Re: [PATCH v4 2/2] rust: add tracepoint support

[Gary Guo <gary@garyguo.net>]

On Fri, 28 Jun 2024 13:23:32 +0000
Alice Ryhl <aliceryhl@google.com> 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@google.com>

The Rust part LGTM. Some comment about the C part.

> ---
>  include/linux/tracepoint.h      | 18 +++++++++++++++-
>  include/trace/define_trace.h    | 12 +++++++++++
>  rust/bindings/bindings_helper.h |  1 +
>  rust/kernel/lib.rs              |  1 +
>  rust/kernel/tracepoint.rs       | 47 +++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 78 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 689b6d71590e..d82af4d77c9f 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -238,6 +238,20 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  #define __DECLARE_TRACE_RCU(name, proto, args, cond)
>  #endif
>  
> +/*
> + * Declare an exported function that Rust code can call to trigger this
> + * tracepoint. This function does not include the static branch; that is done
> + * in Rust to avoid a function call when the tracepoint is disabled.
> + */
> +#define DEFINE_RUST_DO_TRACE(name, proto, args)
> +#define DEFINE_RUST_DO_TRACE_REAL(name, proto, args)			\
> +	notrace void rust_do_trace_##name(proto)			\
> +	{								\
> +		__DO_TRACE(name,					\
> +			TP_ARGS(args),					\
> +			cpu_online(raw_smp_processor_id()), 0);		\

I guess this doesn't support conditions. Currently the conditions are
specified during declaration and not during definition.

Would it make sense to have

	static inline void do_trace_##name(proto)
	{
		__DO_TRACE(name, TP_ARGS(args), TP_CONDITION(cond), 0);
	}

in `__DECLARE_TRACE` and then simply call it in `rust_do_trace_##name`?

> +	}
> +
>  /*
>   * Make sure the alignment of the structure in the __tracepoints section will
>   * not add unwanted padding between the beginning of the section and the
> @@ -253,6 +267,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  	extern int __traceiter_##name(data_proto);			\
>  	DECLARE_STATIC_CALL(tp_func_##name, __traceiter_##name);	\
>  	extern struct tracepoint __tracepoint_##name;			\
> +	extern void rust_do_trace_##name(proto);			\
>  	static inline void trace_##name(proto)				\
>  	{								\
>  		if (static_key_false(&__tracepoint_##name.key))		\
> @@ -337,7 +352,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  	void __probestub_##_name(void *__data, proto)			\
>  	{								\
>  	}								\
> -	DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name);
> +	DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name);	\
> +	DEFINE_RUST_DO_TRACE(_name, TP_PROTO(proto), TP_ARGS(args))
>  
>  #define DEFINE_TRACE(name, proto, args)		\
>  	DEFINE_TRACE_FN(name, NULL, NULL, PARAMS(proto), PARAMS(args));
> diff --git a/include/trace/define_trace.h b/include/trace/define_trace.h
> index 00723935dcc7..08ed5ce63a96 100644
> --- a/include/trace/define_trace.h
> +++ b/include/trace/define_trace.h
> @@ -72,6 +72,13 @@
>  #define DECLARE_TRACE(name, proto, args)	\
>  	DEFINE_TRACE(name, PARAMS(proto), PARAMS(args))
>  
> +/* If requested, create helpers for calling these tracepoints from Rust. */
> +#ifdef CREATE_RUST_TRACE_POINTS
> +#undef DEFINE_RUST_DO_TRACE
> +#define DEFINE_RUST_DO_TRACE(name, proto, args)	\
> +	DEFINE_RUST_DO_TRACE_REAL(name, PARAMS(proto), PARAMS(args))
> +#endif
> +
>  #undef TRACE_INCLUDE
>  #undef __TRACE_INCLUDE
>  
> @@ -129,6 +136,11 @@
>  # undef UNDEF_TRACE_INCLUDE_PATH
>  #endif
>  
> +#ifdef CREATE_RUST_TRACE_POINTS
> +# undef DEFINE_RUST_DO_TRACE
> +# define DEFINE_RUST_DO_TRACE(name, proto, args)
> +#endif
> +
>  /* We may be processing more files */
>  #define CREATE_TRACE_POINTS
>