Re: [PATCH v6 7/7] Add Propeller configuration for kernel build

Kees Cook
2024-10-29T00:15:02Z

On Fri, Oct 25, 2024 at 10:14:09PM -0700, Rong Xu wrote:
> Add the build support for using Clang's Propeller optimizer. Like
> AutoFDO, Propeller 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.
>
> The support requires a Clang compiler LLVM 19 or later, and the
> create_llvm_prof tool
> (https://github.com/google/autofdo/releases/tag/v0.30.1). This
> commit is limited to x86 platforms that support PMU features
> like LBR on Intel machines and AMD Zen3 BRS.
>
> Here is an example workflow for building an AutoFDO+Propeller
> optimized kernel:
>
> 1) Build the kernel on the host machine, with AutoFDO and Propeller
> build config
> CONFIG_AUTOFDO_CLANG=y
> CONFIG_PROPELLER_CLANG=y
> then
> $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<autofdo_profile>
>
> “<autofdo_profile>” is the profile collected when doing a non-Propeller
> AutoFDO build. This step builds a kernel that has the same optimization
> level as AutoFDO, plus a metadata section that records basic block
> information. This kernel image runs as fast as an AutoFDO optimized
> kernel.
>
> 2) Install the kernel on test/production machines.
>
> 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
> # To see if Zen3 support LBR:
> $ cat proc/cpuinfo | grep " brs"
> # To see if Zen4 support LBR:
> $ cat proc/cpuinfo | grep amd_lbr_v2
> # If the result is yes, then collect the profile using:
> $ 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) Generate Propeller profile:
> $ create_llvm_prof --binary=<vmlinux> --profile=<perf_file> \
> --format=propeller --propeller_output_module_name \
> --out=<propeller_profile_prefix>_cc_profile.txt \
> --propeller_symorder=<propeller_profile_prefix>_ld_profile.txt
>
> “create_llvm_prof” is the profile conversion tool, and a prebuilt
> binary for linux can be found on
> https://github.com/google/autofdo/releases/tag/v0.30.1 (can also build
> from source).
>
> "<propeller_profile_prefix>" can be something like
> "/home/user/dir/any_string".
>
> This command generates a pair of Propeller profiles:
> "<propeller_profile_prefix>_cc_profile.txt" and
> "<propeller_profile_prefix>_ld_profile.txt".
>
> 6) Rebuild the kernel using the AutoFDO and Propeller profile files.
> CONFIG_AUTOFDO_CLANG=y
> CONFIG_PROPELLER_CLANG=y
> and
> $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<autofdo_profile> \
> CLANG_PROPELLER_PROFILE_PREFIX=<propeller_profile_prefix>
>
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

Looks good. Similarly isolated like FDO. :)

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

--
Kees Cook