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Date: Thu, 6 Jun 2024 08:49:06 -0700
From: Boqun Feng <boqun.feng@gmail.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Alice Ryhl <aliceryhl@google.com>, Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Jason Baron <jbaron@akamai.com>, Ard Biesheuvel <ardb@kernel.org>,
	Miguel Ojeda <ojeda@kernel.org>,
	Alex Gaynor <alex.gaynor@gmail.com>,
	Wedson Almeida Filho <wedsonaf@gmail.com>,
	Gary Guo <gary@garyguo.net>,
	=?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
	Benno Lossin <benno.lossin@proton.me>,
	Andreas Hindborg <a.hindborg@samsung.com>,
	linux-trace-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] rust: add tracepoint support
Message-ID: <ZmHacqvRwBj7OvWm@boqun-archlinux>
References: <20240606-tracepoint-v1-0-6551627bf51b@google.com>
 <20240606-tracepoint-v1-3-6551627bf51b@google.com>
 <389a8c55-a169-47ef-99c0-48f58003b40c@efficios.com>
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In-Reply-To: <389a8c55-a169-47ef-99c0-48f58003b40c@efficios.com>

On Thu, Jun 06, 2024 at 11:30:03AM -0400, Mathieu Desnoyers 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@google.com>
> 
> [...]
> 
> > 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

Long-term plan is to 1) compile the C helpers in some IR and 2) inline
the helpers with Rust in IR-level, as what Gary has:

	https://lore.kernel.org/rust-for-linux/20240529202817.3641974-1-gary@garyguo.net/

and I use it for the upcoming atomic API support:

	https://github.com/fbq/linux/tree/dev/rust/atomic-rfc

and it works very well.
	
Regards,
Boqun

> 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
>