Re: [PATCH 0/5] perf symbol: Speed up demangling of long Rust symbols

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Sep 28, 2026 at 02:02:22AM +0100, Michal Pluta wrote:
> dso__demangle_sym() demangles Rust v0 symbols itself instead of using
> a helper like the other demanglers. When a demangled name doesn't fit
> the buffer, it grows the buffer by a fixed 32 bytes and reformats the
> whole name from scratch. The number of attempts grows with the name's
> length, so the total work is quadratic. Rust programs with deeply
> nested generics can have very long demangled names, and I noticed this
> as real time spent in perf report on perf.data recorded from launching
> Bevy and Typst.
> 
> Patch 3 is the actual optimisation, doubling the size of the buffer
> instead.  Patch 5 moves the Rust-specific logic into its own function,
> matching how the other demanglers are called.  I think the cleanup is
> worthwhile but I understand it comes down to personal taste and the
> patch can be skipped if maintainers would rather keep
> dso__demangle_sym() as is.
> 
> I used an LLM to review my patches and find the issues with the
> demangle tests and the unterminated buffer bug.
> 
> Testing:
> - 'perf test' passes, including the demangle Java, OCaml and Rust
>   suites (patch 3 adds a case for a symbol that exceeds the buffer
>   limit).
> - 'perf report' output is unchanged before and after this series on
>   every Rust binary I tried it on.
> - Built and tested on x86-64 with a 7.0 kernel.
> 
> The series is based on perf-tools-next, commit 0ae6fc78c5ce ("perf
> timechart: Remove the unused use_old_power_events variable").
> 
> Michal Pluta (5):
>   perf test demangle: Fail when demangling fails
>   perf symbol: Don't return an unterminated Rust demangle buffer
>   perf symbol: Grow the Rust demangle buffer geometrically
>   perf symbol: Shrink the demangled Rust buffer to fit
>   perf symbol: Move Rust demangling into its own function

Reviewed-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung

> 
>  tools/perf/tests/demangle-java-test.c    |  1 +
>  tools/perf/tests/demangle-rust-v0-test.c | 13 +++++
>  tools/perf/util/symbol.c                 | 60 +++++++++++++++---------
>  3 files changed, 52 insertions(+), 22 deletions(-)
> 
> 
> base-commit: 0ae6fc78c5ce0dfd18d8712a50f0fd4602eff103
> -- 
> 2.43.0
>