Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Peter Zijlstra]

On Wed, Oct 02, 2024 at 04:34:00PM -0700, Rong Xu wrote:
> +Preparation
> +===========
> +
> +Configure the kernel with:
> +
> +   .. code-block:: make
> +
> +      CONFIG_AUTOFDO_CLANG=y
> +
> +Customization
> +=============
> +
> +You can enable or disable AutoFDO build for individual file and directories by
> +adding a line similar to the following to the respective kernel Makefile:
> +
> +- For enabling a single file (e.g. foo.o)
> +
> +     .. code-block:: make
> +
> +        AUTOFDO_PROFILE_foo.o := y
> +
> +- For enabling all files in one directory
> +
> +     .. code-block:: make
> +
> +        AUTOFDO_PROFILE := y
> +
> +- For disabling one file
> +
> +     .. code-block:: make
> +
> +        AUTOFDO_PROFILE_foo.o := n
> +
> +- For disabling all files in one directory
> +
> +     .. code-block:: make
> +
> +        AUTOFDO_PROFILE := n
> +
> +
> +Workflow
> +========
> +
> +Here is an example workflow for AutoFDO kernel:
> +
> +
> +
> +1)  Build the kernel on the HOST machine with LLVM enabled, for example,
> +
> +      .. code-block:: make
> +
> +         $ make menuconfig LLVM=1
> +
> +
> +    Turn on AutoFDO build config:
> +
> +      .. code-block:: make
> +
> +         CONFIG_AUTOFDO_CLANG=y
> +
> +    With a configuration that with LLVM enabled, use the following command:
> +
> +      .. code-block:: sh
> +
> +         $ scripts/config -e AUTOFDO_CLANG
> +
> +    After getting the config, build with
> +
> +      .. code-block:: make
> +
> +         $ 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:
> +
> +      .. code-block:: sh
> +
> +         $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c <count> -o <perf_file> -- <loadtest>
> +
> +   - For AMD platforms: For Intel platforms:
> +     The supported systems are: Zen3 with BRS, or Zen4 with amd_lbr_v2. To check,
> +     For Zen3:
> +
> +      .. code-block:: sh
> +
> +         $ cat proc/cpuinfo | grep " brs"
> +
> +      For Zen4:
> +
> +      .. code-block:: sh
> +
> +         $ cat proc/cpuinfo | grep amd_lbr_v2
> +
> +      The following command generated the perf data file:
> +
> +      .. code-block:: sh
> +
> +         $ 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.
> +
> +      .. code-block:: sh
> +
> +         $ llvm-profgen --kernel --binary=<vmlinux> --perfdata=<perf_file> -o <profile_file>
> +
> +   or
> +      .. code-block:: sh
> +
> +         $ create_llvm_prof --binary=<vmlinux> --profile=<perf_file> --format=extbinary -o <profile_file>
> +
> +   Note that multiple AutoFDO profile files can be merged into one via:
> +
> +      .. code-block:: sh
> +
> +         $ 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):
> +
> +      .. code-block:: sh
> +
> +         $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file
> +


Can this be done without the endless ... code-block nonsense?