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

[Kees Cook <kees@kernel.org>]

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