[PATCH v6 0/7] Add AutoFDO and Propeller support for Clang build

[PATCH v6 0/7] Add AutoFDO and Propeller support for Clang build

[Rong Xu]

Hi,

This patch series is to integrate AutoFDO and Propeller support into
the Linux kernel. AutoFDO is a profile-guided optimization technique
that leverages hardware sampling to enhance binary performance.
Unlike Instrumentation-based FDO (iFDO), AutoFDO offers a user-friendly
and straightforward application process. While iFDO generally yields
superior profile quality and performance, our findings reveal that
AutoFDO achieves remarkable effectiveness, bringing performance close
to iFDO for benchmark applications.

Propeller is a profile-guided, post-link optimizer that improves
the performance of large-scale applications compiled with LLVM. It
operates by relinking the binary based on an additional round of runtime
profiles, enabling precise optimizations that are not possible at
compile time.  Similar to AutoFDO, Propeller too utilizes hardware
sampling to collect profiles and apply post-link optimizations to improve
the benchmark’s performance over and above AutoFDO.

Our empirical data demonstrates significant performance improvements
with AutoFDO and Propeller, up to 10% on microbenchmarks and up to 5%
on large warehouse-scale benchmarks. This makes a strong case for their
inclusion as supported features in the upstream kernel.

Background

A significant fraction of fleet processing cycles (excluding idle time)
from data center workloads are attributable to the kernel. Ware-house
scale workloads maximize performance by optimizing the production kernel
using iFDO (a.k.a instrumented PGO, Profile Guided Optimization).

iFDO can significantly enhance application performance but its use
within the kernel has raised concerns. AutoFDO is a variant of FDO that
uses the hardware’s Performance Monitoring Unit (PMU) to collect
profiling data. While AutoFDO typically yields smaller performance
gains than iFDO, it presents unique benefits for optimizing kernels.

AutoFDO eliminates the need for instrumented kernels, allowing a single
optimized kernel to serve both execution and profile collection. It also
minimizes slowdown during profile collection, potentially yielding
higher-fidelity profiling, especially for time-sensitive code, compared
to iFDO. Additionally, AutoFDO profiles can be obtained from production
environments via the hardware’s PMU whereas iFDO profiles require
carefully curated load tests that are representative of real-world
traffic.

AutoFDO facilitates profile collection across diverse targets.
Preliminary studies indicate significant variation in kernel hot spots
within Google’s infrastructure, suggesting potential performance gains
through target-specific kernel customization.

Furthermore, other advanced compiler optimization techniques, including
ThinLTO and Propeller can be stacked on top of AutoFDO, similar to iFDO.
ThinLTO achieves better runtime performance through whole-program
analysis and cross module optimizations. The main difference between
traditional LTO and ThinLTO is that the latter is scalable in time and
memory.

This patch series adds AutoFDO and Propeller support to the kernel. The
actual solution comes in six parts:

[P 1] Add the build support for using AutoFDO in Clang

      Add the basic support for AutoFDO build and provide the
      instructions for using AutoFDO.

[P 2] Fix objtool for bogus warnings when -ffunction-sections is enabled

[P 3] Adjust symbol ordering in text output sections

[P 4] Add markers for text_unlikely and text_hot sections

[P 5] Enable –ffunction-sections for the AutoFDO build

[P 6] Enable Machine Function Split (MFS) optimization for AutoFDO

[P 7] Add Propeller configuration to the kernel build

Patch 1 provides basic AutoFDO build support. Patches 2 to 6 further
enhance the performance of AutoFDO builds and are functionally dependent
on Patch 1. Patch 7 enables support for Propeller and is dependent on
patch 2 to patch 4.

Caveats

AutoFDO is compatible with both GCC and Clang, but the patches in this
series are exclusively applicable to LLVM 17 or newer for AutoFDO and
LLVM 19 or newer for Propeller. For profile conversion, two different
tools could be used, llvm_profgen or create_llvm_prof. llvm_profgen
needs to be the LLVM 19 or newer, or just the LLVM trunk. Alternatively,
create_llvm_prof v0.30.1 or newer can be used instead of llvm-profgen.

Additionally, the build is only supported on x86 platforms equipped
with PMU capabilities, such as LBR on Intel machines. More
specifically:
 * Intel platforms: works on every platform that supports LBR;
   we have tested on Skylake.
 * AMD platforms: tested on AMD Zen3 with the BRS feature. The kernel
   needs to be configured with “CONFIG_PERF_EVENTS_AMD_BRS=y", To
   check, use
   $ cat /proc/cpuinfo | grep “ brs”
   For the AMD Zen4, AMD LBRV2 is supported, but we suspect a bug with
   AMD LBRv2 implementation in Genoa which blocks the usage.

For ARM, we plan to send patches for SPE-based Propeller when
AutoFDO for Arm is ready.

Experiments and Results

Experiments were conducted to compare the performance of AutoFDO-optimized
kernel images (version 6.9.x) against default builds.. The evaluation
encompassed both open source microbenchmarks and real-world production
services from Google and Meta. The selected microbenchmarks included Neper,
a network subsystem benchmark, and UnixBench which is a comprehensive suite
for assessing various kernel operations.

For Neper, AutoFDO optimization resulted in a 6.1% increase in throughput
and a 10.6% reduction in latency. UnixBench saw a 2.2% improvement in its
index score under low system load and a 2.6% improvement under high system
load.

For further details on the improvements observed in Google and Meta's
production services, please refer to the LLVM discourse post:
https://discourse.llvm.org/t/optimizing-the-linux-kernel-with-autofdo-including-thinlto-and-propeller/79108

Thanks,

Rong Xu and Han Shen

Change-Logs in V2:
Rebased to commit e32cde8d2bd7 ("Merge tag 'sched_ext-for-6.12-rc1-fixes-1'
of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext")

1. [P 0]: moved the Propeller description to the top (Peter Zijlstra)
2. [P 1]: (1) Makefile: fixed file order (Masahiro Yamada)
          (2) scripts/Makefile.lib: used is-kernel-object to exclude
              files (Masahiro Yamada)
          (3) scripts/Makefile.autofdo: improved the code (Masahiro Yamada)
          (4) scripts/Makefile.autofdo: handled when DEBUG_INFO disabled
	      (Nick Desaulniers)
3. [P 2]: tools/objtool/elf.c: updated the comments (Peter Zijlstra)
4. [P 3]: include/asm-generic/vmlinux.lds.h:
          (1) explicit set cold text function aligned (Peter Zijlstra and
	      Peter Anvin)
          (2) set hot-text page aligned
5. [P 6]: (1) include/asm-generic/vmlinux.lds.h: made Propeller not
              depending on AutoFDO
          (2) Makefile: fixed file order (Masahiro Yamada)
          (3) scripts/Makefile.lib: used is-kernel-object to exclude
              files (Masahiro Yamada). This removed the change in
              arch/x86/platform/efi/Makefile,
              drivers/firmware/efi/libstub/Makefile, and
              arch/x86/boot/compressed/Makefile.
              And this also addressed the comment from Arnd Bergmann
	      regarding arch/x86/purgatory/Makefile
          (4) scripts/Makefile.propeller: improved the code
	      (Masahiro Yamada)

Change-Logs in V3:
Rebased to commit eb952c47d154 ("Merge tag 'for-6.12-rc2-tag' of
git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux")

Integrated the following changes suggested by Mike Rapoport.
1. [P 1]: autofdo.rst: removed code-block directives and used "::"
2. [P 6]: propeller.rst: removed code-block directives and use "::"

Change-Logs in V4:
1. [P 1]: autofdo.rst: fixed a typo for create_llvm_prof command.

Change-Logs in V5:
Added "Tested-by: Yonghong Song <yonghong.song@linux.dev>" to all patches.

Integrated the following changes suggested by Masahiro Yamada.
1. [P 0]: (1) moved ARM related remark from patch 6 to here
2. [P 1]: (1) autofdo.rst: improved the documentation
          (2) scripts/Makefile.autofdo: improved comments and used ifdef
	      instead of ifeq
3. [P 3]: Make the layout change unconditionally
4. [P 4]: Split the patch into two: this patch only added the markers, and
          the AutoFDO change went to new P_5
5. [P 7]: (1) propeller.rst: improved the documentation
          (2) scripts/Makefile.propeller: improved comments and used ifdef
	      instead of ifeq
	  (3) arch/Kconfig: made Propeller build independent of AutoFDO
	      build
	  (4) moved ARM related remarks to the cover letter

Change-Logs in V6:
Added "Tested-by: Yabin Cui <yabinc@google.com>" to AutoFDO patches.

1.  [P 3]: (1) changed patch title
           (2) fixed the build error in sparc64 reported by kernel test
	       robot <lkp@intel.com>

Rong Xu (7):
  Add AutoFDO support for Clang build
  objtool: Fix unreachable instruction warnings for weak functions
  Adjust symbol ordering in text output section
  Add markers for text_unlikely and text_hot sections
  AutoFDO: Enable -ffunction-sections for the AutoFDO build
  AutoFDO: Enable machine function split optimization for AutoFDO
  Add Propeller configuration for kernel build

 Documentation/dev-tools/autofdo.rst   | 167 ++++++++++++++++++++++++++
 Documentation/dev-tools/index.rst     |   2 +
 Documentation/dev-tools/propeller.rst | 162 +++++++++++++++++++++++++
 MAINTAINERS                           |  14 +++
 Makefile                              |   2 +
 arch/Kconfig                          |  39 ++++++
 arch/sparc/kernel/vmlinux.lds.S       |   5 +
 arch/x86/Kconfig                      |   2 +
 arch/x86/kernel/vmlinux.lds.S         |   4 +
 include/asm-generic/vmlinux.lds.h     |  49 ++++++--
 scripts/Makefile.autofdo              |  24 ++++
 scripts/Makefile.lib                  |  20 +++
 scripts/Makefile.propeller            |  28 +++++
 tools/objtool/check.c                 |   2 +
 tools/objtool/elf.c                   |  15 ++-
 15 files changed, 519 insertions(+), 16 deletions(-)
 create mode 100644 Documentation/dev-tools/autofdo.rst
 create mode 100644 Documentation/dev-tools/propeller.rst
 create mode 100644 scripts/Makefile.autofdo
 create mode 100644 scripts/Makefile.propeller


base-commit: 850925a8133c73c4a2453c360b2c3beb3bab67c9
-- 
2.47.0.163.g1226f6d8fa-goog

[PATCH v6 1/7] Add AutoFDO support for Clang build

[Rong Xu]

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>
Change-Id: I9d11fb49e68e5a74dac90a0af485655980331d16
---
 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

diff --git a/Documentation/dev-tools/autofdo.rst b/Documentation/dev-tools/autofdo.rst
new file mode 100644
index 0000000000000..9d90e6d79781c
--- /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 53d4d124f9c52..6945644f7008a 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 a274079502426..d6ea49433747a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -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>
diff --git a/Makefile b/Makefile
index a9a7d9ffaa98d..f7dee6cee3c29 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 00163e4a237c1..39b5a705aee32 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 2852fcd82cbd8..503a0268155ab 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 0000000000000..ff96a63fea7cd
--- /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 01a9f567d5af4..2d0942c1a0277 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 6604f5d038aad..4c5229991e1e0 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;
 
-- 
2.47.0.163.g1226f6d8fa-goog

Re: [PATCH v6 1/7] Add AutoFDO support for Clang build

[Kees Cook]

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@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

This looks good. Fairly well isolated.

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

[PATCH v6 2/7] objtool: Fix unreachable instruction warnings for weak functions

[Rong Xu]

In the presence of both weak and strong function definitions, the
linker drops the weak symbol in favor of a strong symbol, but
leaves the code in place. Code in ignore_unreachable_insn() has
some heuristics to suppress the warning, but it does not work when
-ffunction-sections is enabled.

Suppose function foo has both strong and weak definitions.
Case 1: The strong definition has an annotated section name,
like .init.text. Only the weak definition will be placed into
.text.foo. But since the section has no symbols, there will be no
"hole" in the section.

Case 2: Both sections are without an annotated section name.
Both will be placed into .text.foo section, but there will be only one
symbol (the strong one). If the weak code is before the strong code,
there is no "hole" as it fails to find the right-most symbol before
the offset.

The fix is to use the first node to compute the hole if hole.sym
is empty. If there is no symbol in the section, the first node
will be NULL, in which case, -1 is returned to skip the whole
section.

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>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Yabin Cui <yabinc@google.com>
Change-Id: Ib3a484b6f056db8ba4f8e91e567e3165bbeb51ea
---
 tools/objtool/elf.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 3d27983dc908d..6f64d611faea9 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -224,12 +224,17 @@ int find_symbol_hole_containing(const struct section *sec, unsigned long offset)
 	if (n)
 		return 0; /* not a hole */
 
-	/* didn't find a symbol for which @offset is after it */
-	if (!hole.sym)
-		return 0; /* not a hole */
+	/*
+	 * @offset >= sym->offset + sym->len, find symbol after it.
+	 * When hole.sym is empty, use the first node to compute the hole.
+	 * If there is no symbol in the section, the first node will be NULL,
+	 * in which case, -1 is returned to skip the whole section.
+	 */
+	if (hole.sym)
+		n = rb_next(&hole.sym->node);
+	else
+		n = rb_first_cached(&sec->symbol_tree);
 
-	/* @offset >= sym->offset + sym->len, find symbol after it */
-	n = rb_next(&hole.sym->node);
 	if (!n)
 		return -1; /* until end of address space */
 
-- 
2.47.0.163.g1226f6d8fa-goog

Re: [PATCH v6 2/7] objtool: Fix unreachable instruction warnings for weak functions

[Kees Cook]

On Fri, Oct 25, 2024 at 10:14:04PM -0700, Rong Xu wrote:
> In the presence of both weak and strong function definitions, the
> linker drops the weak symbol in favor of a strong symbol, but
> leaves the code in place. Code in ignore_unreachable_insn() has
> some heuristics to suppress the warning, but it does not work when
> -ffunction-sections is enabled.
> 
> Suppose function foo has both strong and weak definitions.
> Case 1: The strong definition has an annotated section name,
> like .init.text. Only the weak definition will be placed into
> .text.foo. But since the section has no symbols, there will be no
> "hole" in the section.
> 
> Case 2: Both sections are without an annotated section name.
> Both will be placed into .text.foo section, but there will be only one
> symbol (the strong one). If the weak code is before the strong code,
> there is no "hole" as it fails to find the right-most symbol before
> the offset.
> 
> The fix is to use the first node to compute the hole if hole.sym
> is empty. If there is no symbol in the section, the first node
> will be NULL, in which case, -1 is returned to skip the whole
> section.
> 
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

This seems logically correct to me, but I'd love to see review from Josh
and/or Peter Z on this change too.

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

Re: [PATCH v6 2/7] objtool: Fix unreachable instruction warnings for weak functions

[H. Peter Anvin]

On 10/28/24 17:16, Kees Cook wrote:
> On Fri, Oct 25, 2024 at 10:14:04PM -0700, Rong Xu wrote:
>> In the presence of both weak and strong function definitions, the
>> linker drops the weak symbol in favor of a strong symbol, but
>> leaves the code in place. Code in ignore_unreachable_insn() has
>> some heuristics to suppress the warning, but it does not work when
>> -ffunction-sections is enabled.
>>
>> Suppose function foo has both strong and weak definitions.
>> Case 1: The strong definition has an annotated section name,
>> like .init.text. Only the weak definition will be placed into
>> .text.foo. But since the section has no symbols, there will be no
>> "hole" in the section.
>>
>> Case 2: Both sections are without an annotated section name.
>> Both will be placed into .text.foo section, but there will be only one
>> symbol (the strong one). If the weak code is before the strong code,
>> there is no "hole" as it fails to find the right-most symbol before
>> the offset.
>>
>> The fix is to use the first node to compute the hole if hole.sym
>> is empty. If there is no symbol in the section, the first node
>> will be NULL, in which case, -1 is returned to skip the whole
>> section.
>>
>> Co-developed-by: Han Shen <shenhan@google.com>
>> Signed-off-by: Han Shen <shenhan@google.com>
> 
> This seems logically correct to me, but I'd love to see review from Josh
> and/or Peter Z on this change too.
> 
> Reviewed-by: Kees Cook <kees@kernel.org>
> 

Does this happen even with -Wl,--gc-sections?

	-hpa

Re: [PATCH v6 2/7] objtool: Fix unreachable instruction warnings for weak functions

[Rong Xu]

I think the objtool works on individual object files (or vmlinux.o).
The -Wl,--gc-sections flag, on the other hand, is a linker flag that
acts on the final link -- it's applied after objtool invocations.
Therefore, even with -Wl,--gc-sections, we'll still encounter those
spurious warnings from objtool.

-Rong

On Mon, Oct 28, 2024 at 5:19 PM H. Peter Anvin <hpa@zytor.com> wrote:
>
> On 10/28/24 17:16, Kees Cook wrote:
> > On Fri, Oct 25, 2024 at 10:14:04PM -0700, Rong Xu wrote:
> >> In the presence of both weak and strong function definitions, the
> >> linker drops the weak symbol in favor of a strong symbol, but
> >> leaves the code in place. Code in ignore_unreachable_insn() has
> >> some heuristics to suppress the warning, but it does not work when
> >> -ffunction-sections is enabled.
> >>
> >> Suppose function foo has both strong and weak definitions.
> >> Case 1: The strong definition has an annotated section name,
> >> like .init.text. Only the weak definition will be placed into
> >> .text.foo. But since the section has no symbols, there will be no
> >> "hole" in the section.
> >>
> >> Case 2: Both sections are without an annotated section name.
> >> Both will be placed into .text.foo section, but there will be only one
> >> symbol (the strong one). If the weak code is before the strong code,
> >> there is no "hole" as it fails to find the right-most symbol before
> >> the offset.
> >>
> >> The fix is to use the first node to compute the hole if hole.sym
> >> is empty. If there is no symbol in the section, the first node
> >> will be NULL, in which case, -1 is returned to skip the whole
> >> section.
> >>
> >> Co-developed-by: Han Shen <shenhan@google.com>
> >> Signed-off-by: Han Shen <shenhan@google.com>
> >
> > This seems logically correct to me, but I'd love to see review from Josh
> > and/or Peter Z on this change too.
> >
> > Reviewed-by: Kees Cook <kees@kernel.org>
> >
>
> Does this happen even with -Wl,--gc-sections?
>
>         -hpa
>

Re: [PATCH v6 2/7] objtool: Fix unreachable instruction warnings for weak functions

[Josh Poimboeuf]

On Mon, Oct 28, 2024 at 05:16:44PM -0700, Kees Cook wrote:
> On Fri, Oct 25, 2024 at 10:14:04PM -0700, Rong Xu wrote:
> > In the presence of both weak and strong function definitions, the
> > linker drops the weak symbol in favor of a strong symbol, but
> > leaves the code in place. Code in ignore_unreachable_insn() has
> > some heuristics to suppress the warning, but it does not work when
> > -ffunction-sections is enabled.
> > 
> > Suppose function foo has both strong and weak definitions.
> > Case 1: The strong definition has an annotated section name,
> > like .init.text. Only the weak definition will be placed into
> > .text.foo. But since the section has no symbols, there will be no
> > "hole" in the section.
> > 
> > Case 2: Both sections are without an annotated section name.
> > Both will be placed into .text.foo section, but there will be only one
> > symbol (the strong one). If the weak code is before the strong code,
> > there is no "hole" as it fails to find the right-most symbol before
> > the offset.
> > 
> > The fix is to use the first node to compute the hole if hole.sym
> > is empty. If there is no symbol in the section, the first node
> > will be NULL, in which case, -1 is returned to skip the whole
> > section.
> > 
> > Co-developed-by: Han Shen <shenhan@google.com>
> > Signed-off-by: Han Shen <shenhan@google.com>
> 
> This seems logically correct to me, but I'd love to see review from Josh
> and/or Peter Z on this change too.
> 
> Reviewed-by: Kees Cook <kees@kernel.org>

LGTM, thanks!

Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>

-- 
Josh

[PATCH v6 3/7] Adjust symbol ordering in text output section

[Rong Xu]

When the -ffunction-sections compiler option is enabled, each function
is placed in a separate section named .text.function_name rather than
putting all functions in a single .text section.

However, using -function-sections can cause problems with the
linker script. The comments included in include/asm-generic/vmlinux.lds.h
note these issues.:
  “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
   code elimination is enabled, so these sections should be converted
   to use ".." first.”

It is unclear whether there is a straightforward method for converting
a suffix to "..".

This patch modifies the order of subsections within the text output
section. Specifically, it repositions sections with certain fixed patterns
(for example .text.unlikely) before TEXT_MAIN, ensuring that they are
grouped and matched together. It also places .text.hot section at the
beginning of a page to help the TLB performance.

Note that the limitation arises because the linker script employs glob
patterns instead of regular expressions for string matching. While there
is a method to maintain the current order using complex patterns, this
significantly complicates the pattern and increases the likelihood of
errors.

This patch also changes vmlinux.lds.S for the sparc64 architecture to
accommodate specific symbol placement requirements.

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>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Yabin Cui <yabinc@google.com>
Change-Id: I5202d40bc7e24f93c2bfb2f0d987e9dc57dec1b1
---
 arch/sparc/kernel/vmlinux.lds.S   |  5 +++++
 include/asm-generic/vmlinux.lds.h | 19 ++++++++++++-------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/sparc/kernel/vmlinux.lds.S b/arch/sparc/kernel/vmlinux.lds.S
index d317a843f7ea9..f1b86eb303404 100644
--- a/arch/sparc/kernel/vmlinux.lds.S
+++ b/arch/sparc/kernel/vmlinux.lds.S
@@ -48,6 +48,11 @@ SECTIONS
 	{
 		_text = .;
 		HEAD_TEXT
+	        ALIGN_FUNCTION();
+#ifdef CONFIG_SPARC64
+	        /* Match text section symbols in head_64.S first */
+	        *head_64.o(.text)
+#endif
 		TEXT_TEXT
 		SCHED_TEXT
 		LOCK_TEXT
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index eeadbaeccf88b..fd901951549c0 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -553,19 +553,24 @@
  * .text section. Map to function alignment to avoid address changes
  * during second ld run in second ld pass when generating System.map
  *
- * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
- * code elimination is enabled, so these sections should be converted
- * to use ".." first.
+ * TEXT_MAIN here will match symbols with a fixed pattern (for example,
+ * .text.hot or .text.unlikely) if dead code elimination or
+ * function-section is enabled. Match these symbols first before
+ * TEXT_MAIN to ensure they are grouped together.
+ *
+ * Also placing .text.hot section at the beginning of a page, this
+ * would help the TLB performance.
  */
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
+		*(.text.asan.* .text.tsan.*)				\
+		*(.text.unknown .text.unknown.*)			\
+		*(.text.unlikely .text.unlikely.*)			\
+		. = ALIGN(PAGE_SIZE);					\
 		*(.text.hot .text.hot.*)				\
 		*(TEXT_MAIN .text.fixup)				\
-		*(.text.unlikely .text.unlikely.*)			\
-		*(.text.unknown .text.unknown.*)			\
 		NOINSTR_TEXT						\
-		*(.ref.text)						\
-		*(.text.asan.* .text.tsan.*)
+		*(.ref.text)
 
 
 /* sched.text is aling to function alignment to secure we have same
-- 
2.47.0.163.g1226f6d8fa-goog

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Kees Cook]

On Fri, Oct 25, 2024 at 10:14:05PM -0700, Rong Xu wrote:
> When the -ffunction-sections compiler option is enabled, each function
> is placed in a separate section named .text.function_name rather than
> putting all functions in a single .text section.
> 
> However, using -function-sections can cause problems with the
> linker script. The comments included in include/asm-generic/vmlinux.lds.h
> note these issues.:
>   “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
>    code elimination is enabled, so these sections should be converted
>    to use ".." first.”
> 
> It is unclear whether there is a straightforward method for converting
> a suffix to "..".

While I'm not a fan of the potential loss of a page, I'm not aware of
a way to do this renaming. If we want to keep these as "cold", then this
is probably the best way to do it.

> This patch modifies the order of subsections within the text output
> section. Specifically, it repositions sections with certain fixed patterns
> (for example .text.unlikely) before TEXT_MAIN, ensuring that they are
> grouped and matched together. It also places .text.hot section at the
> beginning of a page to help the TLB performance.
> 
> Note that the limitation arises because the linker script employs glob
> patterns instead of regular expressions for string matching. While there
> is a method to maintain the current order using complex patterns, this
> significantly complicates the pattern and increases the likelihood of
> errors.
> 
> This patch also changes vmlinux.lds.S for the sparc64 architecture to
> accommodate specific symbol placement requirements.
> 
> 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>
> Tested-by: Yonghong Song <yonghong.song@linux.dev>
> Tested-by: Yabin Cui <yabinc@google.com>
> Change-Id: I5202d40bc7e24f93c2bfb2f0d987e9dc57dec1b1

Reviewed-by: Kees Cook <kees@kernel.org>

-Kees

> ---
>  arch/sparc/kernel/vmlinux.lds.S   |  5 +++++
>  include/asm-generic/vmlinux.lds.h | 19 ++++++++++++-------
>  2 files changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/sparc/kernel/vmlinux.lds.S b/arch/sparc/kernel/vmlinux.lds.S
> index d317a843f7ea9..f1b86eb303404 100644
> --- a/arch/sparc/kernel/vmlinux.lds.S
> +++ b/arch/sparc/kernel/vmlinux.lds.S
> @@ -48,6 +48,11 @@ SECTIONS
>  	{
>  		_text = .;
>  		HEAD_TEXT
> +	        ALIGN_FUNCTION();
> +#ifdef CONFIG_SPARC64
> +	        /* Match text section symbols in head_64.S first */
> +	        *head_64.o(.text)
> +#endif
>  		TEXT_TEXT
>  		SCHED_TEXT
>  		LOCK_TEXT
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index eeadbaeccf88b..fd901951549c0 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -553,19 +553,24 @@
>   * .text section. Map to function alignment to avoid address changes
>   * during second ld run in second ld pass when generating System.map
>   *
> - * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> - * code elimination is enabled, so these sections should be converted
> - * to use ".." first.
> + * TEXT_MAIN here will match symbols with a fixed pattern (for example,
> + * .text.hot or .text.unlikely) if dead code elimination or
> + * function-section is enabled. Match these symbols first before
> + * TEXT_MAIN to ensure they are grouped together.
> + *
> + * Also placing .text.hot section at the beginning of a page, this
> + * would help the TLB performance.
>   */
>  #define TEXT_TEXT							\
>  		ALIGN_FUNCTION();					\
> +		*(.text.asan.* .text.tsan.*)				\
> +		*(.text.unknown .text.unknown.*)			\
> +		*(.text.unlikely .text.unlikely.*)			\
> +		. = ALIGN(PAGE_SIZE);					\
>  		*(.text.hot .text.hot.*)				\
>  		*(TEXT_MAIN .text.fixup)				\
> -		*(.text.unlikely .text.unlikely.*)			\
> -		*(.text.unknown .text.unknown.*)			\
>  		NOINSTR_TEXT						\
> -		*(.ref.text)						\
> -		*(.text.asan.* .text.tsan.*)
> +		*(.ref.text)
>  
>  
>  /* sched.text is aling to function alignment to secure we have same
> -- 
> 2.47.0.163.g1226f6d8fa-goog
> 

-- 
Kees Cook

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Masahiro Yamada]

On Sat, Oct 26, 2024 at 7:14 AM Rong Xu <xur@google.com> wrote:
>
> When the -ffunction-sections compiler option is enabled, each function
> is placed in a separate section named .text.function_name rather than
> putting all functions in a single .text section.
>
> However, using -function-sections can cause problems with the
> linker script. The comments included in include/asm-generic/vmlinux.lds.h
> note these issues.:
>   “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
>    code elimination is enabled, so these sections should be converted
>    to use ".." first.”
>
> It is unclear whether there is a straightforward method for converting
> a suffix to "..".
>
> This patch modifies the order of subsections within the text output
> section. Specifically, it repositions sections with certain fixed patterns
> (for example .text.unlikely) before TEXT_MAIN, ensuring that they are
> grouped and matched together. It also places .text.hot section at the
> beginning of a page to help the TLB performance.


The fixed patterns are currently listed in this order:

  .text.hot, .text_unlikely, .text.unknown, .text.asan.

You reorder them to:

  .text.asan, .text.unknown, .text.unlikely, .text.hot


I believe it is better to describe your thoughts
about the reshuffling among the fixed pattern sections.

Otherwise, It is unclear to me.




--
Best Regards
Masahiro Yamada

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Rong Xu]

Current order is:
.text.hot, .text, .text_unlikely, .text.unknown, .text.asan

The patch reorders them to:
.text.asan, .text.unknown, .text_unlikely, .text.hot, .text

The majority of the code resides in three sections: .text.hot, .text, and
 .text.unlikely, with .text.unknown containing a negligible amount.
.text.asan is only generated in ASAN builds.

Our primary goal is to group code segments based on their execution
frequency (hotness).

First, we want to place .text.hot adjacent to .text. Since we cannot put
.text.hot after .text (Due to constraints with -ffunction-sections,
placing .text.hot after .text is problematic), we need to put
.text.hot before .text.

Then it comes to .text.unlikely, we cannot put it after .text
(same -ffunction-sections issue) . Therefore, we'll position .text.unlikely
before .text.hot.

.text.unknown and .tex.asan follow the same logic.

This revised ordering effectively reverses the original arrangement (for
.text.unlikely, .text.unknown, and .tex.asan), maintaining a similar level of
affinity between sections.

I hope this explains the reason for the new ordering.

-Rong

On Fri, Nov 1, 2024 at 11:06 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Sat, Oct 26, 2024 at 7:14 AM Rong Xu <xur@google.com> wrote:
> >
> > When the -ffunction-sections compiler option is enabled, each function
> > is placed in a separate section named .text.function_name rather than
> > putting all functions in a single .text section.
> >
> > However, using -function-sections can cause problems with the
> > linker script. The comments included in include/asm-generic/vmlinux.lds.h
> > note these issues.:
> >   “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> >    code elimination is enabled, so these sections should be converted
> >    to use ".." first.”
> >
> > It is unclear whether there is a straightforward method for converting
> > a suffix to "..".
> >
> > This patch modifies the order of subsections within the text output
> > section. Specifically, it repositions sections with certain fixed patterns
> > (for example .text.unlikely) before TEXT_MAIN, ensuring that they are
> > grouped and matched together. It also places .text.hot section at the
> > beginning of a page to help the TLB performance.
>
>
> The fixed patterns are currently listed in this order:
>
>   .text.hot, .text_unlikely, .text.unknown, .text.asan.
>
> You reorder them to:
>
>   .text.asan, .text.unknown, .text.unlikely, .text.hot
>
>
> I believe it is better to describe your thoughts
> about the reshuffling among the fixed pattern sections.
>
> Otherwise, It is unclear to me.
>
>
>
>
> --
> Best Regards
> Masahiro Yamada

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Masahiro Yamada]

On Sat, Nov 2, 2024 at 3:37 AM Rong Xu <xur@google.com> wrote:
>
> Current order is:
> .text.hot, .text, .text_unlikely, .text.unknown, .text.asan
>
> The patch reorders them to:
> .text.asan, .text.unknown, .text_unlikely, .text.hot, .text
>
> The majority of the code resides in three sections: .text.hot, .text, and
>  .text.unlikely, with .text.unknown containing a negligible amount.
> .text.asan is only generated in ASAN builds.
>
> Our primary goal is to group code segments based on their execution
> frequency (hotness).
>
> First, we want to place .text.hot adjacent to .text. Since we cannot put
> .text.hot after .text (Due to constraints with -ffunction-sections,
> placing .text.hot after .text is problematic), we need to put
> .text.hot before .text.
>
> Then it comes to .text.unlikely, we cannot put it after .text
> (same -ffunction-sections issue) . Therefore, we'll position .text.unlikely
> before .text.hot.
>
> .text.unknown and .tex.asan follow the same logic.
>
> This revised ordering effectively reverses the original arrangement (for
> .text.unlikely, .text.unknown, and .tex.asan), maintaining a similar level of
> affinity between sections.
>
> I hope this explains the reason for the new ordering.


Make sense.

Please describe the above in the commit description.

Then, it will be clearer why you not only moved up fixed patterns
but also reversed the order.







-- 
Best Regards
Masahiro Yamada

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Klara Modin]

[-- Attachment #1: Type: text/plain, Size: 4406 bytes --]

Hi,

On 2024-10-26 07:14, Rong Xu wrote:
> When the -ffunction-sections compiler option is enabled, each function
> is placed in a separate section named .text.function_name rather than
> putting all functions in a single .text section.
> 
> However, using -function-sections can cause problems with the
> linker script. The comments included in include/asm-generic/vmlinux.lds.h
> note these issues.:
>    “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
>     code elimination is enabled, so these sections should be converted
>     to use ".." first.”
> 
> It is unclear whether there is a straightforward method for converting
> a suffix to "..".
> 
> This patch modifies the order of subsections within the text output
> section. Specifically, it repositions sections with certain fixed patterns
> (for example .text.unlikely) before TEXT_MAIN, ensuring that they are
> grouped and matched together. It also places .text.hot section at the
> beginning of a page to help the TLB performance.
> 
> Note that the limitation arises because the linker script employs glob
> patterns instead of regular expressions for string matching. While there
> is a method to maintain the current order using complex patterns, this
> significantly complicates the pattern and increases the likelihood of
> errors.
> 
> This patch also changes vmlinux.lds.S for the sparc64 architecture to
> accommodate specific symbol placement requirements.

With this patch (622240ea8d71a75055399fd4b3cc2b190e44d2e2 in 
next-20241108) my Edgerouter 6P hangs on boot (Cavium Octeon III, 
mips64, running in big endian). It's using device tree passed from the 
vendored u-boot (attached in case it's relevant).

Disabling dead code elimination does not fix the issue.

Please let me know if there's anything else you need.

Regards,
Klara Modin

> 
> 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>
> Tested-by: Yonghong Song <yonghong.song@linux.dev>
> Tested-by: Yabin Cui <yabinc@google.com>
> Change-Id: I5202d40bc7e24f93c2bfb2f0d987e9dc57dec1b1
> ---
>   arch/sparc/kernel/vmlinux.lds.S   |  5 +++++
>   include/asm-generic/vmlinux.lds.h | 19 ++++++++++++-------
>   2 files changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/sparc/kernel/vmlinux.lds.S b/arch/sparc/kernel/vmlinux.lds.S
> index d317a843f7ea9..f1b86eb303404 100644
> --- a/arch/sparc/kernel/vmlinux.lds.S
> +++ b/arch/sparc/kernel/vmlinux.lds.S
> @@ -48,6 +48,11 @@ SECTIONS
>   	{
>   		_text = .;
>   		HEAD_TEXT
> +	        ALIGN_FUNCTION();
> +#ifdef CONFIG_SPARC64
> +	        /* Match text section symbols in head_64.S first */
> +	        *head_64.o(.text)
> +#endif
>   		TEXT_TEXT
>   		SCHED_TEXT
>   		LOCK_TEXT
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index eeadbaeccf88b..fd901951549c0 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -553,19 +553,24 @@
>    * .text section. Map to function alignment to avoid address changes
>    * during second ld run in second ld pass when generating System.map
>    *
> - * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> - * code elimination is enabled, so these sections should be converted
> - * to use ".." first.
> + * TEXT_MAIN here will match symbols with a fixed pattern (for example,
> + * .text.hot or .text.unlikely) if dead code elimination or
> + * function-section is enabled. Match these symbols first before
> + * TEXT_MAIN to ensure they are grouped together.
> + *
> + * Also placing .text.hot section at the beginning of a page, this
> + * would help the TLB performance.
>    */
>   #define TEXT_TEXT							\
>   		ALIGN_FUNCTION();					\
> +		*(.text.asan.* .text.tsan.*)				\
> +		*(.text.unknown .text.unknown.*)			\
> +		*(.text.unlikely .text.unlikely.*)			\
> +		. = ALIGN(PAGE_SIZE);					\
>   		*(.text.hot .text.hot.*)				\
>   		*(TEXT_MAIN .text.fixup)				\
> -		*(.text.unlikely .text.unlikely.*)			\
> -		*(.text.unknown .text.unknown.*)			\
>   		NOINSTR_TEXT						\
> -		*(.ref.text)						\
> -		*(.text.asan.* .text.tsan.*)
> +		*(.ref.text)
>   
>   
>   /* sched.text is aling to function alignment to secure we have same

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Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Rong Xu]

Thanks for reporting this issue!

I'm assuming your kernel build enables dead code elimination and
uses the --ffunction-sections compiler flag. Without this patch, all
the functions
-- I think there are only .text.unlikely.* and .text.* are grouped
together in the
final vmlinux. This patch modifies the linker script to place
.text.unlikely.* functions
 before .text.* functions. I've examined arch/mips/kernel/vmlinux.lds.S, and
haven't found any obvious issue.

Can you send me the following?
(1) the kernel build command
(2) System.map without the patch
(3) System.map with the patch

Best regards,

-Rong

On Sat, Nov 9, 2024 at 7:39 AM Klara Modin <klarasmodin@gmail.com> wrote:
>
> Hi,
>
> On 2024-10-26 07:14, Rong Xu wrote:
> > When the -ffunction-sections compiler option is enabled, each function
> > is placed in a separate section named .text.function_name rather than
> > putting all functions in a single .text section.
> >
> > However, using -function-sections can cause problems with the
> > linker script. The comments included in include/asm-generic/vmlinux.lds.h
> > note these issues.:
> >    “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> >     code elimination is enabled, so these sections should be converted
> >     to use ".." first.”
> >
> > It is unclear whether there is a straightforward method for converting
> > a suffix to "..".
> >
> > This patch modifies the order of subsections within the text output
> > section. Specifically, it repositions sections with certain fixed patterns
> > (for example .text.unlikely) before TEXT_MAIN, ensuring that they are
> > grouped and matched together. It also places .text.hot section at the
> > beginning of a page to help the TLB performance.
> >
> > Note that the limitation arises because the linker script employs glob
> > patterns instead of regular expressions for string matching. While there
> > is a method to maintain the current order using complex patterns, this
> > significantly complicates the pattern and increases the likelihood of
> > errors.
> >
> > This patch also changes vmlinux.lds.S for the sparc64 architecture to
> > accommodate specific symbol placement requirements.
>
> With this patch (622240ea8d71a75055399fd4b3cc2b190e44d2e2 in
> next-20241108) my Edgerouter 6P hangs on boot (Cavium Octeon III,
> mips64, running in big endian). It's using device tree passed from the
> vendored u-boot (attached in case it's relevant).
>
> Disabling dead code elimination does not fix the issue.
>
> Please let me know if there's anything else you need.
>
> Regards,
> Klara Modin
>
> >
> > 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>
> > Tested-by: Yonghong Song <yonghong.song@linux.dev>
> > Tested-by: Yabin Cui <yabinc@google.com>
> > Change-Id: I5202d40bc7e24f93c2bfb2f0d987e9dc57dec1b1
> > ---
> >   arch/sparc/kernel/vmlinux.lds.S   |  5 +++++
> >   include/asm-generic/vmlinux.lds.h | 19 ++++++++++++-------
> >   2 files changed, 17 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/sparc/kernel/vmlinux.lds.S b/arch/sparc/kernel/vmlinux.lds.S
> > index d317a843f7ea9..f1b86eb303404 100644
> > --- a/arch/sparc/kernel/vmlinux.lds.S
> > +++ b/arch/sparc/kernel/vmlinux.lds.S
> > @@ -48,6 +48,11 @@ SECTIONS
> >       {
> >               _text = .;
> >               HEAD_TEXT
> > +             ALIGN_FUNCTION();
> > +#ifdef CONFIG_SPARC64
> > +             /* Match text section symbols in head_64.S first */
> > +             *head_64.o(.text)
> > +#endif
> >               TEXT_TEXT
> >               SCHED_TEXT
> >               LOCK_TEXT
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index eeadbaeccf88b..fd901951549c0 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -553,19 +553,24 @@
> >    * .text section. Map to function alignment to avoid address changes
> >    * during second ld run in second ld pass when generating System.map
> >    *
> > - * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> > - * code elimination is enabled, so these sections should be converted
> > - * to use ".." first.
> > + * TEXT_MAIN here will match symbols with a fixed pattern (for example,
> > + * .text.hot or .text.unlikely) if dead code elimination or
> > + * function-section is enabled. Match these symbols first before
> > + * TEXT_MAIN to ensure they are grouped together.
> > + *
> > + * Also placing .text.hot section at the beginning of a page, this
> > + * would help the TLB performance.
> >    */
> >   #define TEXT_TEXT                                                   \
> >               ALIGN_FUNCTION();                                       \
> > +             *(.text.asan.* .text.tsan.*)                            \
> > +             *(.text.unknown .text.unknown.*)                        \
> > +             *(.text.unlikely .text.unlikely.*)                      \
> > +             . = ALIGN(PAGE_SIZE);                                   \
> >               *(.text.hot .text.hot.*)                                \
> >               *(TEXT_MAIN .text.fixup)                                \
> > -             *(.text.unlikely .text.unlikely.*)                      \
> > -             *(.text.unknown .text.unknown.*)                        \
> >               NOINSTR_TEXT                                            \
> > -             *(.ref.text)                                            \
> > -             *(.text.asan.* .text.tsan.*)
> > +             *(.ref.text)
> >
> >
> >   /* sched.text is aling to function alignment to secure we have same

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Klara Modin]

[-- Attachment #1: Type: text/plain, Size: 1037 bytes --]

On 2024-11-11 21:43, Rong Xu wrote:
> Thanks for reporting this issue!
> 
> I'm assuming your kernel build enables dead code elimination and
> uses the --ffunction-sections compiler flag. Without this patch, all
> the functions
> -- I think there are only .text.unlikely.* and .text.* are grouped
> together in the
> final vmlinux. This patch modifies the linker script to place
> .text.unlikely.* functions
>   before .text.* functions. I've examined arch/mips/kernel/vmlinux.lds.S, and
> haven't found any obvious issue.
> 
> Can you send me the following?
> (1) the kernel build command
> (2) System.map without the patch
> (3) System.map with the patch
> 
> Best regards,
> 
> -Rong
> 
I don't set -ffunction-sections explicitly but it seems to be used when 
I look at the .cmd files. The build command is nothing fancy, I just set 
ARCH=mips CROSS_COMPILE=mips64-unknown-linux-gnuabin32- and build with 
make -j24.

I've attached the System.map, built on next-20241111 as well as it with 
this series reverted.

Regards,
Klara Modin

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Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Rong Xu]

In the new System.map, we have:
ffffffff81112400 T _stext

This looks wrong. It should point to the beginning of the text, like
ffffffff81100400 T _stext

I'll do some debugging on this.

-Rong

On Mon, Nov 11, 2024 at 1:32 PM Klara Modin <klarasmodin@gmail.com> wrote:
>
> On 2024-11-11 21:43, Rong Xu wrote:
> > Thanks for reporting this issue!
> >
> > I'm assuming your kernel build enables dead code elimination and
> > uses the --ffunction-sections compiler flag. Without this patch, all
> > the functions
> > -- I think there are only .text.unlikely.* and .text.* are grouped
> > together in the
> > final vmlinux. This patch modifies the linker script to place
> > .text.unlikely.* functions
> >   before .text.* functions. I've examined arch/mips/kernel/vmlinux.lds.S, and
> > haven't found any obvious issue.
> >
> > Can you send me the following?
> > (1) the kernel build command
> > (2) System.map without the patch
> > (3) System.map with the patch
> >
> > Best regards,
> >
> > -Rong
> >
> I don't set -ffunction-sections explicitly but it seems to be used when
> I look at the .cmd files. The build command is nothing fancy, I just set
> ARCH=mips CROSS_COMPILE=mips64-unknown-linux-gnuabin32- and build with
> make -j24.
>
> I've attached the System.map, built on next-20241111 as well as it with
> this series reverted.
>
> Regards,
> Klara Modin

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Rong Xu]

I compared the System.map files from Klara Modin. The linker script is
doing what I expected: relocating the unlikely executed functions to the
beginning of the .text section.

However, the problem is with the _stext symbol. It belongs to the
.text section, so
it is positioned after the unlikely (or hot) functions. But it really
needs to be
the start of the text section.

I checked all vmlinux.lds.S in arch/, I found that most archs
explicitly assign _stext to the same address as _text, with the
following 3 exceptions:
  arch/sh/kernel/vmlinux.lds.S
  arch/mips/kernel/vmlinux.lds.S
  arch/sparc/kernel/vmlinux.lds.S

Note that we already partially handled arch/sparc/kernel/vmlinux.lds.S
for sparc64.
But we need to handle sparc32 also.

Additionally, the boot/compressed/vmlinux.lds.S also the TEXT_TEXT
template. However,
I presume these files do not generate the .text.unlikely. or
.text.hot.* sections.

I sent the following patch to Klara because I don't have an
environment to build and test.
====================
diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 9ff55cb80a64..5f130af44247 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -61,6 +61,7 @@ SECTIONS
        /* read-only */
        _text = .;      /* Text and read-only data */
        .text : {
+               _stext = .;
                TEXT_TEXT
                SCHED_TEXT
                LOCK_TEXT
======================

If Klara confirms the fix, I will send the patch for review.

Thanks,

-Rong


On Mon, Nov 11, 2024 at 2:39 PM Rong Xu <xur@google.com> wrote:
>
> In the new System.map, we have:
> ffffffff81112400 T _stext
>
> This looks wrong. It should point to the beginning of the text, like
> ffffffff81100400 T _stext
>
> I'll do some debugging on this.
>
> -Rong
>
> On Mon, Nov 11, 2024 at 1:32 PM Klara Modin <klarasmodin@gmail.com> wrote:
> >
> > On 2024-11-11 21:43, Rong Xu wrote:
> > > Thanks for reporting this issue!
> > >
> > > I'm assuming your kernel build enables dead code elimination and
> > > uses the --ffunction-sections compiler flag. Without this patch, all
> > > the functions
> > > -- I think there are only .text.unlikely.* and .text.* are grouped
> > > together in the
> > > final vmlinux. This patch modifies the linker script to place
> > > .text.unlikely.* functions
> > >   before .text.* functions. I've examined arch/mips/kernel/vmlinux.lds.S, and
> > > haven't found any obvious issue.
> > >
> > > Can you send me the following?
> > > (1) the kernel build command
> > > (2) System.map without the patch
> > > (3) System.map with the patch
> > >
> > > Best regards,
> > >
> > > -Rong
> > >
> > I don't set -ffunction-sections explicitly but it seems to be used when
> > I look at the .cmd files. The build command is nothing fancy, I just set
> > ARCH=mips CROSS_COMPILE=mips64-unknown-linux-gnuabin32- and build with
> > make -j24.
> >
> > I've attached the System.map, built on next-20241111 as well as it with
> > this series reverted.
> >
> > Regards,
> > Klara Modin

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Klara Modin]

On 2024-11-12 06:38, Rong Xu wrote:
> I compared the System.map files from Klara Modin. The linker script is
> doing what I expected: relocating the unlikely executed functions to the
> beginning of the .text section.
> 
> However, the problem is with the _stext symbol. It belongs to the
> .text section, so
> it is positioned after the unlikely (or hot) functions. But it really
> needs to be
> the start of the text section.
> 
> I checked all vmlinux.lds.S in arch/, I found that most archs
> explicitly assign _stext to the same address as _text, with the
> following 3 exceptions:
>    arch/sh/kernel/vmlinux.lds.S
>    arch/mips/kernel/vmlinux.lds.S
>    arch/sparc/kernel/vmlinux.lds.S
> 
> Note that we already partially handled arch/sparc/kernel/vmlinux.lds.S
> for sparc64.
> But we need to handle sparc32 also.
> 
> Additionally, the boot/compressed/vmlinux.lds.S also the TEXT_TEXT
> template. However,
> I presume these files do not generate the .text.unlikely. or
> .text.hot.* sections.
> 
> I sent the following patch to Klara because I don't have an
> environment to build and test.
> ====================
> diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
> index 9ff55cb80a64..5f130af44247 100644
> --- a/arch/mips/kernel/vmlinux.lds.S
> +++ b/arch/mips/kernel/vmlinux.lds.S
> @@ -61,6 +61,7 @@ SECTIONS
>          /* read-only */
>          _text = .;      /* Text and read-only data */
>          .text : {
> +               _stext = .;
>                  TEXT_TEXT
>                  SCHED_TEXT
>                  LOCK_TEXT
> ======================
> 
> If Klara confirms the fix, I will send the patch for review.
> 
> Thanks,
> 
> -Rong
> 

This does indeed fix the issue for me.

Thanks,
Tested-by: Klara Modin <klarasmodin@gmail.com>

Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Rong Xu]

I sent the following patch for review:
https://lkml.org/lkml/2024/11/12/1565

Thanks!

-Rong

On Mon, Nov 11, 2024 at 11:45 PM Klara Modin <klarasmodin@gmail.com> wrote:
>
> On 2024-11-12 06:38, Rong Xu wrote:
> > I compared the System.map files from Klara Modin. The linker script is
> > doing what I expected: relocating the unlikely executed functions to the
> > beginning of the .text section.
> >
> > However, the problem is with the _stext symbol. It belongs to the
> > .text section, so
> > it is positioned after the unlikely (or hot) functions. But it really
> > needs to be
> > the start of the text section.
> >
> > I checked all vmlinux.lds.S in arch/, I found that most archs
> > explicitly assign _stext to the same address as _text, with the
> > following 3 exceptions:
> >    arch/sh/kernel/vmlinux.lds.S
> >    arch/mips/kernel/vmlinux.lds.S
> >    arch/sparc/kernel/vmlinux.lds.S
> >
> > Note that we already partially handled arch/sparc/kernel/vmlinux.lds.S
> > for sparc64.
> > But we need to handle sparc32 also.
> >
> > Additionally, the boot/compressed/vmlinux.lds.S also the TEXT_TEXT
> > template. However,
> > I presume these files do not generate the .text.unlikely. or
> > .text.hot.* sections.
> >
> > I sent the following patch to Klara because I don't have an
> > environment to build and test.
> > ====================
> > diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
> > index 9ff55cb80a64..5f130af44247 100644
> > --- a/arch/mips/kernel/vmlinux.lds.S
> > +++ b/arch/mips/kernel/vmlinux.lds.S
> > @@ -61,6 +61,7 @@ SECTIONS
> >          /* read-only */
> >          _text = .;      /* Text and read-only data */
> >          .text : {
> > +               _stext = .;
> >                  TEXT_TEXT
> >                  SCHED_TEXT
> >                  LOCK_TEXT
> > ======================
> >
> > If Klara confirms the fix, I will send the patch for review.
> >
> > Thanks,
> >
> > -Rong
> >
>
> This does indeed fix the issue for me.
>
> Thanks,
> Tested-by: Klara Modin <klarasmodin@gmail.com>

[PATCH v6 4/7] Add markers for text_unlikely and text_hot sections

[Rong Xu]

Add markers like __hot_text_start, __hot_text_end, __unlikely_text_start,
and __unlikely_text_end which will be included in System.map. These markers
indicate how the compiler groups functions, providing valuable information
to developers about the layout and optimization of the code.

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>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Yabin Cui <yabinc@google.com>
Change-Id: Ie7688587adf89a14661ceca108680903b546d5d9
---
 include/asm-generic/vmlinux.lds.h | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index fd901951549c0..e02973f3b4189 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -549,6 +549,16 @@
 		__cpuidle_text_end = .;					\
 		__noinstr_text_end = .;
 
+#define TEXT_UNLIKELY							\
+		__unlikely_text_start = .;				\
+		*(.text.unlikely .text.unlikely.*)			\
+		__unlikely_text_end = .;
+
+#define TEXT_HOT							\
+		__hot_text_start = .;					\
+		*(.text.hot .text.hot.*)				\
+		__hot_text_end = .;
+
 /*
  * .text section. Map to function alignment to avoid address changes
  * during second ld run in second ld pass when generating System.map
@@ -565,9 +575,9 @@
 		ALIGN_FUNCTION();					\
 		*(.text.asan.* .text.tsan.*)				\
 		*(.text.unknown .text.unknown.*)			\
-		*(.text.unlikely .text.unlikely.*)			\
+		TEXT_UNLIKELY						\
 		. = ALIGN(PAGE_SIZE);					\
-		*(.text.hot .text.hot.*)				\
+		TEXT_HOT						\
 		*(TEXT_MAIN .text.fixup)				\
 		NOINSTR_TEXT						\
 		*(.ref.text)
-- 
2.47.0.163.g1226f6d8fa-goog

Re: [PATCH v6 4/7] Add markers for text_unlikely and text_hot sections

[Kees Cook]

On Fri, Oct 25, 2024 at 10:14:06PM -0700, Rong Xu wrote:
> Add markers like __hot_text_start, __hot_text_end, __unlikely_text_start,
> and __unlikely_text_end which will be included in System.map. These markers
> indicate how the compiler groups functions, providing valuable information
> to developers about the layout and optimization of the code.
> 
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

Yup, this is good.

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

Re: [PATCH v6 4/7] Add markers for text_unlikely and text_hot sections

[Kees Cook]

On Fri, Oct 25, 2024 at 10:14:06PM -0700, Rong Xu wrote:
> Add markers like __hot_text_start, __hot_text_end, __unlikely_text_start,
> and __unlikely_text_end which will be included in System.map. These markers
> indicate how the compiler groups functions, providing valuable information
> to developers about the layout and optimization of the code.
> 
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

[PATCH v6 5/7] AutoFDO: Enable -ffunction-sections for the AutoFDO build

[Rong Xu]

Enable -ffunction-sections by default for the AutoFDO build.

With -ffunction-sections, the compiler places each function in its own
section named .text.function_name instead of placing all functions in
the .text section. In the AutoFDO build, this allows the linker to
utilize profile information to reorganize functions for improved
utilization of iCache and iTLB.

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>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Yabin Cui <yabinc@google.com>
Change-Id: I6dc9150c46983a576f4d4f1415dbdcab78a48021
---
 include/asm-generic/vmlinux.lds.h | 11 +++++++++--
 scripts/Makefile.autofdo          |  2 +-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e02973f3b4189..bd64fdedabd2f 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -95,18 +95,25 @@
  * 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.
+ *
  * 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)
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) || \
+defined(CONFIG_AUTOFDO_CLANG)
 #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
+#else
+#define TEXT_MAIN .text
+#endif
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
 #define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral* .data.$__unnamed_* .data.$L*
 #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
 #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*
 #define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..L* .bss..compoundliteral*
 #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
 #else
-#define TEXT_MAIN .text
 #define DATA_MAIN .data
 #define SDATA_MAIN .sdata
 #define RODATA_MAIN .rodata
diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
index ff96a63fea7cd..6155d6fc4ca7f 100644
--- a/scripts/Makefile.autofdo
+++ b/scripts/Makefile.autofdo
@@ -9,7 +9,7 @@ ifndef CONFIG_DEBUG_INFO
 endif
 
 ifdef CLANG_AUTOFDO_PROFILE
-  CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE)
+  CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
 endif
 
 ifdef CONFIG_LTO_CLANG_THIN
-- 
2.47.0.163.g1226f6d8fa-goog

Re: [PATCH v6 5/7] AutoFDO: Enable -ffunction-sections for the AutoFDO build

[Kees Cook]

On Fri, Oct 25, 2024 at 10:14:07PM -0700, Rong Xu wrote:
> Enable -ffunction-sections by default for the AutoFDO build.
> 
> With -ffunction-sections, the compiler places each function in its own
> section named .text.function_name instead of placing all functions in
> the .text section. In the AutoFDO build, this allows the linker to
> utilize profile information to reorganize functions for improved
> utilization of iCache and iTLB.
> 
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

[PATCH v6 6/7] AutoFDO: Enable machine function split optimization for AutoFDO

[Rong Xu]

Enable the machine function split optimization for AutoFDO in Clang.

Machine function split (MFS) is a pass in the Clang compiler that
splits a function into hot and cold parts. The linker groups all
cold blocks across functions together. This decreases hot code
fragmentation and improves iCache and iTLB utilization.

MFS requires a profile so this is enabled only for the AutoFDO builds.

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>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Tested-by: Yabin Cui <yabinc@google.com>
Change-Id: Iece9b36d37162c0cf66b8efba7e81d04b4768254
---
 include/asm-generic/vmlinux.lds.h | 7 ++++++-
 scripts/Makefile.autofdo          | 2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index bd64fdedabd2f..8a0bb3946cf05 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -556,6 +556,11 @@ defined(CONFIG_AUTOFDO_CLANG)
 		__cpuidle_text_end = .;					\
 		__noinstr_text_end = .;
 
+#define TEXT_SPLIT							\
+		__split_text_start = .;					\
+		*(.text.split .text.split.[0-9a-zA-Z_]*)		\
+		__split_text_end = .;
+
 #define TEXT_UNLIKELY							\
 		__unlikely_text_start = .;				\
 		*(.text.unlikely .text.unlikely.*)			\
@@ -582,6 +587,7 @@ defined(CONFIG_AUTOFDO_CLANG)
 		ALIGN_FUNCTION();					\
 		*(.text.asan.* .text.tsan.*)				\
 		*(.text.unknown .text.unknown.*)			\
+		TEXT_SPLIT						\
 		TEXT_UNLIKELY						\
 		. = ALIGN(PAGE_SIZE);					\
 		TEXT_HOT						\
@@ -589,7 +595,6 @@ defined(CONFIG_AUTOFDO_CLANG)
 		NOINSTR_TEXT						\
 		*(.ref.text)
 
-
 /* sched.text is aling to function alignment to secure we have same
  * address even at second ld pass when generating System.map */
 #define SCHED_TEXT							\
diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
index 6155d6fc4ca7f..1caf2457e585c 100644
--- a/scripts/Makefile.autofdo
+++ b/scripts/Makefile.autofdo
@@ -10,6 +10,7 @@ endif
 
 ifdef CLANG_AUTOFDO_PROFILE
   CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
+  CFLAGS_AUTOFDO_CLANG += -fsplit-machine-functions
 endif
 
 ifdef CONFIG_LTO_CLANG_THIN
@@ -17,6 +18,7 @@ ifdef CONFIG_LTO_CLANG_THIN
     KBUILD_LDFLAGS += --lto-sample-profile=$(CLANG_AUTOFDO_PROFILE)
   endif
   KBUILD_LDFLAGS += --mllvm=-enable-fs-discriminator=true --mllvm=-improved-fs-discriminator=true -plugin-opt=thinlto
+  KBUILD_LDFLAGS += -plugin-opt=-split-machine-functions
 endif
 
 export CFLAGS_AUTOFDO_CLANG
-- 
2.47.0.163.g1226f6d8fa-goog

Re: [PATCH v6 6/7] AutoFDO: Enable machine function split optimization for AutoFDO

[Kees Cook]

On Fri, Oct 25, 2024 at 10:14:08PM -0700, Rong Xu wrote:
> Enable the machine function split optimization for AutoFDO in Clang.
> 
> Machine function split (MFS) is a pass in the Clang compiler that
> splits a function into hot and cold parts. The linker groups all
> cold blocks across functions together. This decreases hot code
> fragmentation and improves iCache and iTLB utilization.
> 
> MFS requires a profile so this is enabled only for the AutoFDO builds.
> 
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

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

[Rong Xu]

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

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

[Kees Cook]

On Fri, Oct 25, 2024 at 10:14:09PM -0700, Rong Xu wrote:
> 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>

Looks good. Similarly isolated like FDO. :)

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

Re: [PATCH v6 0/7] Add AutoFDO and Propeller support for Clang build

[Nathan Chancellor]

Hi Rong,

I tested this series by following the documentation added in the series
using Clang 19 and my standard distribution configuration on an Intel
platform with the combinations of

  * AutoFDO
  * AutoFDO + ThinLTO
  * AutoFDO + Propeller
  * AutoFDO + ThinLTO + Propeller

and I noticed no issues (it would be great to see create_llvm_prof
somewhere in LLVM upstream for ease of access but that's a small
complaint).

I did not do any real benchmarking to see if those combinations were
actually quicker but I think it is pretty clear from the cover letter
that any sort of gains are going to depend on the profiling and test
case so for time's sake, I did not bother.

Tested-by: Nathan Chancellor <nathan@kernel.org>

Cheers,
Nathan