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

[Boqun Feng]

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@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

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@xxxxxxxxxxx/

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
>