Apply thread labels to internal worker threads

[bgamari]

Various functions in base are implemented via the use of auxiliary threads. Recently a user requested that GHC apply thread labels to these threads to make these threads clearly identifiable in the event log and listThreads output.

Specifically, I propose that the following functions' implementations be augmented with thread labels where appropriate:

• Control.Concurrent.threadWaitRead
• Control.Concurrent.threadWaitWrite
• Control.Concurrent.threadWaitReadSTM
• Control.Concurrent.threadWaitWriteSTM
• System.Timeout.timeout
• GHC.Conc.Signal.runHandlers

The proposed thread labels are static and consequently should have negligible impact on runtime.

Implementation: !13566

[tomjaguarpaw]

How would this work with listThreads exactly? We have listThreads :: IO [ThreadId], and we can label a thread with labelThread (presumably overwriting any existing label). But I don't see a way of reading the label. How would one do that?

[bgamari]

threadLabel :: ThreadId -> IO (Maybe String) exposed by GHC.Conc.Sync allows one to retrieve the label of a thread (IMHO these operations should be exposed from a more sensibly named module).

[bgamari]

Gentle ping. From my perspective this proposal is ready for a vote.

[bgamari]

Is there anything I can do to move this along? I would very much appreciate being able to get this off my desk.

[tomjaguarpaw]

@Bodigrim, what's the next step?

[Bodigrim]

(Sorry, I'm massively overwhelmed for the past month or so)

Related discussion:

• https://kazu-yamamoto.hatenablog.jp/entry/2024/11/20/160218
• https://www.reddit.com/r/haskell/comments/1gvxl5e/labeling_threads_in_haskell/

@tomjaguarpaw following the Reddit discussion, do you have any specific concerns about the proposal at hand? I understand that you are somewhat unhappy about the existence of listThreads at all, but labelling threads does not seem to make it materially worse.

[tomjaguarpaw]

but labelling threads does not seem to make it materially worse.

I agree. I don't see the labels themselves as a problem. This proposal is fine by me.

[Bodigrim]

Dear CLC members, let's vote on the proposal to annotate internal worker threads in base with helpful labels as detailed in https://gitlab.haskell.org/ghc/ghc/-/merge_requests/13566/diffs. This is purely a quality-of-life change.

@tomjaguarpaw @velveteer @mixphix @hasufell @angerman @parsonsmatt

---

+1 from me.

[parsonsmatt]

+1

[tomjaguarpaw]

+1

[hasufell]

+1

[Bodigrim]

Thanks all, that's enough votes to approve.