Re: [PATCH 3/3] rust: add tracepoint support

[Mathieu Desnoyers]

On 2024-06-06 12:16, Alice Ryhl wrote:
> On Thu, Jun 6, 2024 at 5:29 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > On 2024-06-06 11:05, Alice Ryhl wrote:
> > > Make it possible to have Rust code call into tracepoints defined by C
> > > code. It is still required that the tracepoint is declared in a C
> > > header, and that this header is included in the input to bindgen.
> > >
> > > Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> >
> > [...]
> >
> > > diff --git a/rust/helpers.c b/rust/helpers.c
> > > index 2c37a0f5d7a8..0560cc2a512a 100644
> > > --- a/rust/helpers.c
> > > +++ b/rust/helpers.c
> > > @@ -165,6 +165,30 @@ rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
> > >    }
> > >    EXPORT_SYMBOL_GPL(rust_helper_krealloc);
> > >
> > > +void rust_helper_preempt_enable_notrace(void)
> > > +{
> > > +     preempt_enable_notrace();
> > > +}
> > > +EXPORT_SYMBOL_GPL(rust_helper_preempt_enable_notrace);
> > > +
> > > +void rust_helper_preempt_disable_notrace(void)
> > > +{
> > > +     preempt_disable_notrace();
> > > +}
> > > +EXPORT_SYMBOL_GPL(rust_helper_preempt_disable_notrace);
> > > +
> > > +bool rust_helper_current_cpu_online(void)
> > > +{
> > > +     return cpu_online(raw_smp_processor_id());
> > > +}
> > > +EXPORT_SYMBOL_GPL(rust_helper_current_cpu_online);
> > > +
> > > +void *rust_helper___rcu_dereference_raw(void **p)
> > > +{
> > > +     return rcu_dereference_raw(p);
> > > +}
> > > +EXPORT_SYMBOL_GPL(rust_helper___rcu_dereference_raw);
> >
> > Ouch. Doing a function call for each of those small operations will
> > have a rather large performance impact when tracing is active. If it is
> > not possible to inline those in Rust, then implementing __DO_TRACE in
> > a C function would at least allow Rust to only do a single call to C
> > rather than go back and forth between Rust and C.
> >
> > What prevents inlining those helpers in Rust ?
>
> There's nothing fundamental that prevents it. But they contain a large
> amount of architecture specific code, which makes them a significant
> amount of work to reimplement in Rust.
>
> For example, rcu_dereference_raw calls into READ_ONCE. READ_ONCE is
> usually a volatile load, but under arm64+LTO, you get a bunch of
> inline assembly that relies on ALTERNATIVE for feature detection:
> https://elixir.bootlin.com/linux/v6.9/source/arch/arm64/include/asm/rwonce.h#L36
>
> And preempt_enable_notrace has a similar story.
>
> The solution that Boqun mentions is nice, but it relies on rustc and
> clang using the same version of LLVM. You are unlikely to have
> compilers with matching LLVMs unless you intentionally take steps to
> make that happen.
>
> But yes, perhaps these helpers are an argument to have a single call
> for __DO_TRACE instead.

If those helpers end up being inlined into Rust with the solution
pointed to by Boqun, then it makes sense to implement __DO_TRACE
in Rust. Otherwise doing a single call to C would be more efficient
than calling each of the helpers individually.

Thanks,

Mathieu

> Alice

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