Merge bitcoin#22006: tracing: first tracepoints and documentation on …

…User-Space, Statically Defined Tracing (USDT)

8f37f5c tracing: Tracepoint for connected blocks (0xb10c)
4224dec tracing: Tracepoints for in- and outbound P2P msgs (0xb10c)
469b71a doc: document systemtap dependency (0xb10c)
84ace9a 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 bitcoin#19866 earlier this year. Issue bitcoin#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 bitcoin@933ab8a
  - [ ] release notes?

ACKs for top commit:
  laanwj:
    re-ACK 8f37f5c
  jb55:
    ACK 8f37f5c

Tree-SHA512: a92a8a2dfcd28465f58a6e5f50d39486817ef5f51214ec40bdb02a6843b9c08ea154fadb31558825ff3a4687477b90f2a5da5d6451989eef978e128a264c289d

contrib/tracing/README.md

@@ -0,0 +1,241 @@
+Example scripts for User-space, Statically Defined Tracing (USDT)
+=================================================================
+
+This directory contains scripts showcasing User-space, Statically Defined
+Tracing (USDT) support for Bitcoin Core on Linux using. For more information on
+USDT support in Bitcoin Core see the [USDT documentation].
+
+[USDT documentation]: ../../doc/tracing.md
+
+
+Examples for the two main eBPF front-ends, [bpftrace] and
+[BPF Compiler Collection (BCC)], with support for USDT, are listed. BCC is used
+for complex tools and daemons and `bpftrace` is preferred for one-liners and
+shorter scripts.
+
+[bpftrace]: https://github.com/iovisor/bpftrace
+[BPF Compiler Collection (BCC)]: https://github.com/iovisor/bcc
+
+
+To develop and run bpftrace and BCC scripts you need to install the
+corresponding packages. See [installing bpftrace] and [installing BCC] for more
+information. For development there exist a [bpftrace Reference Guide], a
+[BCC Reference Guide], and a [bcc Python Developer Tutorial].
+
+[installing bpftrace]: https://github.com/iovisor/bpftrace/blob/master/INSTALL.md
+[installing BCC]: https://github.com/iovisor/bcc/blob/master/INSTALL.md
+[bpftrace Reference Guide]: https://github.com/iovisor/bpftrace/blob/master/docs/reference_guide.md
+[BCC Reference Guide]: https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md
+[bcc Python Developer Tutorial]: https://github.com/iovisor/bcc/blob/master/docs/tutorial_bcc_python_developer.md
+
+## Examples
+
+The bpftrace examples contain a relative path to the `bitcoind` binary. By
+default, the scripts should be run from the repository-root and assume a
+self-compiled `bitcoind` binary. The paths in the examples can be changed, for
+example, to point to release builds if needed. See the
+[Bitcoin Core USDT documentation] on how to list available tracepoints in your
+`bitcoind` binary.
+
+[Bitcoin Core USDT documentation]: ../../doc/tracing.md#listing-available-tracepoints
+
+**WARNING: eBPF programs require root privileges to be loaded into a Linux
+kernel VM. This means the bpftrace and BCC examples must be executed with root
+privileges. Make sure to carefully review any scripts that you run with root
+privileges first!**
+
+### log_p2p_traffic.bt
+
+A bpftrace script logging information about inbound and outbound P2P network
+messages. Based on the `net:inbound_message` and `net:outbound_message`
+tracepoints.
+
+By default, `bpftrace` limits strings to 64 bytes due to the limited stack size
+in the eBPF VM. For example, Tor v3 addresses exceed the string size limit which
+results in the port being cut off during logging. The string size limit can be
+increased with the `BPFTRACE_STRLEN` environment variable (`BPFTRACE_STRLEN=70`
+works fine).
+
+```
+$ bpftrace contrib/tracing/log_p2p_traffic.bt
+```
+
+Output
+```
+outbound 'ping' msg to peer 11 (outbound-full-relay, [2a02:b10c:f747:1:ef:fake:ipv6:addr]:8333) with 8 bytes
+inbound 'pong' msg from peer 11 (outbound-full-relay, [2a02:b10c:f747:1:ef:fake:ipv6:addr]:8333) with 8 bytes
+inbound 'inv' msg from peer 16 (outbound-full-relay, XX.XX.XXX.121:8333) with 37 bytes
+outbound 'getdata' msg to peer 16 (outbound-full-relay, XX.XX.XXX.121:8333) with 37 bytes
+inbound 'tx' msg from peer 16 (outbound-full-relay, XX.XX.XXX.121:8333) with 222 bytes
+outbound 'inv' msg to peer 9 (outbound-full-relay, faketorv3addressa2ufa6odvoi3s77j4uegey0xb10csyfyve2t33curbyd.onion:8333) with 37 bytes
+outbound 'inv' msg to peer 7 (outbound-full-relay, XX.XX.XXX.242:8333) with 37 bytes
+…
+```
+
+### p2p_monitor.py
+
+A BCC Python script using curses for an interactive P2P message monitor. Based
+on the `net:inbound_message` and `net:outbound_message` tracepoints.
+
+Inbound and outbound traffic is listed for each peer together with information
+about the connection. Peers can be selected individually to view recent P2P
+messages.
+
+```
+$ python3 contrib/tracing/p2p_monitor.py ./src/bitcoind
+```
+
+Lists selectable peers and traffic and connection information.
+```
+ P2P Message Monitor
+ Navigate with UP/DOWN or J/K and select a peer with ENTER or SPACE to see individual P2P messages
+
+ PEER  OUTBOUND              INBOUND               TYPE                   ADDR
+    0  46          398 byte  61      1407590 byte  block-relay-only       XX.XX.XXX.196:8333
+   11  1156     253570 byte  3431    2394924 byte  outbound-full-relay    XXX.X.XX.179:8333
+   13  3425    1809620 byte  1236     305458 byte  inbound                XXX.X.X.X:60380
+   16  1046     241633 byte  1589    1199220 byte  outbound-full-relay    4faketorv2pbfu7x.onion:8333
+   19  577      181679 byte  390      148951 byte  outbound-full-relay    kfake4vctorjv2o2.onion:8333
+   20  11         1248 byte  13         1283 byte  block-relay-only       [2600:fake:64d9:b10c:4436:aaaa:fe:bb]:8333
+   21  11         1248 byte  13         1299 byte  block-relay-only       XX.XXX.X.155:8333
+   22  5           103 byte  1           102 byte  feeler                 XX.XX.XXX.173:8333
+   23  11         1248 byte  12         1255 byte  block-relay-only       XX.XXX.XXX.220:8333
+   24  3           103 byte  1           102 byte  feeler                 XXX.XXX.XXX.64:8333
+…
+```
+
+Showing recent P2P messages between our node and a selected peer.
+
+```
+    ----------------------------------------------------------------------
+    |                PEER 16 (4faketorv2pbfu7x.onion:8333)               |
+    | OUR NODE                outbound-full-relay                   PEER |
+    |                                           <--- sendcmpct (9 bytes) |
+    | inv (37 byte) --->                                                 |
+    |                                                <--- ping (8 bytes) |
+    | pong (8 byte) --->                                                 |
+    | inv (37 byte) --->                                                 |
+    |                                               <--- addr (31 bytes) |
+    | inv (37 byte) --->                                                 |
+    |                                       <--- getheaders (1029 bytes) |
+    | headers (1 byte) --->                                              |
+    |                                           <--- feefilter (8 bytes) |
+    |                                                <--- pong (8 bytes) |
+    |                                            <--- headers (82 bytes) |
+    |                                            <--- addr (30003 bytes) |
+    | inv (1261 byte) --->                                               |
+    |                                 …                                  |
+
+```
+
+### log_raw_p2p_msgs.py
+
+A BCC Python script showcasing eBPF and USDT limitations when passing data
+larger than about 32kb. Based on the `net:inbound_message` and
+`net:outbound_message` tracepoints.
+
+Bitcoin P2P messages can be larger than 32kb (e.g. `tx`, `block`, ...). The
+eBPF VM's stack is limited to 512 bytes, and we can't allocate more than about
+32kb for a P2P message in the eBPF VM. The **message data is cut off** when the
+message is larger than MAX_MSG_DATA_LENGTH (see script). This can be detected
+in user-space by comparing the data length to the message length variable. The
+message is cut off when the data length is smaller than the message length.
+A warning is included with the printed message data.
+
+Data is submitted to user-space (i.e. to this script) via a ring buffer. The
+throughput of the ring buffer is limited. Each p2p_message is about 32kb in
+size. In- or outbound messages submitted to the ring buffer in rapid
+succession fill the ring buffer faster than it can be read. Some messages are
+lost. BCC prints: `Possibly lost 2 samples` on lost messages.
+
+
+```
+$ python3 contrib/tracing/log_raw_p2p_msgs.py ./src/bitcoind
+```
+
+```
+Logging raw P2P messages.
+Messages larger that about 32kb will be cut off!
+Some messages might be lost!
+ outbound msg 'inv' from peer 4 (outbound-full-relay, XX.XXX.XX.4:8333) with 253 bytes: 0705000000be2245c8f844c9f763748e1a7…
+…
+Warning: incomplete message (only 32568 out of 53552 bytes)! inbound msg 'tx' from peer 32 (outbound-full-relay, XX.XXX.XXX.43:8333) with 53552 bytes: 020000000001fd3c01939c85ad6756ed9fc…
+…
+Possibly lost 2 samples
+```
+
+### connectblock_benchmark.bt
+
+A `bpftrace` script to benchmark the `ConnectBlock()` function during, for
+example, a blockchain re-index. Based on the `validation:block_connected` USDT
+tracepoint.
+
+The script takes three positional arguments. The first two arguments, the start,
+and end height indicate between which blocks the benchmark should be run. The
+third acts as a duration threshold in milliseconds. When the `ConnectBlock()`
+function takes longer than the threshold, information about the block, is
+printed. For more details, see the header comment in the script.
+
+By default, `bpftrace` limits strings to 64 bytes due to the limited stack size
+in the kernel VM. Block hashes as zero-terminated hex strings are 65 bytes which
+exceed the string limit. The string size limit can be set to 65 bytes with the
+environment variable `BPFTRACE_STRLEN`.
+
+The following command can be used to benchmark, for example, `ConnectBlock()`
+between height 20000 and 38000 on SigNet while logging all blocks that take
+longer than 25ms to connect.
+
+```
+$ BPFTRACE_STRLEN=65 bpftrace contrib/tracing/connectblock_benchmark.bt 20000 38000 25
+```
+
+In a different terminal, starting Bitcoin Core in SigNet mode and with
+re-indexing enabled.
+
+```
+$ ./src/bitcoind -signet -reindex
+```
+
+This produces the following output.
+```
+Attaching 5 probes...
+ConnectBlock Benchmark between height 20000 and 38000 inclusive
+Logging blocks taking longer than 25 ms to connect.
+Starting Connect Block Benchmark between height 20000 and 38000.
+BENCH   39 blk/s     59 tx/s      59 inputs/s       20 sigops/s (height 20038)
+Block 20492 (000000f555653bb05e2f3c6e79925e01a20dd57033f4dc7c354b46e34735d32b)    20 tx   2319 ins   2318 sigops  took   38 ms
+BENCH 1840 blk/s   2117 tx/s    4478 inputs/s     2471 sigops/s (height 21879)
+BENCH 1816 blk/s   4972 tx/s    4982 inputs/s      125 sigops/s (height 23695)
+BENCH 2095 blk/s   2890 tx/s    2910 inputs/s      152 sigops/s (height 25790)
+BENCH 1684 blk/s   3979 tx/s    4053 inputs/s      288 sigops/s (height 27474)
+BENCH 1155 blk/s   3216 tx/s    3252 inputs/s      115 sigops/s (height 28629)
+BENCH 1797 blk/s   2488 tx/s    2503 inputs/s      111 sigops/s (height 30426)
+BENCH 1849 blk/s   6318 tx/s    6569 inputs/s    12189 sigops/s (height 32275)
+BENCH  946 blk/s  20209 tx/s   20775 inputs/s    83809 sigops/s (height 33221)
+Block 33406 (0000002adfe4a15cfcd53bd890a89bbae836e5bb7f38bac566f61ad4548c87f6)    25 tx   2045 ins   2090 sigops  took   29 ms
+Block 33687 (00000073231307a9828e5607ceb8156b402efe56747271a4442e75eb5b77cd36)    52 tx   1797 ins   1826 sigops  took   26 ms
+BENCH  582 blk/s  21581 tx/s   27673 inputs/s    60345 sigops/s (height 33803)
+BENCH 1035 blk/s  19735 tx/s   19776 inputs/s    51355 sigops/s (height 34838)
+Block 35625 (0000006b00b347390c4768ea9df2655e9ff4b120f29d78594a2a702f8a02c997)    20 tx   3374 ins   3371 sigops  took   49 ms
+BENCH  887 blk/s  17857 tx/s   22191 inputs/s    24404 sigops/s (height 35725)
+Block 35937 (000000d816d13d6e39b471cd4368db60463a764ba1f29168606b04a22b81ea57)    75 tx   3943 ins   3940 sigops  took   61 ms
+BENCH  823 blk/s  16298 tx/s   21031 inputs/s    18440 sigops/s (height 36548)
+Block 36583 (000000c3e260556dbf42968aae3f904dba8b8c1ff96a6f6e3aa5365d2e3ad317)    24 tx   2198 ins   2194 sigops  took   34 ms
+Block 36700 (000000b3b173de9e65a3cfa738d976af6347aaf83fa17ab3f2a4d2ede3ddfac4)    73 tx   1615 ins   1611 sigops  took   31 ms
+Block 36832 (0000007859578c02c1ac37dabd1b9ec19b98f350b56935f5dd3a41e9f79f836e)    34 tx   1440 ins   1436 sigops  took   26 ms
+BENCH  613 blk/s  16718 tx/s   25074 inputs/s    23022 sigops/s (height 37161)
+Block 37870 (000000f5c1086291ba2d943fb0c3bc82e71c5ee341ee117681d1456fbf6c6c38)    25 tx   1517 ins   1514 sigops  took   29 ms
+BENCH  811 blk/s  16031 tx/s   20921 inputs/s    18696 sigops/s (height 37972)
+
+Took 14055 ms to connect the blocks between height 20000 and 38000.
+
+Histogram of block connection times in milliseconds (ms).
+@durations:
+[0]                16838 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
+[1]                  882 |@@                                                  |
+[2, 4)               236 |                                                    |
+[4, 8)                23 |                                                    |
+[8, 16)                9 |                                                    |
+[16, 32)               9 |                                                    |
+[32, 64)               4 |                                                    |
+```

contrib/tracing/connectblock_benchmark.bt

@@ -0,0 +1,150 @@
+#!/usr/bin/env bpftrace
+
+/*
+
+  USAGE:
+
+  BPFTRACE_STRLEN=65 bpftrace contrib/tracing/connectblock_benchmark.bt <start height> <end height> <logging threshold in ms>
+
+  - The environment variable BPFTRACE_STRLEN needs to be set to 65 chars as
+    strings are limited to 64 chars by default. Hex strings with Bitcoin block
+    hashes are 64 hex chars + 1 null-termination char.
+  - <start height> sets the height at which the benchmark should start. Setting
+    the start height to 0 starts the benchmark immediately, even before the
+    first block is connected.
+  - <end height> sets the height after which the benchmark should end. Setting
+    the end height to 0 disables the benchmark. The script only logs blocks
+    over <logging threshold in ms>.
+  - Threshold <logging threshold in ms>
+
+  This script requires a 'bitcoind' binary compiled with eBPF support and the
+  'validation:block_connected' USDT. By default, it's assumed that 'bitcoind' is
+  located in './src/bitcoind'. This can be modified in the script below.
+
+  EXAMPLES:
+
+  BPFTRACE_STRLEN=65 bpftrace contrib/tracing/connectblock_benchmark.bt 300000 680000 1000
+
+  When run together 'bitcoind -reindex', this benchmarks the time it takes to
+  connect the blocks between height 300.000 and 680.000 (inclusive) and prints
+  details about all blocks that take longer than 1000ms to connect. Prints a
+  histogram with block connection times when the benchmark is finished.
+
+
+  BPFTRACE_STRLEN=65 bpftrace contrib/tracing/connectblock_benchmark.bt 0 0 500
+
+  When running together 'bitcoind', all newly connected blocks that
+  take longer than 500ms to connect are logged. A histogram with block
+  connection times is shown when the script is terminated.
+
+*/
+
+BEGIN
+{
+  $start_height = $1;
+  $end_height = $2;
+  $logging_threshold_ms = $3;
+
+  if ($end_height < $start_height) {
+    printf("Error: start height (%d) larger than end height (%d)!\n", $start_height, $end_height);
+    exit();
+  }
+
+  if ($end_height > 0) {
+    printf("ConnectBlock benchmark between height %d and %d inclusive\n", $start_height, $end_height);
+  } else {
+    printf("ConnectBlock logging starting at height %d\n", $start_height);
+  }
+
+  if ($logging_threshold_ms > 0) {
+    printf("Logging blocks taking longer than %d ms to connect.\n", $3);
+  }
+
+  if ($start_height == 0) {
+    @start = nsecs;
+  }
+}
+
+/*
+  Attaches to the 'validation:block_connected' USDT and collects stats when the
+  connected block is between the start and end height (or the end height is
+  unset).
+*/
+usdt:./src/bitcoind:validation:block_connected /arg1 >= $1 && (arg1 <= $2 || $2 == 0 )/
+{
+  $height = arg1;
+  $transactions = arg2;
+  $inputs = arg3;
+  $sigops =  arg4;
+  $duration = (uint64) arg5;
+
+  @height = $height;
+
+  @blocks = @blocks + 1;
+  @transactions = @transactions + $transactions;
+  @inputs = @inputs + $inputs;
+  @sigops = @sigops + $sigops;
+
+  @durations = hist($duration / 1000);
+
+  if ($height == $1 && $height != 0) {
+    @start = nsecs;
+    printf("Starting Connect Block Benchmark between height %d and %d.\n", $1, $2);
+  }
+
+  if ($2 > 0 && $height >= $2) {
+    @end = nsecs;
+    $duration = @end - @start;
+    printf("\nTook %d ms to connect the blocks between height %d and %d.\n", $duration / 1000000, $1, $2);
+    exit();
+  }
+}
+
+/*
+  Attaches to the 'validation:block_connected' USDT and logs information about
+  blocks where the time it took to connect the block is above the
+  <logging threshold in ms>.
+*/
+usdt:./src/bitcoind:validation:block_connected / (uint64) arg5 / 1000> $3 /
+{
+  $hash_str = str(arg0);
+  $height = (int32) arg1;
+  $transactions = (uint64) arg2;
+  $inputs = (int32) arg3;
+  $sigops = (int64) arg4;
+  $duration = (int64) arg5;
+
+  printf("Block %d (%s)  %4d tx  %5d ins  %5d sigops  took %4d ms\n", $height, $hash_str, $transactions, $inputs, $sigops, (uint64) $duration / 1000);
+}
+
+
+/*
+  Prints stats about the blocks, transactions, inputs, and sigops processed in
+  the last second (if any).
+*/
+interval:s:1 {
+  if (@blocks > 0) {
+    printf("BENCH %4d blk/s %6d tx/s %7d inputs/s %8d sigops/s (height %d)\n", @blocks, @transactions, @inputs, @sigops, @height);
+
+    zero(@blocks);
+    zero(@transactions);
+    zero(@inputs);
+    zero(@sigops);
+  }
+}
+
+END
+{
+  printf("\nHistogram of block connection times in milliseconds (ms).\n");
+  print(@durations);
+
+  clear(@durations);
+  clear(@blocks);
+  clear(@transactions);
+  clear(@inputs);
+  clear(@sigops);
+  clear(@height);
+  clear(@start);
+  clear(@end);
+}
+