Re: [PATCH v6 1/7] Add AutoFDO support for Clang build

[Kees Cook <kees@kernel.org>]

On Fri, Oct 25, 2024 at 10:14:03PM -0700, Rong Xu 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>

This looks good. Fairly well isolated.

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook