| Message ID | 20241023224409.201771-2-xur@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Add AutoFDO and Propeller support for Clang build | expand |
On Thu, Oct 24, 2024 at 7:44 AM Rong Xu <xur@google.com> 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@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> > +Workflow > +======== > + > +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 with 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:: I am not sure if this double-colon is needed when the next line is not code. > + The supported systems are: Zen3 with BRS, or Zen4 with amd_lbr_v2. To check, > + For Zen3:: > + > + $ cat proc/cpuinfo | grep " brs" > + > + For Zen4:: > + > + $ cat proc/cpuinfo | grep amd_lbr_v2 > + > + The following command generated the perf data file:: > + > + $ 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 of Clang > + or later, or just 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_2> ... <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> > + Trailing blank line. .git/rebase-apply/patch:187: new blank line at EOF. -- Best Regards Masahiro Yamada
On Fri, Nov 1, 2024 at 11:02 AM Masahiro Yamada <masahiroy@kernel.org> wrote: > > On Thu, Oct 24, 2024 at 7:44 AM Rong Xu <xur@google.com> 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@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> > > > > > > +Workflow > > +======== > > + > > +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 with 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:: > > I am not sure if this double-colon is needed > when the next line is not code. Thanks for catching this. We don't mean to use "::" here. It should be ":" and there is supposed to be a blank line after this. Also a blank line before "For Zen3::". I will fix this in the patch. > > > > > + The supported systems are: Zen3 with BRS, or Zen4 with amd_lbr_v2. To check, > > + For Zen3:: > > + > > + $ cat proc/cpuinfo | grep " brs" > > + > > + For Zen4:: > > + > > + $ cat proc/cpuinfo | grep amd_lbr_v2 > > + > > + The following command generated the perf data file:: > > + > > + $ 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 of Clang > > + or later, or just 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_2> ... <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> > > + > > Trailing blank line. > > .git/rebase-apply/patch:187: new blank line at EOF. Will remote the blank line. > > > > > > -- > Best Regards > Masahiro Yamada
diff --git a/Documentation/dev-tools/autofdo.rst b/Documentation/dev-tools/autofdo.rst new file mode 100644 index 000000000000..9d90e6d79781 --- /dev/null +++ b/Documentation/dev-tools/autofdo.rst @@ -0,0 +1,167 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=================================== +Using AutoFDO with the Linux kernel +=================================== + +This enables AutoFDO build support for the kernel when using +the Clang compiler. AutoFDO (Auto-Feedback-Directed Optimization) +is a type of profile-guided optimization (PGO) used to enhance the +performance of binary executables. It gathers information about the +frequency of execution of various code paths within a binary using +hardware sampling. This data is then used to guide the compiler's +optimization decisions, resulting in a more efficient binary. AutoFDO +is a powerful optimization technique, and data indicates that it can +significantly improve kernel performance. It's especially beneficial +for workloads affected by front-end stalls. + +For AutoFDO builds, unlike non-FDO builds, the user must supply a +profile. Acquiring an AutoFDO profile can be done in several ways. +AutoFDO profiles are created by converting hardware sampling using +the "perf" tool. It is crucial that the workload used to create these +perf files is representative; they must exhibit runtime +characteristics similar to the workloads that are intended to be +optimized. Failure to do so will result in the compiler optimizing +for the wrong objective. + +The AutoFDO profile often encapsulates the program's behavior. If the +performance-critical codes are architecture-independent, the profile +can be applied across platforms to achieve performance gains. For +instance, using the profile generated on Intel architecture to build +a kernel for AMD architecture can also yield performance improvements. + +There are two methods for acquiring a representative profile: +(1) Sample real workloads using a production environment. +(2) Generate the profile using a representative load test. +When enabling the AutoFDO build configuration without providing an +AutoFDO profile, the compiler only modifies the dwarf information in +the kernel without impacting runtime performance. It's advisable to +use a kernel binary built with the same AutoFDO configuration to +collect the perf profile. While it's possible to use a kernel built +with different options, it may result in inferior performance. + +One can collect profiles using AutoFDO build for the previous kernel. +AutoFDO employs relative line numbers to match the profiles, offering +some tolerance for source changes. This mode is commonly used in a +production environment for profile collection. + +In a profile collection based on a load test, the AutoFDO collection +process consists of the following steps: + +#. Initial build: The kernel is built with AutoFDO options + without a profile. + +#. 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. AutoFDO is most + effective on platforms supporting advanced PMU features like + LBR on Intel machines. + +#. AutoFDO profile generation: Perf output file is converted to + the AutoFDO profile via offline tools. + +The support requires a Clang compiler LLVM 17 or later. + +Preparation +=========== + +Configure the kernel with:: + + CONFIG_AUTOFDO_CLANG=y + +Customization +============= + +The default CONFIG_AUTOFDO_CLANG setting covers kernel space objects for +AutoFDO builds. One can, however, enable or disable AutoFDO 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) :: + + AUTOFDO_PROFILE_foo.o := y + +- For enabling all files in one directory :: + + AUTOFDO_PROFILE := y + +- For disabling one file :: + + AUTOFDO_PROFILE_foo.o := n + +- For disabling all files in one directory :: + + AUTOFDO_PROFILE := n + +Workflow +======== + +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 with 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 systems are: Zen3 with BRS, or Zen4 with amd_lbr_v2. To check, + For Zen3:: + + $ cat proc/cpuinfo | grep " brs" + + For Zen4:: + + $ cat proc/cpuinfo | grep amd_lbr_v2 + + The following command generated the perf data file:: + + $ 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 of Clang + or later, or just 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_2> ... <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> + diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst index 53d4d124f9c5..6945644f7008 100644 --- a/Documentation/dev-tools/index.rst +++ b/Documentation/dev-tools/index.rst @@ -34,6 +34,7 @@ Documentation/dev-tools/testing-overview.rst ktap checkuapi gpio-sloppy-logic-analyzer + autofdo .. only:: subproject and html diff --git a/MAINTAINERS b/MAINTAINERS index d01256208c9f..1b8db863031f 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -3678,6 +3678,13 @@ F: kernel/audit* F: lib/*audit.c K: \baudit_[a-z_0-9]\+\b +AUTOFDO BUILD +M: Rong Xu <xur@google.com> +M: Han Shen <shenhan@google.com> +S: Supported +F: Documentation/dev-tools/autofdo.rst +F: scripts/Makefile.autofdo + AUXILIARY BUS DRIVER M: Greg Kroah-Hartman <gregkh@linuxfoundation.org> R: Dave Ertman <david.m.ertman@intel.com> diff --git a/Makefile b/Makefile index c5493c0c0ca1..bbb6ec68f5dc 100644 --- a/Makefile +++ b/Makefile @@ -1018,6 +1018,7 @@ include-$(CONFIG_KMSAN) += scripts/Makefile.kmsan include-$(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_GCC_PLUGINS) += scripts/Makefile.gcc-plugins include $(addprefix $(srctree)/, $(include-y)) diff --git a/arch/Kconfig b/arch/Kconfig index 8af374ea1adc..5e9604960cbb 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -811,6 +811,26 @@ config LTO_CLANG_THIN If unsure, say Y. endchoice +config ARCH_SUPPORTS_AUTOFDO_CLANG + bool + +config AUTOFDO_CLANG + bool "Enable Clang's AutoFDO build (EXPERIMENTAL)" + depends on ARCH_SUPPORTS_AUTOFDO_CLANG + depends on CC_IS_CLANG && CLANG_VERSION >= 170000 + help + This option enables Clang’s AutoFDO build. When + an AutoFDO profile is specified in variable + CLANG_AUTOFDO_PROFILE during the build process, + Clang uses the profile to optimize the kernel. + + If no profile is specified, AutoFDO options are + still passed to Clang to facilitate the collection + of perf data for creating an AutoFDO profile in + subsequent builds. + + If unsure, say N. + config ARCH_SUPPORTS_CFI_CLANG bool help diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 2852fcd82cbd..503a0268155a 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -126,6 +126,7 @@ config X86 select ARCH_SUPPORTS_LTO_CLANG select ARCH_SUPPORTS_LTO_CLANG_THIN select ARCH_SUPPORTS_RT + select ARCH_SUPPORTS_AUTOFDO_CLANG select ARCH_USE_BUILTIN_BSWAP select ARCH_USE_CMPXCHG_LOCKREF if X86_CMPXCHG64 select ARCH_USE_MEMTEST diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo new file mode 100644 index 000000000000..ff96a63fea7c --- /dev/null +++ b/scripts/Makefile.autofdo @@ -0,0 +1,22 @@ +# SPDX-License-Identifier: GPL-2.0 + +# Enable available and selected Clang AutoFDO features. + +CFLAGS_AUTOFDO_CLANG := -fdebug-info-for-profiling -mllvm -enable-fs-discriminator=true -mllvm -improved-fs-discriminator=true + +ifndef CONFIG_DEBUG_INFO + CFLAGS_AUTOFDO_CLANG += -gmlt +endif + +ifdef CLANG_AUTOFDO_PROFILE + CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) +endif + +ifdef CONFIG_LTO_CLANG_THIN + ifdef CLANG_AUTOFDO_PROFILE + KBUILD_LDFLAGS += --lto-sample-profile=$(CLANG_AUTOFDO_PROFILE) + endif + KBUILD_LDFLAGS += --mllvm=-enable-fs-discriminator=true --mllvm=-improved-fs-discriminator=true -plugin-opt=thinlto +endif + +export CFLAGS_AUTOFDO_CLANG diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib index 01a9f567d5af..2d0942c1a027 100644 --- a/scripts/Makefile.lib +++ b/scripts/Makefile.lib @@ -191,6 +191,16 @@ _c_flags += $(if $(patsubst n%,, \ -D__KCSAN_INSTRUMENT_BARRIERS__) endif +# +# Enable AutoFDO build flags except some files or directories we don't want to +# enable (depends on variables AUTOFDO_PROFILE_obj.o and AUTOFDO_PROFILE). +# +ifeq ($(CONFIG_AUTOFDO_CLANG),y) +_c_flags += $(if $(patsubst n%,, \ + $(AUTOFDO_PROFILE_$(target-stem).o)$(AUTOFDO_PROFILE)$(is-kernel-object)), \ + $(CFLAGS_AUTOFDO_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/tools/objtool/check.c b/tools/objtool/check.c index 6604f5d038aa..4c5229991e1e 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -4557,6 +4557,7 @@ static int validate_ibt(struct objtool_file *file) !strcmp(sec->name, "__jump_table") || !strcmp(sec->name, "__mcount_loc") || !strcmp(sec->name, ".kcfi_traps") || + !strcmp(sec->name, ".llvm.call-graph-profile") || strstr(sec->name, "__patchable_function_entries")) continue;
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@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/autofdo.rst | 167 ++++++++++++++++++++++++++++ Documentation/dev-tools/index.rst | 1 + MAINTAINERS | 7 ++ Makefile | 1 + arch/Kconfig | 20 ++++ arch/x86/Kconfig | 1 + scripts/Makefile.autofdo | 22 ++++ scripts/Makefile.lib | 10 ++ tools/objtool/check.c | 1 + 9 files changed, 230 insertions(+) create mode 100644 Documentation/dev-tools/autofdo.rst create mode 100644 scripts/Makefile.autofdo