linux-kernel - Re: [PATCH v2] rust: sync: rcu: Mark Guard methods as inline

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Message-ID: <20250321204056.GA121221@joelnvbox>
Date: Fri, 21 Mar 2025 16:40:56 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: I Hsin Cheng <richard120310@...il.com>
Cc: rostedt@...dmis.org, paulmck@...nel.org, frederic@...nel.org,
	neeraj.upadhyay@...nel.org, joel@...lfernandes.org,
	josh@...htriplett.org, boqun.feng@...il.com, urezki@...il.com,
	mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
	qiang.zhang1211@...il.com, ojeda@...nel.org, alex.gaynor@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	dakr@...nel.org, rcu@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev,
	jserv@...s.ncku.edu.tw
Subject: Re: [PATCH v2] rust: sync: rcu: Mark Guard methods as inline

On Wed, Mar 19, 2025 at 10:26:40AM +0800, I Hsin Cheng wrote:
> Currently the implementation of "Guard" methods are basically wrappers
> around rcu's function within kernel. Building the kernel with llvm
> 18.1.8 on x86_64 machine will generate the following symbols:
> 
> $ nm vmlinux | grep ' _R'.*Guard | rustfilt
> ffffffff817b6c90 T <kernel::sync::rcu::Guard>::new
> ffffffff817b6cb0 T <kernel::sync::rcu::Guard>::unlock
> ffffffff817b6cd0 T <kernel::sync::rcu::Guard as core::ops::drop::Drop>::drop
> ffffffff817b6c90 T <kernel::sync::rcu::Guard as core::default::Default>::default
> 
> These Rust symbols are basically wrappers around functions
> "rcu_read_lock" and "rcu_read_unlock". Marking them as inline can
> reduce the generation of these symbols, and saves the size of code
> generation for 132 bytes.
> 
> $ ./scripts/bloat-o-meter vmlinux_old vmlinux_new

[..]
I am queuing this version for 6.16, please in the future add all reviewed-by
and acked-by tags on resubmissions. This time I added it.

Also I fixed up the demanging to make the change log look less ugly, let me
know if it doesn't look Ok.

------------8<-------------

From: I Hsin Cheng <richard120310@...il.com>
Subject: [PATCH v2] rust: sync: rcu: Mark Guard methods as inline

Currently the implementation of "Guard" methods are basically wrappers
around rcu's function within kernel. Building the kernel with llvm
18.1.8 on x86_64 machine will generate the following symbols:

$ nm vmlinux | grep ' _R'.*Guard | rustfilt
ffffffff817b6c90 T <kernel::sync::rcu::Guard>::new
ffffffff817b6cb0 T <kernel::sync::rcu::Guard>::unlock
ffffffff817b6cd0 T <kernel::sync::rcu::Guard as core::ops::drop::Drop>::drop
ffffffff817b6c90 T <kernel::sync::rcu::Guard as core::default::Default>::default

These Rust symbols are basically wrappers around functions
"rcu_read_lock" and "rcu_read_unlock". Marking them as inline can
reduce the generation of these symbols, and saves the size of code
generation for 132 bytes.

$ ./scripts/bloat-o-meter vmlinux_old vmlinux_new
(Output is demangled for readability)

add/remove: 0/10 grow/shrink: 0/1 up/down: 0/-132 (-132)
Function                                     old     new   delta
rust_driver_pci::SampleDriver::probe        1041    1034      -7
kernel::sync::rcu::Guard::default             9       -      -9
kernel::sync::rcu::Guard::drop                9       -      -9
kernel::sync::rcu::read_lock                  9       -      -9
kernel::sync::rcu::Guard::unlock              9       -      -9
kernel::sync::rcu::Guard::new                 9       -      -9
__pfx__kernel::sync::rcu::Guard::default     16       -     -16
__pfx__kernel::sync::rcu::Guard::drop        16       -     -16
__pfx__kernel::sync::rcu::read_lock          16       -     -16
__pfx__kernel::sync::rcu::Guard::unlock      16       -     -16
__pfx__kernel::sync::rcu::Guard::new         16       -     -16
Total: Before=23365955, After=23365823, chg -0.00%

Link: https://github.com/Rust-for-Linux/linux/issues/1145
Signed-off-by: I Hsin Cheng <richard120310@...il.com>
Reviewed-by: Joel Fernandes <joelagnelf@...dia.com>
Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
Reviewed-by: Charalampos Mitrodimas <charmitro@...teo.net>
Acked-by: Miguel Ojeda <ojeda@...nel.org>
Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
---
 rust/kernel/sync/rcu.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
index b51d9150ffe2..a32bef6e490b 100644
--- a/rust/kernel/sync/rcu.rs
+++ b/rust/kernel/sync/rcu.rs
@@ -17,6 +17,7 @@
 
 impl Guard {
     /// Acquires the RCU read side lock and returns a guard.
+    #[inline]
     pub fn new() -> Self {
         // SAFETY: An FFI call with no additional requirements.
         unsafe { bindings::rcu_read_lock() };
@@ -25,16 +26,19 @@ pub fn new() -> Self {
     }
 
     /// Explicitly releases the RCU read side lock.
+    #[inline]
     pub fn unlock(self) {}
 }
 
 impl Default for Guard {
+    #[inline]
     fn default() -> Self {
         Self::new()
     }
 }
 
 impl Drop for Guard {
+    #[inline]
     fn drop(&mut self) {
         // SAFETY: By the type invariants, the RCU read side is locked, so it is ok to unlock it.
         unsafe { bindings::rcu_read_unlock() };
@@ -42,6 +46,7 @@ fn drop(&mut self) {
 }
 
 /// Acquires the RCU read side lock.
+#[inline]
 pub fn read_lock() -> Guard {
     Guard::new()
 }
-- 
2.43.0