Re: [PATCH v5 1/7] Add AutoFDO support for Clang build

[Rong Xu <xur@google.com>]

On Fri, Nov 1, 2024 at 11:02 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Thu, Oct 24, 2024 at 7:44 AM Rong Xu <xur@google.com> wrote:
> >
> > Add the build support for using Clang's AutoFDO. Building the kernel
> > with AutoFDO does not reduce the optimization level from the
> > compiler. AutoFDO uses hardware sampling to gather information about
> > the frequency of execution of different code paths within a binary.
> > This information is then used to guide the compiler's optimization
> > decisions, resulting in a more efficient binary. Experiments
> > showed that the kernel can improve up to 10% in latency.
> >
> > The support requires a Clang compiler after LLVM 17. This submission
> > is limited to x86 platforms that support PMU features like LBR on
> > Intel machines and AMD Zen3 BRS. Support for SPE on ARM 1,
> >  and BRBE on ARM 1 is part of planned future work.
> >
> > Here is an example workflow for AutoFDO kernel:
> >
> > 1) Build the kernel on the host machine with LLVM enabled, for example,
> >        $ make menuconfig LLVM=1
> >     Turn on AutoFDO build config:
> >       CONFIG_AUTOFDO_CLANG=y
> >     With a configuration that has LLVM enabled, use the following
> >     command:
> >        scripts/config -e AUTOFDO_CLANG
> >     After getting the config, build with
> >       $ make LLVM=1
> >
> > 2) Install the kernel on the test machine.
> >
> > 3) Run the load tests. The '-c' option in perf specifies the sample
> >    event period. We suggest     using a suitable prime number,
> >    like 500009, for this purpose.
> >    For Intel platforms:
> >       $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c <count> \
> >         -o <perf_file> -- <loadtest>
> >    For AMD platforms:
> >       The supported system are: Zen3 with BRS, or Zen4 with amd_lbr_v2
> >      For Zen3:
> >       $ cat proc/cpuinfo | grep " brs"
> >       For Zen4:
> >       $ cat proc/cpuinfo | grep amd_lbr_v2
> >       $ perf record --pfm-events RETIRED_TAKEN_BRANCH_INSTRUCTIONS:k -a \
> >         -N -b -c <count> -o <perf_file> -- <loadtest>
> >
> > 4) (Optional) Download the raw perf file to the host machine.
> >
> > 5) To generate an AutoFDO profile, two offline tools are available:
> >    create_llvm_prof and llvm_profgen. The create_llvm_prof tool is part
> >    of the AutoFDO project and can be found on GitHub
> >    (https://github.com/google/autofdo), version v0.30.1 or later. The
> >    llvm_profgen tool is included in the LLVM compiler itself. It's
> >    important to note that the version of llvm_profgen doesn't need to
> >    match the version of Clang. It needs to be the LLVM 19 release or
> >    later, or from the LLVM trunk.
> >       $ llvm-profgen --kernel --binary=<vmlinux> --perfdata=<perf_file> \
> >         -o <profile_file>
> >    or
> >       $ create_llvm_prof --binary=<vmlinux> --profile=<perf_file> \
> >         --format=extbinary --out=<profile_file>
> >
> >    Note that multiple AutoFDO profile files can be merged into one via:
> >       $ llvm-profdata merge -o <profile_file>  <profile_1> ... <profile_n>
> >
> > 6) Rebuild the kernel using the AutoFDO profile file with the same config
> >    as step 1, (Note CONFIG_AUTOFDO_CLANG needs to be enabled):
> >       $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file>
> >
> > Co-developed-by: Han Shen <shenhan@google.com>
> > Signed-off-by: Han Shen <shenhan@google.com>
> > Signed-off-by: Rong Xu <xur@google.com>
> > Suggested-by: Sriraman Tallam <tmsriram@google.com>
> > Suggested-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
> > Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Suggested-by: Stephane Eranian <eranian@google.com>
> > Tested-by: Yonghong Song <yonghong.song@linux.dev>
>
>
>
>
> > +Workflow
> > +========
> > +
> > +Here is an example workflow for AutoFDO kernel:
> > +
> > +1)  Build the kernel on the host machine with LLVM enabled,
> > +    for example, ::
> > +
> > +      $ make menuconfig LLVM=1
> > +
> > +    Turn on AutoFDO build config::
> > +
> > +      CONFIG_AUTOFDO_CLANG=y
> > +
> > +    With a configuration that with LLVM enabled, use the following command::
> > +
> > +      $ scripts/config -e AUTOFDO_CLANG
> > +
> > +    After getting the config, build with ::
> > +
> > +      $ make LLVM=1
> > +
> > +2) Install the kernel on the test machine.
> > +
> > +3) Run the load tests. The '-c' option in perf specifies the sample
> > +   event period. We suggest using a suitable prime number, like 500009,
> > +   for this purpose.
> > +
> > +   - For Intel platforms::
> > +
> > +      $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c <count> -o <perf_file> -- <loadtest>
> > +
> > +   - For AMD platforms::
>
> I am not sure if this double-colon is needed
> when the next line is not code.

Thanks for catching this. We don't mean to use "::" here. It should be
":" and there is supposed to be a blank line after this.
Also a blank line before "For Zen3::". I will fix this in the patch.

>
>
>
> > +     The supported systems are: Zen3 with BRS, or Zen4 with amd_lbr_v2. To check,
> > +     For Zen3::
> > +
> > +      $ cat proc/cpuinfo | grep " brs"
> > +
> > +     For Zen4::
> > +
> > +      $ cat proc/cpuinfo | grep amd_lbr_v2
> > +
> > +     The following command generated the perf data file::
> > +
> > +      $ perf record --pfm-events RETIRED_TAKEN_BRANCH_INSTRUCTIONS:k -a -N -b -c <count> -o <perf_file> -- <loadtest>
> > +
> > +4) (Optional) Download the raw perf file to the host machine.
> > +
> > +5) To generate an AutoFDO profile, two offline tools are available:
> > +   create_llvm_prof and llvm_profgen. The create_llvm_prof tool is part
> > +   of the AutoFDO project and can be found on GitHub
> > +   (https://github.com/google/autofdo), version v0.30.1 or later.
> > +   The llvm_profgen tool is included in the LLVM compiler itself. It's
> > +   important to note that the version of llvm_profgen doesn't need to match
> > +   the version of Clang. It needs to be the LLVM 19 release of Clang
> > +   or later, or just from the LLVM trunk. ::
> > +
> > +      $ llvm-profgen --kernel --binary=<vmlinux> --perfdata=<perf_file> -o <profile_file>
> > +
> > +   or ::
> > +
> > +      $ create_llvm_prof --binary=<vmlinux> --profile=<perf_file> --format=extbinary --out=<profile_file>
> > +
> > +   Note that multiple AutoFDO profile files can be merged into one via::
> > +
> > +      $ llvm-profdata merge -o <profile_file> <profile_1> <profile_2> ... <profile_n>
> > +
> > +6) Rebuild the kernel using the AutoFDO profile file with the same config as step 1,
> > +   (Note CONFIG_AUTOFDO_CLANG needs to be enabled)::
> > +
> > +      $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file>
> > +
>
> Trailing blank line.
>
> .git/rebase-apply/patch:187: new blank line at EOF.

Will remote the blank line.

>
>
>
>
>
> --
> Best Regards
> Masahiro Yamada