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

[Mathieu Desnoyers]

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 ?

Thanks,

Mathieu

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