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

[Nick Desaulniers]

On Fri, Nov 6, 2026 at 11:38 AM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> 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>

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Maybe we can credit a few folks with reported by tags, to encourage to
report issues upstream more in the future?

Reported-by: Han Wang <nesthanwang@xxxxxxxxxx>
Reported-by: Yabin Cui <yabinc@xxxxxxxxxx>

> ---
> 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
>


--
Thanks,
~Nick Desaulniers