[PATCH 20/23] kbuild: rust: parallelise rustc front end

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Rust crates are compiled in a serial chain on the critical path - core,
bindings, kernel crates and then the drivers - each a rustc invocation.

With CONFIG_RUST set and every rust driver enabled, the entire chain takes
around 25 seconds on a 128-thread machine, with the associated C code
taking 24 seconds.

Each time a change is made to any of the bindings, it triggers a rebuild.

Use the -Zthreads option to have rustc perform builds in parallel. This has
been available since rust 1.84 (rust-lang/rust#132282), and the kernel
requires rust 1.85 or above.

A future -j/--jobs option is planned for
rustc:(rust-lang/compiler-team#1005), so check to see if this available and
if so use it.

If the user's rustc supports neither, then it falls back gracefully and
neither are used.

The threads are taken from make's jobserver, so a parallel build is not
oversubscribed. It was found that benefits level off at 8 threads (16 was
found to be around the same, and 32 slower).

Generated output was confirmed byte-for-byte identical.

Observed build time changes (using rustc 1.98, clang):

                                        before    after
  core.o                                   7.7s	    4.9s
  bindings.o                               4.3s     3.1s
  kernel.o                                 2.0s     1.4s
  clean build                             39.3s    34.6s
  touch rust/kernel/lib.rs                12.9s    12.1s
  touch rust/bindings/bindings_helper.h   19.5s    17.5s

Whole build, 128-thread Threadripper 9980X, best of N runs:

                                         before   after     delta
                                         -------------------------------
  x86 defconfig+RUST, clean               41.0s    36.4s     -4.6s (-11%)
  x86 defconfig+RUST, touch lib.rs         8.3s     7.7s    -0.56s (-7%)

Assisted-by: LLM
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 Makefile     | 4 ++++
 init/Kconfig | 7 +++++++
 2 files changed, 11 insertions(+)

diff --git a/Makefile b/Makefile
index 997fc7ebe096..7d23d7b64be3 100644
--- a/Makefile
+++ b/Makefile
@@ -1204,6 +1204,10 @@ KBUILD_RUSTFLAGS += --remap-path-prefix=$(srcroot)/= --remap-path-scope=macro
 endif
 endif
 
+ifdef CONFIG_RUST
+KBUILD_RUSTFLAGS += $(if $(CONFIG_RUSTC_HAS_JOBS),-j8,$(if $(CONFIG_RUSTC_HAS_ZTHREADS),-Zthreads=8))
+endif
+
 # include additional Makefiles when needed
 include-y			:= scripts/Makefile.warn
 include-$(CONFIG_DEBUG_INFO)	+= scripts/Makefile.debug
diff --git a/init/Kconfig b/init/Kconfig
index 3c92c87254a3..4096a306e2ab 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -231,6 +231,13 @@ config RUSTC_HAS_FILE_AS_C_STR
 config RUSTC_HAS_SUSPICIOUS_RUNTIME_SYMBOL_DEFINITIONS
 	def_bool RUSTC_VERSION >= 109800
 
+# rustc's parallel front end: -j is the coming spelling, -Zthreads the current one
+config RUSTC_HAS_JOBS
+	def_bool $(rustc-option,-j8)
+
+config RUSTC_HAS_ZTHREADS
+	def_bool $(rustc-option,-Zthreads=8)
+
 config PAHOLE_VERSION
 	int
 	default "$(PAHOLE_VERSION)"

-- 
2.55.0