> but that isn't efficient [...]

That's right.  We have had a rough design for this, but kind of forgot
to finish the job.  Your note and PR10995 should bring attention back
to the problem.  (One drawback of actually disarming/rearming probes
is that because of the unavoidable latency between intended rearming
and actual rearming, the script may experience unknown missed probes.)

> Also nice would be a way to (efficiently!) say "run this probe every
> N times the location is hit".

Since this would require counting each location hit, and thus the
activation of the corresponding low-level probe, this would be
equivalent to adding something like this into the probe handler

           if (count++ % 100) next;

- FChE