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

[Mathieu Desnoyers]

On 2024-06-06 11:46, Alice Ryhl wrote:
> On Thu, Jun 6, 2024 at 5:25 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> 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.

As long as it is only __DO_TRACE that is duplicated between C and Rust,
I don't see it as a large burden.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com