[PATCH v2] kbuild: AutoFDO: Disable Profi (-fno-sample-profile-use-profi)

[Douglas Anderson]

Starting in Clang 21 (LLVM commit 2fc4a4a9d3a8 ("[Driver][SamplePGO]
Enable -fsample-profile-use-profi (#146795)") [1]), the Clang driver
automatically passes -mllvm -sample-profile-use-profi whenever
-fprofile-sample-use is specified, enabling LLVM's flow-based profile
inference algorithm (Profi) by default.

However, as Rong Xu noted [2], the kernel's AutoFDO implementation uses
flow-sensitive discriminators (FS-AFDO), and Profi has known issues with
FS-AFDO that degrade runtime performance compared to LLVM's original
profile propagation algorithm. In addition, Profi's min-cost max-flow
solver (applyFlowInference) can hit pathological compile times (30+
minutes or hangs) on certain kernel control-flow graphs when profiles
drift slightly or when combined with GCOV [3].

Pass -fno-sample-profile-use-profi to disable Profi and restore the
original profile propagation behavior that kernel AutoFDO was tested
with. Guard the flag with clang-min-version 210000 since it was
introduced in Clang 21 alongside the default change, whereas
CONFIG_AUTOFDO_CLANG supports Clang 17+. Note that cc-option cannot be
used to probe for the flag because cc-option invokes Clang with -Werror
but without -fprofile-sample-use, causing Clang to fail the probe with
-Werror,-Wunused-command-line-argument.

Although SampleProfileUseProfi currently defaults to false in the LLVM
backend and is only enabled by the Clang driver, explicitly pass
-Cllvm-args=-sample-profile-use-profi=false in RUSTFLAGS_AUTOFDO_CLANG
and --mllvm=-sample-profile-use-profi=false in KBUILD_LDFLAGS as well so
that C, Rust, and ThinLTO flags remain consistent even if rustc or LLVM
defaults change in the future. Unlike the Clang driver flag
-fno-sample-profile-use-profi (added in Clang 21), the underlying LLVM
-sample-profile-use-profi option was introduced in LLVM 14, which
predates the LLVM 17+ requirement of CONFIG_AUTOFDO_CLANG, so no version
checks are needed for the Rust or linker flags.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 315ad8780a12 ("kbuild: Add AutoFDO support for Clang build")
Suggested-by: Rong Xu <xur@xxxxxxxxxx>
Reviewed-by: Rong Xu <xur@xxxxxxxxxx>
Reviewed-by: Snehasish Kumar <snehasishk@xxxxxxxxxx>
Link: https://github.com/llvm/llvm-project/commit/2fc4a4a9d3a87a9e11c48ef25522aa17c2a71003 [1]
Link: https://lore.kernel.org/r/CAF1bQ=S52KWHopqxksctbBirFEr5Mz1dKKMAoWh-dqLHii3Nig@xxxxxxxxxxxxxx [2]
Link: https://github.com/ClangBuiltLinux/linux/issues/2188 [3]
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---
v1: https://lore.kernel.org/r/20261105151159.1.I0b27923381ef34cc99cdf8b65fea2e1bafce6370@changeid

Changes in v2:
- Explicitly disable Profi in RUSTFLAGS_AUTOFDO_CLANG and KBUILD_LDFLAGS
  as well (Rong Xu, Nick Desaulniers)

 scripts/Makefile.autofdo | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
index 57a3aba6ffc2..55fda66ad960 100644
--- a/scripts/Makefile.autofdo
+++ b/scripts/Makefile.autofdo
@@ -13,13 +13,16 @@ endif
 ifdef CLANG_AUTOFDO_PROFILE
   CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
   CFLAGS_AUTOFDO_CLANG += -fsplit-machine-functions
+  CFLAGS_AUTOFDO_CLANG += $(if $(call clang-min-version,210000),-fno-sample-profile-use-profi)
   RUSTFLAGS_AUTOFDO_CLANG += -Zprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -Zfunction-sections=y
   RUSTFLAGS_AUTOFDO_CLANG += -Cllvm-args=-split-machine-functions
+  RUSTFLAGS_AUTOFDO_CLANG += -Cllvm-args=-sample-profile-use-profi=false
 endif
 
 ifdef CONFIG_LTO_CLANG_THIN
   ifdef CLANG_AUTOFDO_PROFILE
     KBUILD_LDFLAGS += --lto-sample-profile=$(CLANG_AUTOFDO_PROFILE)
+    KBUILD_LDFLAGS += --mllvm=-sample-profile-use-profi=false
   endif
   KBUILD_LDFLAGS += --mllvm=-enable-fs-discriminator=true --mllvm=-improved-fs-discriminator=true -plugin-opt=thinlto
   KBUILD_LDFLAGS += $(call ld-option,-plugin-opt=-function-splitting=all,-plugin-opt=-split-machine-functions)
-- 
2.56.0.455.g6c5a503b38-goog