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kbuild: Add AutoFDO support for Clang build

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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>
Tested-by: Yabin Cui <yabinc@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Tested-by: Peter Jung <ptr1337@cachyos.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
This commit is contained in:
Rong Xu 2024-11-02 10:51:08 -07:00 committed by Masahiro Yamada
parent 397a479b51
commit 315ad8780a
9 changed files with 231 additions and 0 deletions
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168
Documentation/dev-tools/autofdo.rst Normal file
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@ -0,0 +1,168 @@
.. 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>

1
Documentation/dev-tools/index.rst
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@ -34,6 +34,7 @@ Documentation/dev-tools/testing-overview.rst
ktap
checkuapi
gpio-sloppy-logic-analyzer
autofdo
.. only:: subproject and html

7
MAINTAINERS
View File

@ -3665,6 +3665,13 @@ 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>

1
Makefile
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@ -1023,6 +1023,7 @@ 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))

20
arch/Kconfig
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@ -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

1
arch/x86/Kconfig
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@ -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

22
scripts/Makefile.autofdo Normal file
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@ -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

10
scripts/Makefile.lib
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@ -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),)

1
tools/objtool/check.c
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@ -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;