Re: [PATCH 0/3] Tracepoints and static branch/call in Rust

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Jun 6, 2024 at 5:25 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> On 2024-06-06 11:05, Alice Ryhl wrote:
> > This implementation implements support for static keys in Rust so that
> > the actual static branch will end up in the Rust object file. However,
> > it would also be possible to just wrap the trace_##name generated by
> > __DECLARE_TRACE in an extern C function and then call that from Rust.
> > This will simplify the Rust code by removing the need for static
> > branches and calls, but it places the static branch behind an external
> > call, which has performance implications.
>
> The tracepoints try very hard to minimize overhead of dormant tracepoints
> so it is not frowned-upon to have them built into production binaries.
> This is needed to make sure distribution vendors keep those tracepoints
> in the kernel binaries that reach end-users.
>
> Adding a function call before evaluation of the static branch goes against
> this major goal.
>
> >
> > A possible middle ground would be to place just the __DO_TRACE body in
> > an extern C function and to implement the Rust wrapper by doing the
> > static branch in Rust, and then calling into C the code that contains
> > __DO_TRACE when the tracepoint is active. However, this would need some
> > changes to include/linux/tracepoint.h to generate and export a function
> > containing the body of __DO_TRACE when the tracepoint should be callable
> > from Rust.
>
> This tradeoff is more acceptable than having a function call before
> evaluation of the static branch, but I wonder what is the upside of
> this tradeoff compared to inlining the whole __DO_TRACE in Rust ?
>
> > So in general, there is a tradeoff between placing parts of the
> > tracepoint (which is perf sensitive) behind an external call, and having
> > code duplicated in both C and Rust (which must be kept in sync when
> > changes are made). This is an important point that I would like feedback
> > on from the C maintainers.
>
> I don't see how the duplication happens there: __DO_TRACE is meant to be
> inlined into each C tracepoint caller site, so the code is already meant
> to be duplicated. Having an explicit function wrapping the tracepoint
> for Rust would just create an extra instance of __DO_TRACE if it happens
> to be also inlined into C code.
>
> Or do you meant you would like to prevent having to duplicate the
> implementation of __DO_TRACE in both C and Rust ?
>
> I'm not sure if you mean to prevent source code duplication between
> C and Rust or duplication of binary code (instructions).

It's a question of maintenance burden. If you change how __DO_TRACE is
implemented, then those changes must also be reflected in the Rust
version. There's no issue in the binary code.

Alice