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@xxxxxxxxxx> > Signed-off-by: Han Shen <shenhan@xxxxxxxxxx> This looks good. Fairly well isolated. Reviewed-by: Kees Cook <kees@xxxxxxxxxx> -- Kees Cook
