61cefde7a7
8f37f5c2a562c38c83fc40234ade9c301fc4e685 tracing: Tracepoint for connected blocks (0xb10c)
4224dec22baa66547303840707cf1d4f15a49b20 tracing: Tracepoints for in- and outbound P2P msgs (0xb10c)
469b71ae629228b2591a55831817a0e5fad89360 doc: document systemtap dependency (0xb10c)
84ace9aef116a05e034730f2bb2f109d1d77aac7 doc: Add initial USDT documentation (0xb10c)
Pull request description:
This PR adds documentation for User-Space, Statically Defined Tracing (USDT) as well as three tracepoints (including documentation and usage examples).
## Context
The `TRACEx` macros for tracepoints and build system changes for USDT were merged in https://github.com/bitcoin/bitcoin/pull/19866 earlier this year. Issue https://github.com/bitcoin/bitcoin/issues/20981 contains discussion about potential tracepoints and guidelines for adding them (also documented with this PR). USDT was a topic in a [core-dev-meeting discussion](https://bitcoin.jonasschnelli.ch/ircmeetings/logs/bitcoin-core-dev/2021/bitcoin-core-dev.2021-01-21-19.00.moin.txt) on 21st Jan, 2021.
- [collabora.com: An eBPF overview, part 1: Introduction](https://www.collabora.com/news-and-blog/blog/2019/04/05/an-ebpf-overview-part-1-introduction/)
- [collabora.com: An eBPF overview, part 2: Machine & bytecode](https://www.collabora.com/news-and-blog/blog/2019/04/15/an-ebpf-overview-part-2-machine-and-bytecode/)
- [Brendan D. Gregg's blog posts, and book on on eBPF](http://www.brendangregg.com/)
- [Brendan D. Gregg: Linux bcc/BPF Node.js USDT Tracing](http://www.brendangregg.com/blog/2016-10-12/linux-bcc-nodejs-usdt.html)
## USDT? Stablecoin?
User-space, Statically Defined Tracing (USDT) allows for more observability during development, debugging, code review, and production usage. The tracepoints make it possible to keep track of custom statistics and enable detailed monitoring of otherwise hidden internals and have little to no performance impact when unused. Linux kernels (4.x or newer) can hook into the tracepoints and execute [eBPF] programs in a kernel VM once the tracepoint is called.
This PR includes, for example, tracepoints for in- and outbound P2P messages.
```
USDT and eBPF Overview
======================
┌──────────────────┐ ┌──────────────┐
│ tracing script │ │ bitcoind │
│==================│ 2. │==============│
│ eBPF │ tracing │ hooks │ │
│ code │ logic │ into┌─┤►tracepoint 1─┼───┐ 3.
└────┬───┴──▲──────┘ ├─┤►tracepoint 2 │ │ pass args
1. │ │ 4. │ │ ... │ │ to eBPF
User compiles │ │ pass data to │ └──────────────┘ │ program
Space & loads │ │ tracing script │ │
─────────────────┼──────┼─────────────────┼────────────────────┼───
Kernel │ │ │ │
Space ┌──┬─▼──────┴─────────────────┴────────────┐ │
│ │ eBPF program │◄──────┘
│ └───────────────────────────────────────┤
│ eBPF kernel Virtual Machine (sandboxed) │
└──────────────────────────────────────────┘
1. The tracing script compiles the eBPF code and loads the eBFP program into a kernel VM
2. The eBPF program hooks into one or more tracepoints
3. When the tracepoint is called, the arguments are passed to the eBPF program
4. The eBPF program processes the arguments and returns data to the tracing script
```
The two main [eBPF] front-ends with support for USDT are [bpftrace] an [BPF Compiler Collection (BCC)]. BCC is used for complex tools and daemons and `bpftrace` is preferred for one-liners and shorter scripts. Example tracing scripts for both are provided with this PR.
[eBPF]: https://ebpf.io/
[bpftrace]: https://github.com/iovisor/bpftrace
[BPF Compiler Collection (BCC)]: https://github.com/iovisor/bcc
This PR adds three tracepoints:
- `net:inbound_message`
- `net:outbound_message`
- `valildation:block_connected`
See `doc/tracing.md` and `contrib/tracing/` for documentation and example tracing scripts.
## Open Questions (Not in scope for this PR)
- How to use these tracepoints under macOS?
- Release builds with USDT support?
- Should and can the tracepoints be automatically tested?
## Todo (before undraft)
- [x] bcc example showing how to read raw P2P messages up to 32kb
- [x] document that you need `sys/sdt.h` from `systemtap` for USDT support in Bitcoin Core (`apt install systemtap-sdt-dev` on debian-like). See 933ab8a720
- [ ] release notes?
ACKs for top commit:
laanwj:
re-ACK 8f37f5c2a562c38c83fc40234ade9c301fc4e685
jb55:
ACK 8f37f5c2a562c38c83fc40234ade9c301fc4e685
Tree-SHA512: a92a8a2dfcd28465f58a6e5f50d39486817ef5f51214ec40bdb02a6843b9c08ea154fadb31558825ff3a4687477b90f2a5da5d6451989eef978e128a264c289d
Bitcoin Core integration/staging tree
For an immediately usable, binary version of the Bitcoin Core software, see https://bitcoincore.org/en/download/.
Further information about Bitcoin Core is available in the doc folder.
What is Bitcoin?
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