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

Rong Xu
2024-10-26T05:14:28Z

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>
---
Documentation/dev-tools/index.rst | 1 +
Documentation/dev-tools/propeller.rst | 162 ++++++++++++++++++++++++++
MAINTAINERS | 7 ++
Makefile | 1 +
arch/Kconfig | 19 +++
arch/x86/Kconfig | 1 +
arch/x86/kernel/vmlinux.lds.S | 4 +
include/asm-generic/vmlinux.lds.h | 6 +-
scripts/Makefile.lib | 10 ++
scripts/Makefile.propeller | 28 +++++
tools/objtool/check.c | 1 +
11 files changed, 237 insertions(+), 3 deletions(-)
create mode 100644 Documentation/dev-tools/propeller.rst
create mode 100644 scripts/Makefile.propeller

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 6945644f7008a..3c0ac08b27091 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 0000000000000..92195958e3dbc
--- /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>
diff --git a/MAINTAINERS b/MAINTAINERS
index d6ea49433747a..42e3af0791e15 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18449,6 +18449,13 @@ S: Maintained
F: include/linux/psi*
F: kernel/sched/psi.c

+PROPELLER BUILD
+M: Rong Xu <xur@google.com>
+M: Han Shen <shenhan@google.com>
+S: Supported
+F: Documentation/dev-tools/propeller.rst
+F: scripts/Makefile.propeller
+
PRINTK
M: Petr Mladek <pmladek@suse.com>
R: Steven Rostedt <rostedt@goodmis.org>
diff --git a/Makefile b/Makefile
index f7dee6cee3c29..facf889fe0480 100644
--- a/Makefile
+++ b/Makefile
@@ -1019,6 +1019,7 @@ include-$(CONFIG_UBSAN) += scripts/Makefile.ubsan
include-$(CONFIG_KCOV) += scripts/Makefile.kcov
include-$(CONFIG_RANDSTRUCT) += scripts/Makefile.randstruct
include-$(CONFIG_AUTOFDO_CLANG) += scripts/Makefile.autofdo
+include-$(CONFIG_PROPELLER_CLANG) += scripts/Makefile.propeller
include-$(CONFIG_GCC_PLUGINS) += scripts/Makefile.gcc-plugins

include $(addprefix $(srctree)/, $(include-y))
diff --git a/arch/Kconfig b/arch/Kconfig
index 39b5a705aee32..faceb9b733cfb 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -831,6 +831,25 @@ config AUTOFDO_CLANG

If unsure, say N.

+config ARCH_SUPPORTS_PROPELLER_CLANG
+ bool
+
+config PROPELLER_CLANG
+ bool "Enable Clang's Propeller build"
+ depends on ARCH_SUPPORTS_PROPELLER_CLANG
+ depends on CC_IS_CLANG && CLANG_VERSION >= 190000
+ help
+ This option enables Clang’s Propeller build. When the Propeller
+ profiles is specified in variable CLANG_PROPELLER_PROFILE_PREFIX
+ during the build process, Clang uses the profiles to optimize
+ the kernel.
+
+ If no profile is specified, Propeller options are still passed
+ to Clang to facilitate the collection of perf data for creating
+ the Propeller profiles in subsequent builds.
+
+ If unsure, say N.
+
config ARCH_SUPPORTS_CFI_CLANG
bool
help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 503a0268155ab..da47164bfddc3 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -127,6 +127,7 @@ config X86
select ARCH_SUPPORTS_LTO_CLANG_THIN
select ARCH_SUPPORTS_RT
select ARCH_SUPPORTS_AUTOFDO_CLANG
+ select ARCH_SUPPORTS_PROPELLER_CLANG if X86_64
select ARCH_USE_BUILTIN_BSWAP
select ARCH_USE_CMPXCHG_LOCKREF if X86_CMPXCHG64
select ARCH_USE_MEMTEST
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index b8c5741d2fb48..cf22081601ed6 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -443,6 +443,10 @@ SECTIONS

STABS_DEBUG
DWARF_DEBUG
+#ifdef CONFIG_PROPELLER_CLANG
+ .llvm_bb_addr_map : { *(.llvm_bb_addr_map) }
+#endif
+
ELF_DETAILS

DISCARDS
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 8a0bb3946cf05..c995474e4c649 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -95,14 +95,14 @@
* With LTO_CLANG, the linker also splits sections by default, so we need
* these macros to combine the sections during the final link.
*
- * With AUTOFDO_CLANG, by default, the linker splits text sections and
- * regroups functions into subsections.
+ * With AUTOFDO_CLANG and PROPELLER_CLANG, by default, the linker splits
+ * text sections and regroups functions into subsections.
*
* RODATA_MAIN is not used because existing code already defines .rodata.x
* sections to be brought in with rodata.
*/
#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) || \
-defined(CONFIG_AUTOFDO_CLANG)
+defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
#else
#define TEXT_MAIN .text
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 2d0942c1a0277..e7859ad90224a 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -201,6 +201,16 @@ _c_flags += $(if $(patsubst n%,, \
$(CFLAGS_AUTOFDO_CLANG))
endif

+#
+# Enable Propeller build flags except some files or directories we don't want to
+# enable (depends on variables AUTOFDO_PROPELLER_obj.o and PROPELLER_PROFILE).
+#
+ifdef CONFIG_PROPELLER_CLANG
+_c_flags += $(if $(patsubst n%,, \
+ $(AUTOFDO_PROFILE_$(target-stem).o)$(AUTOFDO_PROFILE)$(PROPELLER_PROFILE))$(is-kernel-object), \
+ $(CFLAGS_PROPELLER_CLANG))
+endif
+
# $(src) for including checkin headers from generated source files
# $(obj) for including generated headers from checkin source files
ifeq ($(KBUILD_EXTMOD),)
diff --git a/scripts/Makefile.propeller b/scripts/Makefile.propeller
new file mode 100644
index 0000000000000..344190717e471
--- /dev/null
+++ b/scripts/Makefile.propeller
@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: GPL-2.0
+
+# Enable available and selected Clang Propeller features.
+ifdef CLANG_PROPELLER_PROFILE_PREFIX
+ CFLAGS_PROPELLER_CLANG := -fbasic-block-sections=list=$(CLANG_PROPELLER_PROFILE_PREFIX)_cc_profile.txt -ffunction-sections
+ KBUILD_LDFLAGS += --symbol-ordering-file=$(CLANG_PROPELLER_PROFILE_PREFIX)_ld_profile.txt --no-warn-symbol-ordering
+else
+ CFLAGS_PROPELLER_CLANG := -fbasic-block-sections=labels
+endif
+
+# Propeller requires debug information to embed module names in the profiles.
+# If CONFIG_DEBUG_INFO is not enabled, set -gmlt option. Skip this for AutoFDO,
+# as the option should already be set.
+ifndef CONFIG_DEBUG_INFO
+ ifndef CONFIG_AUTOFDO_CLANG
+ CFLAGS_PROPELLER_CLANG += -gmlt
+ endif
+endif
+
+ifdef CONFIG_LTO_CLANG_THIN
+ ifdef CLANG_PROPELLER_PROFILE_PREFIX
+ KBUILD_LDFLAGS += --lto-basic-block-sections=$(CLANG_PROPELLER_PROFILE_PREFIX)_cc_profile.txt
+ else
+ KBUILD_LDFLAGS += --lto-basic-block-sections=labels
+ endif
+endif
+
+export CFLAGS_PROPELLER_CLANG
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 4c5229991e1e0..05a0fb4a3d1a0 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4558,6 +4558,7 @@ static int validate_ibt(struct objtool_file *file)
!strcmp(sec->name, "__mcount_loc") ||
!strcmp(sec->name, ".kcfi_traps") ||
!strcmp(sec->name, ".llvm.call-graph-profile") ||
+ !strcmp(sec->name, ".llvm_bb_addr_map") ||
strstr(sec->name, "__patchable_function_entries"))
continue;

--
2.47.0.163.g1226f6d8fa-goog