kbuild: Add Propeller configuration for kernel build

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>
Signed-off-by: Rong Xu <xur@google.com>
Suggested-by: Sriraman Tallam <tmsriram@google.com>
Suggested-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Suggested-by: Stephane Eranian <eranian@google.com>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

2 files changed, 163 insertions, 0 deletions

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 6945644f7008..3c0ac08b2709 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -35,6 +35,7 @@ Documentation/dev-tools/testing-overview.rst
    checkuapi
    gpio-sloppy-logic-analyzer
    autofdo
+   propeller
 
 
 .. only::  subproject and html
diff --git a/Documentation/dev-tools/propeller.rst b/Documentation/dev-tools/propeller.rst
new file mode 100644
index 000000000000..92195958e3db
--- /dev/null
+++ b/Documentation/dev-tools/propeller.rst
@@ -0,0 +1,162 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+Using Propeller with the Linux kernel
+=====================================
+
+This enables Propeller build support for the kernel when using Clang
+compiler. Propeller is a profile-guided optimization (PGO) method used
+to optimize binary executables. Like AutoFDO, it utilizes hardware
+sampling to gather information about the frequency of execution of
+different code paths within a binary. Unlike AutoFDO, this information
+is then used right before linking phase to optimize (among others)
+block layout within and across functions.
+
+A few important notes about adopting Propeller optimization:
+
+#. Although it can be used as a standalone optimization step, it is
+   strongly recommended to apply Propeller on top of AutoFDO,
+   AutoFDO+ThinLTO or Instrument FDO. The rest of this document
+   assumes this paradigm.
+
+#. Propeller uses another round of profiling on top of
+   AutoFDO/AutoFDO+ThinLTO/iFDO. The whole build process involves
+   "build-afdo - train-afdo - build-propeller - train-propeller -
+   build-optimized".
+
+#. Propeller requires LLVM 19 release or later for Clang/Clang++
+   and the linker(ld.lld).
+
+#. In addition to LLVM toolchain, Propeller requires a profiling
+   conversion tool: https://github.com/google/autofdo with a release
+   after v0.30.1: https://github.com/google/autofdo/releases/tag/v0.30.1.
+
+The Propeller optimization process involves the following steps:
+
+#. Initial building: Build the AutoFDO or AutoFDO+ThinLTO binary as
+   you would normally do, but with a set of compile-time / link-time
+   flags, so that a special metadata section is created within the
+   kernel binary. The special section is only intend to be used by the
+   profiling tool, it is not part of the runtime image, nor does it
+   change kernel run time text sections.
+
+#. Profiling: The above kernel is then run with a representative
+   workload to gather execution frequency data. This data is collected
+   using hardware sampling, via perf. Propeller is most effective on
+   platforms supporting advanced PMU features like LBR on Intel
+   machines. This step is the same as profiling the kernel for AutoFDO
+   (the exact perf parameters can be different).
+
+#. Propeller profile generation: Perf output file is converted to a
+   pair of Propeller profiles via an offline tool.
+
+#. Optimized build: Build the AutoFDO or AutoFDO+ThinLTO optimized
+   binary as you would normally do, but with a compile-time /
+   link-time flag to pick up the Propeller compile time and link time
+   profiles. This build step uses 3 profiles - the AutoFDO profile,
+   the Propeller compile-time profile and the Propeller link-time
+   profile.
+
+#. Deployment: The optimized kernel binary is deployed and used
+   in production environments, providing improved performance
+   and reduced latency.
+
+Preparation
+===========
+
+Configure the kernel with::
+
+   CONFIG_AUTOFDO_CLANG=y
+   CONFIG_PROPELLER_CLANG=y
+
+Customization
+=============
+
+The default CONFIG_PROPELLER_CLANG setting covers kernel space objects
+for Propeller builds. One can, however, enable or disable Propeller build
+for individual files and directories by adding a line similar to the
+following to the respective kernel Makefile:
+
+- For enabling a single file (e.g. foo.o)::
+
+   PROPELLER_PROFILE_foo.o := y
+
+- For enabling all files in one directory::
+
+   PROPELLER_PROFILE := y
+
+- For disabling one file::
+
+   PROPELLER_PROFILE_foo.o := n
+
+- For disabling all files in one directory::
+
+   PROPELLER__PROFILE := n
+
+
+Workflow
+========
+
+Here is an example workflow for building an AutoFDO+Propeller kernel:
+
+1) Assuming an AutoFDO profile is already collected following
+   instructions in the AutoFDO document, build the kernel on the host
+   machine, with AutoFDO and Propeller build configs ::
+
+      CONFIG_AUTOFDO_CLANG=y
+      CONFIG_PROPELLER_CLANG=y
+
+   and ::
+
+      $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<autofdo-profile-name>
+
+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::
+
+      $ perf record --pfm-event RETIRED_TAKEN_BRANCH_INSTRUCTIONS:k -a -N -b -c <count> -o <perf_file> -- <loadtest>
+
+   Note you can repeat the above steps to collect multiple <perf_file>s.
+
+4) (Optional) Download the raw perf file(s) to the host machine.
+
+5) Use the create_llvm_prof tool (https://github.com/google/autofdo) to
+   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
+
+   "<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".
+
+   If there are more than 1 perf_file collected in the previous step,
+   you can create a temp list file "<perf_file_list>" with each line
+   containing one perf file name and run::
+
+      $ create_llvm_prof --binary=<vmlinux> --profile=@<perf_file_list>
+                         --format=propeller --propeller_output_module_name
+                         --out=<propeller_profile_prefix>_cc_profile.txt
+                         --propeller_symorder=<propeller_profile_prefix>_ld_profile.txt
+
+6) Rebuild the kernel using the AutoFDO and Propeller
+   profiles. ::
+
+      CONFIG_AUTOFDO_CLANG=y
+      CONFIG_PROPELLER_CLANG=y
+
+   and ::
+
+      $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file> CLANG_PROPELLER_PROFILE_PREFIX=<propeller_profile_prefix>