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.
rustc has a parallel front end, enabled with -Zthreads, available since
rust 1.84 [0] and therefore on every rustc the kernel supports.
It is not yet stable and its output is not reproducible, so it cannot be
used by default - stabilisation is being worked on [1].
Add optional build parameter KBUILD_RUST_THREADS, which when set passes
that many threads to rustc.
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 with KBUILD_RUST_THREADS=8 (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, KBUILD_RUST_THREADS=8, 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%)
Link: https://github.com/rust-lang/rust/pull/132282 [0]
Link: https://github.com/rust-lang/compiler-team/issues/1005 [1]
Assisted-by: LLM
Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
---
Documentation/kbuild/kbuild.rst | 7 +++++++
Makefile | 4 ++++
2 files changed, 11 insertions(+)
diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index 5a9013bacfb7..9b2b127676c3 100644
--- a/Documentation/kbuild/kbuild.rst
+++ b/Documentation/kbuild/kbuild.rst
@@ -134,6 +134,13 @@ See make help for the full list.
Setting "V=..." takes precedence over KBUILD_VERBOSE.
+KBUILD_RUST_THREADS
+-------------------
+The number of threads rustc's parallel front end may use, passed to it as
+``-Zthreads``. Unset by default, as the parallel front end is not yet
+stable and its output is not reproducible. The gain levels off at 8
+threads.
+
KBUILD_EXTMOD
-------------
Set the directory to look for the kernel source when building external
diff --git a/Makefile b/Makefile
index 8263fc8a86b3..4ef504c1ace2 100644
--- a/Makefile
+++ b/Makefile
@@ -1206,6 +1206,10 @@ KBUILD_RUSTFLAGS += --remap-path-prefix=$(srcroot)/=
--remap-path-scope=macro
endif
endif
+ifneq ($(KBUILD_RUST_THREADS),)
+KBUILD_RUSTFLAGS += -Zthreads=$(KBUILD_RUST_THREADS)
+endif
+
# include additional Makefiles when needed
include-y := scripts/Makefile.warn
include-$(CONFIG_DEBUG_INFO) += scripts/Makefile.debug
--
2.55.0
