Merge #14519: tests: add utility to easily profile node performance with perf

13782b8ba8 docs: add perf section to developer docs (James O'Beirne)
58180b5fd4 tests: add utility to easily profile node performance with perf (James O'Beirne)

Pull request description:

  Adds a context manager to easily (and selectively) profile node performance during functional test execution using `perf`.

  While writing some tests, I encountered some odd bitcoind slowness. I wrote up a utility (`TestNode.profile_with_perf`) that generates performance diagnostics for a node by running `perf` during the execution of a particular region of test code.

  `perf` usage is detailed in the excellent (and sadly unmerged) https://github.com/bitcoin/bitcoin/pull/12649; all due props to @eklitzke.

  ### Example

  ```python
  with node.profile_with_perf("large-msgs"):
      for i in range(200):
          node.p2p.send_message(some_large_msg)
      node.p2p.sync_with_ping()
  ```

  This generates a perf data file in the test node's datadir (`/tmp/testtxmpod0y/node0/node-0-TestName-large-msgs.perf.data`).

  Running `perf report` generates nice output about where the node spent most of its time while running that part of the test:

  ```bash
  $ perf report -i /tmp/testtxmpod0y/node0/node-0-TestName-large-msgs.perf.data --stdio \
    | c++filt \
    | less

  # To display the perf.data header info, please use --header/--header-only options.
  #
  #
  # Total Lost Samples: 0
  #
  # Samples: 135  of event 'cycles:pp'
  # Event count (approx.): 1458205679493582
  #
  # Children      Self  Command          Shared Object        Symbol
  # ........  ........  ...............  ...................  ........................................................................................................................................................................................................................................................................
  #
      70.14%     0.00%  bitcoin-net      bitcoind             [.] CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)
                  |
                  ---CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)

      70.14%     0.00%  bitcoin-net      bitcoind             [.] CNetMessage::readData(char const*, unsigned int)
                  |
                  ---CNetMessage::readData(char const*, unsigned int)
                     CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)

      35.52%     0.00%  bitcoin-net      bitcoind             [.] std::vector<char, zero_after_free_allocator<char> >::_M_fill_insert(__gnu_cxx::__normal_iterator<char*, std::vector<char, zero_after_free_allocator<char> > >, unsigned long, char const&)
                  |
                  ---std::vector<char, zero_after_free_allocator<char> >::_M_fill_insert(__gnu_cxx::__normal_iterator<char*, std::vector<char, zero_after_free_allocator<char> > >, unsigned long, char const&)
                     CNetMessage::readData(char const*, unsigned int)
                     CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)

  ...
  ```

Tree-SHA512: 9ac4ceaa88818d5eca00994e8e3c8ad42ae019550d6583972a0a4f7b0c4f61032e3d0c476b4ae58756bc5eb8f8015a19a7fc26c095bd588f31d49a37ed0c6b3e

test/functional/README.md

@@ -122,3 +122,36 @@ Helpers for script.py
 
 #### [test_framework/blocktools.py](test_framework/blocktools.py)
 Helper functions for creating blocks and transactions.
+
+### Benchmarking with perf
+
+An easy way to profile node performance during functional tests is provided
+for Linux platforms using `perf`.
+
+Perf will sample the running node and will generate profile data in the node's
+datadir. The profile data can then be presented using `perf report` or a graphical
+tool like [hotspot](https://github.com/KDAB/hotspot).
+
+There are two ways of invoking perf: one is to use the `--perf` flag when
+running tests, which will profile each node during the entire test run: perf
+begins to profile when the node starts and ends when it shuts down. The other
+way is the use the `profile_with_perf` context manager, e.g.
+
+```python
+with node.profile_with_perf("send-big-msgs"):
+    # Perform activity on the node you're interested in profiling, e.g.:
+    for _ in range(10000):
+        node.p2p.send_message(some_large_message)
+```
+
+To see useful textual output, run
+
+```sh
+perf report -i /path/to/datadir/send-big-msgs.perf.data.xxxx --stdio | c++filt | less
+```
+
+#### See also:
+
+- [Installing perf](https://askubuntu.com/q/50145)
+- [Perf examples](http://www.brendangregg.com/perf.html)
+- [Hotspot](https://github.com/KDAB/hotspot): a GUI for perf output analysis