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

Kees Cook
2024-10-29T00:08:47Z

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