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

[Rong Xu]

Look good to me.

Reviewed-by: Rong Xu <xur@xxxxxxxxxx>

-Rong

On Fri, Nov 6, 2026 at 11:44 AM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> 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