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

From: Mathieu Desnoyers
Date: Thu Jun 06 2024 - 12:23:01 EST


On 2024-06-06 11:49, Boqun Feng wrote:
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.

Thanks for the pointers, it makes sense.

Thanks,

Mathieu


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


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