Missing DTrace probes

[furkanonder]

Bug report

On Linux, you can list available DTrace probes by running /bcc/tools/tplist.py tool;

$ ~/bcc/tools/tplist.py -l cpython/python 
cpython/python python:function__entry
cpython/python python:function__return
cpython/python python:line
cpython/python python:import__find__load__done
cpython/python python:import__find__load__start
cpython/python python:audit
cpython/python python:gc__start
cpython/python python:gc__done

The function__entry, function__return and line probes appear to be missing after the GH-103082: Implementation of PEP 669: Low Impact Monitoring for CPython update.

$ ~/bcc/tools/tplist.py -l cpython/python
cpython/python python:import__find__load__done
cpython/python python:import__find__load__start
cpython/python python:audit
cpython/python python:gc__start
cpython/python python:gc__done

Your environment

• CPython versions tested on: Python 3.12.0a7+
• Operating system and architecture: Arch Linux x86_64

Linked PRs

• gh-104280: Add test cases for DTrace probes #107125
• [3.12] gh-104280: Add test cases for DTrace probes (GH-107125) #107489
• [3.11] gh-104280: Add test cases for DTrace probes (GH-107125) #107492
• gh-104280: remove deprecated DTrace from docs #110571

[sunmy2019]

CC: @markshannon @gaogaotiantian

[gaogaotiantian]

I think DTrace was intentionally removed because it was not being used. I forgot where I read that, but Mark will have a better idea of it.

[brandtbucher]

I think a bigger issue is that I'm pretty sure it isn't tested on any CI jobs or buildbots, so stuff can break (like #98894) without us noticing.

If it's not tested, it effectively isn't supported. Not sure whether the right path forward is for some motivated person to fix and maintain it, for us to remove the broken stuff and keep the working stuff (with a big disclaimer in the docs), or to just rip it out entirely.

Anecdotally, it's always been a bit uncomfortable to touch this stuff when modifying the interpreter, since I'm not very sure how it works or how to test it.

@ambv might have an idea of what the right path forward is here.

[furkanonder]

I think DTrace was intentionally removed because it was not being used. I forgot where I read that, but Mark will have a better idea of it.

Dtrace support was actually being used. DTrace markers, also known as "probes", are not only used by DTtrace. For example, these probes can be used with bpftrace.

There are tools written using these probes. I will even give a talk about these probes at EuroPython 2023 :)

[kchoudhu]

Just chiming in here: we use these probes in anger in production, and their absence is a blocker preventing us from upgrading to 3.11.

If tests are needed to make sure dtrace support is accounted for in the release process, I am happy to contribute time to write tests and set them up.

[furkanonder]

@ambv @brandtbucher I wrote a test case to check for the presence of probes. Could you review the PR?