doc: Add tracing.md, documenting eBPF tracing #20960

1. 
   laanwj commented at 9:17 pm on January 18, 2021: member

   Add documentation for #19866.
2. laanwj added the label Docs on Jan 18, 2021
3. doc: Add tracing.md, documenting eBPF tracing

   Add documentation for #19866.

   3accbc906a
4. laanwj force-pushed on Jan 18, 2021
5. 
   jonatack commented at 9:58 pm on January 18, 2021: member

   Wow! Concept ACK.
6. 
   practicalswift commented at 9:59 pm on January 18, 2021: contributor

   Concept ACK
7. RiccardoMasutti approved
8. 
   RiccardoMasutti commented at 9:05 am on January 19, 2021: contributor

   ACK
9. 0xB10C changes_requested
10. 
    0xB10C commented at 9:50 am on January 19, 2021: member

    Concept ACK for tracing documentation and USDTs in general.

    However, NACK on this PR in it’s current form. This adds documentation for USDT tracepoints that aren’t merged in master. #19866 did only add build system detection of sys/sdt.h and the tracing framework, but not the mentioned tracepoints.

    The two needed commits adding the documented tracepoints would be 16ed9bc2488a96d895a6e277f284f24f4b9ba8ec and ea608d9adc7f41f929b8f6726774f9f09e26099c from this branch laanwj:usdt-probes. Did you mean to merge them first?

11. 
    laanwj commented at 11:25 am on January 19, 2021: member

    The two needed commits adding the documented tracepoints would be 16ed9bc and ea608d9 from this branch laanwj:usdt-probes. Did you mean to merge them first?

    No, that’s not my work, it’s @jb55’s. I’m really confused here.

12. laanwj closed this on Jan 19, 2021

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13. 
    jb55 commented at 3:29 pm on January 19, 2021: member

    “Wladimir J. van der Laan” notifications@github.com writes:

    The two needed commits adding the documented tracepoints would be 16ed9bc and ea608d9 from this branch laanwj:usdt-probes. Did you mean to merge them first?

    No, that’s not my work, it’s @jb55’s. I’m really confused here.

    We only merged the build support for static traces. You mentioned that we should get that in first.

    I don’t think we decided what arguments the static traces should have yet. In my connect_block example I was only passing the height, but we probably want to pass a few more things.

    Some goals for static traces should be:

    • Somewhat stable API (order of arguments don’t change) so people can rely on these for scripting

    • Some motivational use case @0xB10C’s connected_block trace looks useful as well.

    Cheers, Will

14. 
    jnewbery commented at 11:02 am on January 21, 2021: member

    I think we should probably get a concept ACK from other contributors on whether we should start adding tracepoints all over the code. I can see the benefit that they’re very powerful and add almost no overhead when not used. On the other hand, there’s almost an unlimited number of things that could be traced, adding TRACE() macros in lots of places could potentially make the source less readable, and it’s possible to do a lot of useful tracing with uprobes that don’t require any changes to the source.

    Do we know what other projects are doing? Is it common to put lots of USDTs into the master branch? Perhaps we should discuss what our approach should be in a core dev meeting.

15. 
    jb55 commented at 7:49 pm on January 21, 2021: member

    John Newbery notifications@github.com writes:

    adding TRACE() macros in lots of places could potentially make the source less readable, and it’s possible to do a lot of useful tracing with uprobes that don’t require any changes to the source.

    agreed, USDTs should accompanied by motivating examples and scripts. No point adding USDTs that noone uses.

16. 
    0xB10C commented at 5:16 pm on May 20, 2021: member

    picked this up in https://github.com/bitcoin/bitcoin/pull/22006
17. DrahtBot locked this on Aug 18, 2022

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laanwj jonatack practicalswift RiccardoMasutti 0xB10C jb55 jnewbery

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