rust: add a pointer for profiling to the README

As figuring out how to get useful profiles is not obvious.

Differential Revision: https://phab.mercurial-scm.org/D8603

rust/README.rst

@@ -1,84 +1,99 b''
 ===================
 Mercurial Rust Code
 ===================
 
 This directory contains various Rust code for the Mercurial project.
 Rust is not required to use (or build) Mercurial, but using it
 improves performance in some areas.
 
 There are currently three independent rust projects:
 - chg. An implementation of chg, in rust instead of C.
 - hgcli. A project that provide a (mostly) self-contained "hg" binary,
   for ease of deployment and a bit of speed, using PyOxidizer. See
   hgcli/README.md.
 - hg-core (and hg-cpython): implementation of some
   functionality of mercurial in rust, e.g. ancestry computations in
   revision graphs, status or pull discovery. The top-level ``Cargo.toml`` file
   defines a workspace containing these crates.
 
 Using Rust code
 ===============
 
 Local use (you need to clean previous build artifacts if you have
 built without rust previously)::
 
   $ make PURE=--rust local # to use ./hg
   $ ./tests/run-tests.py --rust # to run all tests
   $ ./hg debuginstall | grep -i rust # to validate rust is in use
   checking Rust extensions (installed)
   checking module policy (rust+c-allow)
 
 If the environment variable ``HGWITHRUSTEXT=cpython`` is set, the Rust
 extension will be used by default unless ``--no-rust``.
 
 One day we may use this environment variable to switch to new experimental
 binding crates like a hypothetical ``HGWITHRUSTEXT=hpy``.
 
+Profiling
+=========
+
+Setting the environment variable ``RUST_LOG=trace`` will make hg print
+a few high level rust-related performance numbers. It can also
+indicate why the rust code cannot be used (say, using lookarounds in
+hgignore).
+
+``py-spy`` (https://github.com/benfred/py-spy) can be used to
+construct a single profile with rust functions and python functions
+(as opposed to ``hg --profile``, which attributes time spent in rust
+to some unlucky python code running shortly after the rust code, and
+as opposed to tools for native code like ``perf``, which attribute
+time to the python interpreter instead of python functions).
+
 Developing Rust
 ===============
 
 The current version of Rust in use is ``1.34.2``, because it's what Debian
 stable has. You can use ``rustup override set 1.34.2`` at the root of the repo
 to make it easier on you.
 
 Go to the ``hg-cpython`` folder::
 
   $ cd rust/hg-cpython
 
 Or, only the ``hg-core`` folder. Be careful not to break compatibility::
 
   $ cd rust/hg-core
 
 Simply run::
 
    $ cargo build --release
 
 It is possible to build without ``--release``, but it is not
 recommended if performance is of any interest: there can be an order
 of magnitude of degradation when removing ``--release``.
 
 For faster builds, you may want to skip code generation::
 
   $ cargo check
 
 For even faster typing::
 
   $ cargo c
 
 You can run only the rust-specific tests (as opposed to tests of
 mercurial as a whole) with::
 
   $ cargo test --all
 
 Formatting the code
 -------------------
 
 We use ``rustfmt`` to keep the code formatted at all times. For now, we are
 using the nightly version because it has been stable enough and provides
 comment folding.
 
 To format the entire Rust workspace::
 
   $ cargo +nightly fmt
 
 This requires you to have the nightly toolchain installed.