[PATCH v2 0/6] Add AutoFDO and Propeller support for Clang build

[PATCH v2 0/6] 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] Change the subsection ordering when -ffunction-sections is enabled

[P 4] Enable –ffunction-sections for the AutoFDO build

[P 5] Enable Machine Function Split (MFS) optimization for AutoFDO

[P 6] Add Propeller configuration to the kernel build

Patch 1 provides basic AutoFDO build support. Patches 2 to 5 further
enhance the performance of AutoFDO builds and are functionally dependent
on Patch 1. Patch 6 enables support for Propeller and is dependent on
patch 2 and patch 3.

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.

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 the source to e32cde8d2bd7 (Merge tag 'sched_ext-for-6.12-rc1-fixes-1')
1. Cover-letter: 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)


Rong Xu (6):
  Add AutoFDO support for Clang build
  objtool: Fix unreachable instruction warnings for weak funcitons
  Change the symbols order when --ffuntion-sections is enabled
  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   | 203 ++++++++++++++++++++++++++
 Documentation/dev-tools/index.rst     |   2 +
 Documentation/dev-tools/propeller.rst | 188 ++++++++++++++++++++++++
 MAINTAINERS                           |  14 ++
 Makefile                              |   2 +
 arch/Kconfig                          |  42 ++++++
 arch/x86/Kconfig                      |   2 +
 arch/x86/kernel/vmlinux.lds.S         |   4 +
 include/asm-generic/vmlinux.lds.h     |  54 ++++++-
 scripts/Makefile.autofdo              |  25 ++++
 scripts/Makefile.lib                  |  20 +++
 scripts/Makefile.propeller            |  22 +++
 tools/objtool/check.c                 |   2 +
 tools/objtool/elf.c                   |  15 +-
 14 files changed, 583 insertions(+), 12 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

-- 
2.46.1.824.gd892dcdcdd-goog

[PATCH v2 1/6] 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 -o <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>
---
 Documentation/dev-tools/autofdo.rst | 203 ++++++++++++++++++++++++++++
 Documentation/dev-tools/index.rst   |   1 +
 MAINTAINERS                         |   7 +
 Makefile                            |   1 +
 arch/Kconfig                        |  20 +++
 arch/x86/Kconfig                    |   1 +
 scripts/Makefile.autofdo            |  23 ++++
 scripts/Makefile.lib                |  10 ++
 tools/objtool/check.c               |   1 +
 9 files changed, 267 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 000000000000..e3f7ee8ee6bb
--- /dev/null
+++ b/Documentation/dev-tools/autofdo.rst
@@ -0,0 +1,203 @@
+.. 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:
+
+   .. code-block:: make
+
+      CONFIG_AUTOFDO_CLANG=y
+
+Customization
+=============
+
+You can enable or disable AutoFDO build for individual file and directories by
+adding a line similar to the following to the respective kernel Makefile:
+
+- For enabling a single file (e.g. foo.o)
+
+     .. code-block:: make
+
+        AUTOFDO_PROFILE_foo.o := y
+
+- For enabling all files in one directory
+
+     .. code-block:: make
+
+        AUTOFDO_PROFILE := y
+
+- For disabling one file
+
+     .. code-block:: make
+
+        AUTOFDO_PROFILE_foo.o := n
+
+- For disabling all files in one directory
+
+     .. code-block:: make
+
+        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,
+
+      .. code-block:: make
+
+         $ make menuconfig LLVM=1
+
+
+    Turn on AutoFDO build config:
+
+      .. code-block:: make
+
+         CONFIG_AUTOFDO_CLANG=y
+
+    With a configuration that with LLVM enabled, use the following command:
+
+      .. code-block:: sh
+
+         $ scripts/config -e AUTOFDO_CLANG
+
+    After getting the config, build with
+
+      .. code-block:: make
+
+         $ 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:
+
+      .. code-block:: sh
+
+         $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c <count> -o <perf_file> -- <loadtest>
+
+   - For AMD platforms: For Intel platforms:
+     The supported systems are: Zen3 with BRS, or Zen4 with amd_lbr_v2. To check,
+     For Zen3:
+
+      .. code-block:: sh
+
+         $ cat proc/cpuinfo | grep " brs"
+
+      For Zen4:
+
+      .. code-block:: sh
+
+         $ cat proc/cpuinfo | grep amd_lbr_v2
+
+      The following command generated the perf data file:
+
+      .. code-block:: sh
+
+         $ 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.
+
+      .. code-block:: sh
+
+         $ llvm-profgen --kernel --binary=<vmlinux> --perfdata=<perf_file> -o <profile_file>
+
+   or
+      .. code-block:: sh
+
+         $ create_llvm_prof --binary=<vmlinux> --profile=<perf_file> --format=extbinary -o <profile_file>
+
+   Note that multiple AutoFDO profile files can be merged into one via:
+
+      .. code-block:: sh
+
+         $ 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):
+
+      .. code-block:: sh
+
+         $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file
+
diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 53d4d124f9c5..6945644f7008 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -34,6 +34,7 @@ Documentation/dev-tools/testing-overview.rst
    ktap
    checkuapi
    gpio-sloppy-logic-analyzer
+   autofdo
 
 
 .. only::  subproject and html
diff --git a/MAINTAINERS b/MAINTAINERS
index c27f3190737f..62b798c20128 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3678,6 +3678,13 @@ F:	kernel/audit*
 F:	lib/*audit.c
 K:	\baudit_[a-z_0-9]\+\b
 
+AUTOFDO BUILD
+M:	Rong Xu <xur@google.com>
+M:	Han Shen <shenhan@google.com>
+S:	Supported
+F:	Documentation/dev-tools/autofdo.rst
+F:	scripts/Makefile.autofdo
+
 AUXILIARY BUS DRIVER
 M:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 R:	Dave Ertman <david.m.ertman@intel.com>
diff --git a/Makefile b/Makefile
index 187a4ce2728e..55d19c81b382 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 98157b38f5cf..106d09fc42ce 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -811,6 +811,26 @@ config LTO_CLANG_THIN
 	  If unsure, say Y.
 endchoice
 
+config ARCH_SUPPORTS_AUTOFDO_CLANG
+	bool
+
+config AUTOFDO_CLANG
+	bool "Enable Clang's AutoFDO build (EXPERIMENTAL)"
+	depends on ARCH_SUPPORTS_AUTOFDO_CLANG
+	depends on CC_IS_CLANG && CLANG_VERSION >= 170000
+	help
+	  This option enables Clang’s AutoFDO build. When
+	  an AutoFDO profile is specified in variable
+	  CLANG_AUTOFDO_PROFILE during the build process,
+	  Clang uses the profile to optimize the kernel.
+
+	  If no profile is specified, AutoFDO options are
+	  still passed to Clang to facilitate the collection
+	  of perf data for creating an AutoFDO profile in
+	  subsequent builds.
+
+	  If unsure, say N.
+
 config ARCH_SUPPORTS_CFI_CLANG
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2852fcd82cbd..503a0268155a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -126,6 +126,7 @@ config X86
 	select ARCH_SUPPORTS_LTO_CLANG
 	select ARCH_SUPPORTS_LTO_CLANG_THIN
 	select ARCH_SUPPORTS_RT
+	select ARCH_SUPPORTS_AUTOFDO_CLANG
 	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_USE_CMPXCHG_LOCKREF		if X86_CMPXCHG64
 	select ARCH_USE_MEMTEST
diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
new file mode 100644
index 000000000000..1c9f224bc221
--- /dev/null
+++ b/scripts/Makefile.autofdo
@@ -0,0 +1,23 @@
+# 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
+
+# If CONFIG_DEBUG_INFO is not enabled, set -gmlt option.
+ifndef CONFIG_DEBUG_INFO
+  CFLAGS_AUTOFDO_CLANG += -gmlt
+endif
+
+ifdef CLANG_AUTOFDO_PROFILE
+  CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE)
+endif
+
+ifdef CONFIG_LTO_CLANG_THIN
+  ifdef CLANG_AUTOFDO_PROFILE
+    KBUILD_LDFLAGS += --lto-sample-profile=$(CLANG_AUTOFDO_PROFILE)
+  endif
+  KBUILD_LDFLAGS += --mllvm=-enable-fs-discriminator=true --mllvm=-improved-fs-discriminator=true -plugin-opt=thinlto
+endif
+
+export CFLAGS_AUTOFDO_CLANG
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 01a9f567d5af..e85d6ac31bd9 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -191,6 +191,16 @@ _c_flags += $(if $(patsubst n%,, \
 	-D__KCSAN_INSTRUMENT_BARRIERS__)
 endif
 
+#
+# Enable Clang's AutoFDO build flags for a file or directory depending on
+# variables AUTOFDO_PROFILE_obj.o and AUTOFDO_PROFILE.
+#
+ifeq ($(CONFIG_AUTOFDO_CLANG),y)
+_c_flags += $(if $(patsubst n%,, \
+	$(AUTOFDO_PROFILE_$(target-stem).o)$(AUTOFDO_PROFILE)$(is-kernel-object)), \
+	$(CFLAGS_AUTOFDO_CLANG))
+endif
+
 # $(src) for including checkin headers from generated source files
 # $(obj) for including generated headers from checkin source files
 ifeq ($(KBUILD_EXTMOD),)
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 6604f5d038aa..4c5229991e1e 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4557,6 +4557,7 @@ static int validate_ibt(struct objtool_file *file)
 		    !strcmp(sec->name, "__jump_table")			||
 		    !strcmp(sec->name, "__mcount_loc")			||
 		    !strcmp(sec->name, ".kcfi_traps")			||
+		    !strcmp(sec->name, ".llvm.call-graph-profile")	||
 		    strstr(sec->name, "__patchable_function_entries"))
 			continue;
 
-- 
2.46.1.824.gd892dcdcdd-goog

[PATCH v2 2/6] objtool: Fix unreachable instruction warnings for weak funcitons

[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>
---
 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 3d27983dc908..6f64d611faea 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 */
 
-- 
2.46.1.824.gd892dcdcdd-goog

[PATCH v2 3/6] Change the symbols order when --ffuntion-sections is enabled

[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 when the -ffunction-sections flag is enabled.
Specifically, it repositions sections with certain fixed patterns (for
example .text.unlikely) before TEXT_MAIN, ensuring that they are grouped
and matched together.

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.

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>
---
 include/asm-generic/vmlinux.lds.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index eeadbaeccf88..5df589c60401 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -554,9 +554,21 @@
  * 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.
+ * code elimination or function-section is enabled. Match these symbols
+ * first when in these builds.
  */
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
+#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)				\
+		NOINSTR_TEXT						\
+		*(.ref.text)
+#else
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
 		*(.text.hot .text.hot.*)				\
@@ -566,6 +578,7 @@
 		NOINSTR_TEXT						\
 		*(.ref.text)						\
 		*(.text.asan.* .text.tsan.*)
+#endif
 
 
 /* sched.text is aling to function alignment to secure we have same
-- 
2.46.1.824.gd892dcdcdd-goog

[PATCH v2 4/6] 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>
---
 include/asm-generic/vmlinux.lds.h | 37 ++++++++++++++++++++++++-------
 scripts/Makefile.autofdo          |  2 +-
 2 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 5df589c60401..ace617d1af9b 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 LTO_CLANG, the linker also splits sections by default, so we need
+ * these macros to combine the sections during the final link.
+ *
  * 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
@@ -549,6 +556,20 @@
 		__cpuidle_text_end = .;					\
 		__noinstr_text_end = .;
 
+#ifdef CONFIG_AUTOFDO_CLANG
+#define TEXT_HOT							\
+		__hot_text_start = .;					\
+		*(.text.hot .text.hot.*)				\
+		__hot_text_end = .;
+#define TEXT_UNLIKELY							\
+		__unlikely_text_start = .;				\
+		*(.text.unlikely .text.unlikely.*)			\
+		__unlikely_text_end = .;
+#else
+#define TEXT_HOT *(.text.hot .text.hot.*)
+#define TEXT_UNLIKELY *(.text.unlikely .text.unlikely.*)
+#endif
+
 /*
  * .text section. Map to function alignment to avoid address changes
  * during second ld run in second ld pass when generating System.map
@@ -557,30 +578,30 @@
  * code elimination or function-section is enabled. Match these symbols
  * first when in these builds.
  */
-#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_TEXT							\
 		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)
 #else
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
-		*(.text.hot .text.hot.*)				\
+		TEXT_HOT						\
 		*(TEXT_MAIN .text.fixup)				\
-		*(.text.unlikely .text.unlikely.*)			\
+		TEXT_UNLIKELY						\
 		*(.text.unknown .text.unknown.*)			\
 		NOINSTR_TEXT						\
 		*(.ref.text)						\
 		*(.text.asan.* .text.tsan.*)
 #endif
 
-
 /* 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 1c9f224bc221..9c9a530ef090 100644
--- a/scripts/Makefile.autofdo
+++ b/scripts/Makefile.autofdo
@@ -10,7 +10,7 @@ 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
-- 
2.46.1.824.gd892dcdcdd-goog

[PATCH v2 5/6] 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>
---
 include/asm-generic/vmlinux.lds.h | 6 ++++++
 scripts/Makefile.autofdo          | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index ace617d1af9b..20e46c0917db 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -565,9 +565,14 @@ defined(CONFIG_AUTOFDO_CLANG)
 		__unlikely_text_start = .;				\
 		*(.text.unlikely .text.unlikely.*)			\
 		__unlikely_text_end = .;
+#define TEXT_SPLIT							\
+		__split_text_start = .;					\
+		*(.text.split .text.split.[0-9a-zA-Z_]*)		\
+		__split_text_end = .;
 #else
 #define TEXT_HOT *(.text.hot .text.hot.*)
 #define TEXT_UNLIKELY *(.text.unlikely .text.unlikely.*)
+#define TEXT_SPLIT
 #endif
 
 /*
@@ -584,6 +589,7 @@ defined(CONFIG_AUTOFDO_CLANG)
 		ALIGN_FUNCTION();					\
 		*(.text.asan.* .text.tsan.*)				\
 		*(.text.unknown .text.unknown.*)			\
+		TEXT_SPLIT						\
 		TEXT_UNLIKELY						\
 		. = ALIGN(PAGE_SIZE);					\
 		TEXT_HOT						\
diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
index 9c9a530ef090..380042a301cc 100644
--- a/scripts/Makefile.autofdo
+++ b/scripts/Makefile.autofdo
@@ -11,6 +11,7 @@ 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
@@ -18,6 +19,7 @@ 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
-- 
2.46.1.824.gd892dcdcdd-goog

[PATCH v2 6/6] 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
submission is limited to x86 platforms that support PMU features
like LBR on Intel machines and AMD Zen3 BRS.

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

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>
---
 Documentation/dev-tools/index.rst     |   1 +
 Documentation/dev-tools/propeller.rst | 188 ++++++++++++++++++++++++++
 MAINTAINERS                           |   7 +
 Makefile                              |   1 +
 arch/Kconfig                          |  22 +++
 arch/x86/Kconfig                      |   1 +
 arch/x86/kernel/vmlinux.lds.S         |   4 +
 include/asm-generic/vmlinux.lds.h     |  10 +-
 scripts/Makefile.lib                  |  10 ++
 scripts/Makefile.propeller            |  28 ++++
 tools/objtool/check.c                 |   1 +
 11 files changed, 268 insertions(+), 5 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 6945644f7008..3c0ac08b2709 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -35,6 +35,7 @@ Documentation/dev-tools/testing-overview.rst
    checkuapi
    gpio-sloppy-logic-analyzer
    autofdo
+   propeller
 
 
 .. only::  subproject and html
diff --git a/Documentation/dev-tools/propeller.rst b/Documentation/dev-tools/propeller.rst
new file mode 100644
index 000000000000..15ef0e6d973e
--- /dev/null
+++ b/Documentation/dev-tools/propeller.rst
@@ -0,0 +1,188 @@
+.. 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:
+
+   .. code-block:: make
+
+      CONFIG_AUTOFDO_CLANG=y
+      CONFIG_PROPELLER_CLANG=y
+
+Customization
+=============
+
+You can enable or disable Propeller build for individual file and
+directories by adding a line similar to the following to the
+respective kernel Makefile:
+
+- For enabling a single file (e.g. foo.o)
+
+     .. code-block:: make
+
+        PROPELLER_PROFILE_foo.o := y
+
+- For enabling all files in one directory
+
+     .. code-block:: make
+
+        PROPELLER_PROFILE := y
+
+- For disabling one file
+
+     .. code-block:: make
+
+        PROPELLER_PROFILE_foo.o := n
+
+- For disabling all files in one directory
+
+     .. code-block:: make
+
+        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:
+
+      .. code-block:: make
+
+         CONFIG_AUTOFDO_CLANG=y
+         CONFIG_PROPELLER_CLANG=y
+
+   and
+
+      .. code-block:: sh
+
+         $ 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:
+
+      .. code-block:: sh
+
+         $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c \
+           <count> -o <perf_file> -- <loadtest>
+
+   - For AMD platforms:
+
+      .. code-block:: sh
+
+         $ 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.
+
+      .. code-block:: sh
+
+         $ 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:
+
+      .. code-block:: sh
+
+         $ 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.
+
+      .. code-block:: make
+
+         CONFIG_AUTOFDO_CLANG=y
+         CONFIG_PROPELLER_CLANG=y
+
+   and
+
+      .. code-block:: sh
+
+         $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file> CLANG_PROPELLER_PROFILE_PREFIX=<propeller_profile_prefix>
diff --git a/MAINTAINERS b/MAINTAINERS
index 62b798c20128..022b8d233558 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18556,6 +18556,13 @@ 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 55d19c81b382..0fa6d35602a4 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 106d09fc42ce..5aacd9c8a0d2 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -831,6 +831,28 @@ 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 AUTOFDO_CLANG
+	depends on CC_IS_CLANG && CLANG_VERSION >= 190000
+	help
+	  This option enables Clang’s Propeller build which
+	  is on top of AutoFDO 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, Proepller 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 503a0268155a..da47164bfddc 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 6726be89b7a6..7ecc21c569be 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -442,6 +442,10 @@ 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 20e46c0917db..5986dd4cfb14 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 LTO_CLANG, the linker also splits sections by default, so we need
- * these macros to combine the sections during the final link.
+ * CONFIG_AUTOFD_CLANG and CONFIG_PROPELLER_CLANG will also split text sections
+ * and cluster them in the linking time.
  *
  * 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
@@ -556,7 +556,7 @@ defined(CONFIG_AUTOFDO_CLANG)
 		__cpuidle_text_end = .;					\
 		__noinstr_text_end = .;
 
-#ifdef CONFIG_AUTOFDO_CLANG
+#if defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 #define TEXT_HOT							\
 		__hot_text_start = .;					\
 		*(.text.hot .text.hot.*)				\
@@ -584,7 +584,7 @@ defined(CONFIG_AUTOFDO_CLANG)
  * first when in these builds.
  */
 #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_TEXT							\
 		ALIGN_FUNCTION();					\
 		*(.text.asan.* .text.tsan.*)				\
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index e85d6ac31bd9..60354c476956 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -201,6 +201,16 @@ _c_flags += $(if $(patsubst n%,, \
 	$(CFLAGS_AUTOFDO_CLANG))
 endif
 
+#
+# Enable Clang's Propeller build flags for a file or directory depending on
+# variables AUTOFDO_PROPELLER_obj.o and PROPELLER_PROFILE.
+#
+ifeq ($(CONFIG_PROPELLER_CLANG),y)
+_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 000000000000..344190717e47
--- /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 4c5229991e1e..05a0fb4a3d1a 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;
 
-- 
2.46.1.824.gd892dcdcdd-goog

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Peter Zijlstra]

On Wed, Oct 02, 2024 at 04:34:00PM -0700, Rong Xu wrote:
> +Preparation
> +===========
> +
> +Configure the kernel with:
> +
> +   .. code-block:: make
> +
> +      CONFIG_AUTOFDO_CLANG=y
> +
> +Customization
> +=============
> +
> +You can enable or disable AutoFDO build for individual file and directories by
> +adding a line similar to the following to the respective kernel Makefile:
> +
> +- For enabling a single file (e.g. foo.o)
> +
> +     .. code-block:: make
> +
> +        AUTOFDO_PROFILE_foo.o := y
> +
> +- For enabling all files in one directory
> +
> +     .. code-block:: make
> +
> +        AUTOFDO_PROFILE := y
> +
> +- For disabling one file
> +
> +     .. code-block:: make
> +
> +        AUTOFDO_PROFILE_foo.o := n
> +
> +- For disabling all files in one directory
> +
> +     .. code-block:: make
> +
> +        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,
> +
> +      .. code-block:: make
> +
> +         $ make menuconfig LLVM=1
> +
> +
> +    Turn on AutoFDO build config:
> +
> +      .. code-block:: make
> +
> +         CONFIG_AUTOFDO_CLANG=y
> +
> +    With a configuration that with LLVM enabled, use the following command:
> +
> +      .. code-block:: sh
> +
> +         $ scripts/config -e AUTOFDO_CLANG
> +
> +    After getting the config, build with
> +
> +      .. code-block:: make
> +
> +         $ 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:
> +
> +      .. code-block:: sh
> +
> +         $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c <count> -o <perf_file> -- <loadtest>
> +
> +   - For AMD platforms: For Intel platforms:
> +     The supported systems are: Zen3 with BRS, or Zen4 with amd_lbr_v2. To check,
> +     For Zen3:
> +
> +      .. code-block:: sh
> +
> +         $ cat proc/cpuinfo | grep " brs"
> +
> +      For Zen4:
> +
> +      .. code-block:: sh
> +
> +         $ cat proc/cpuinfo | grep amd_lbr_v2
> +
> +      The following command generated the perf data file:
> +
> +      .. code-block:: sh
> +
> +         $ 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.
> +
> +      .. code-block:: sh
> +
> +         $ llvm-profgen --kernel --binary=<vmlinux> --perfdata=<perf_file> -o <profile_file>
> +
> +   or
> +      .. code-block:: sh
> +
> +         $ create_llvm_prof --binary=<vmlinux> --profile=<perf_file> --format=extbinary -o <profile_file>
> +
> +   Note that multiple AutoFDO profile files can be merged into one via:
> +
> +      .. code-block:: sh
> +
> +         $ 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):
> +
> +      .. code-block:: sh
> +
> +         $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file
> +


Can this be done without the endless ... code-block nonsense?

Re: [PATCH v2 3/6] Change the symbols order when --ffuntion-sections is enabled

[Peter Zijlstra]

On Wed, Oct 02, 2024 at 04:34:02PM -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 "..". This patch modifies the order of subsections within the
> text output section when the -ffunction-sections flag is enabled.
> Specifically, it repositions sections with certain fixed patterns (for
> example .text.unlikely) before TEXT_MAIN, ensuring that they are grouped
> and matched together.
> 
> 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.

Is there a down-side to using the new order unconditionally?

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Nick Desaulniers]

On Thu, Oct 3, 2024 at 8:42 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Oct 02, 2024 at 04:34:00PM -0700, Rong Xu wrote:
> > +6) Rebuild the kernel using the AutoFDO profile file with the same config as step 1,
> > +    (Note CONFIG_AUTOFDO_CLANG needs to be enabled):
> > +
> > +      .. code-block:: sh
> > +
> > +         $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file
> > +
>
>
> Can this be done without the endless ... code-block nonsense?

Dunno, I think it looks pretty nice once rendered. Makes it
straightforward for a user to copy+paste. I asked Rong explicitly to
make sure the documentation made it so that non-googler or folks not
working on autofdo or propellor could reproduce (since we'll probably
end up standing up CI for these newer configs, and BOLT).
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Peter Zijlstra]

On Thu, Oct 03, 2024 at 08:51:51AM -0700, Nick Desaulniers wrote:
> On Thu, Oct 3, 2024 at 8:42 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Oct 02, 2024 at 04:34:00PM -0700, Rong Xu wrote:
> > > +6) Rebuild the kernel using the AutoFDO profile file with the same config as step 1,
> > > +    (Note CONFIG_AUTOFDO_CLANG needs to be enabled):
> > > +
> > > +      .. code-block:: sh
> > > +
> > > +         $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file
> > > +
> >
> >
> > Can this be done without the endless ... code-block nonsense?
> 
> Dunno, I think it looks pretty nice once rendered. Makes it

It makes it absolute crap for all of us who 'render' text documents
using less or vi.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Nick Desaulniers]

On Thu, Oct 3, 2024 at 9:03 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Oct 03, 2024 at 08:51:51AM -0700, Nick Desaulniers wrote:
> > On Thu, Oct 3, 2024 at 8:42 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Wed, Oct 02, 2024 at 04:34:00PM -0700, Rong Xu wrote:
> > > > +6) Rebuild the kernel using the AutoFDO profile file with the same config as step 1,
> > > > +    (Note CONFIG_AUTOFDO_CLANG needs to be enabled):
> > > > +
> > > > +      .. code-block:: sh
> > > > +
> > > > +         $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file
> > > > +
> > >
> > >
> > > Can this be done without the endless ... code-block nonsense?
> >
> > Dunno, I think it looks pretty nice once rendered. Makes it
>
> It makes it absolute crap for all of us who 'render' text documents
> using less or vi.

"It hurts when I punch myself in the face."

https://docs.kernel.org/ is where we point people to, and what we
should be optimizing for (IMO).  Jonathan, do we have a policy about
using code-block consistently throughout the kernel docs or not?
Otherwise, this complaint smells like "non-technical non-sense" to me.

-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Peter Zijlstra]

On Thu, Oct 03, 2024 at 09:11:34AM -0700, Nick Desaulniers wrote:

> > It makes it absolute crap for all of us who 'render' text documents
> > using less or vi.
> 
> "It hurts when I punch myself in the face."

Weirdly enough I have a job that entails staring at text documents in
text editors all day every day :-) sorry for thinking that's a sane
thing to do.

Re: [PATCH v2 3/6] Change the symbols order when --ffuntion-sections is enabled

[Rong Xu]

In principle, I don't see a problem using the new order unconditionally.

This new ordering of sections (.text.unlikely, .text.hot, then .text)
differs from the typical user-space ordering when no link-script is
used. Usually, the order is .text, .text.hot, and then .text.unlikely
(when -z keep-text-section-prefix is used).

However, for normal kernel builds that don't use FDO (iFDO and
AutoFDO), this change has minimal impact. This is because the
.text.unlikely section is very small, containing only functions
specifically annotated as cold by the user.

When using FDO, either with iFDO or AutoFDO, this new section ordering
(.text.unlikely, .text.hot, then .text) should be used. These builds
should enable -ffunction-sections and use the new order for
function-level grouping.

This new ordering also affects the placement of ASan and TSan code.
While I expect that this change won't cause issues for them, sanitizer
developers should confirm this.

We've tested this new ordering with iFDO (PGO), AutoFDO, and standard
non-FDO builds. But I think more extensive testing is needed before
using it unconditionally.

-Rong

On Thu, Oct 3, 2024 at 8:43 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Oct 02, 2024 at 04:34:02PM -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 "..". This patch modifies the order of subsections within the
> > text output section when the -ffunction-sections flag is enabled.
> > Specifically, it repositions sections with certain fixed patterns (for
> > example .text.unlikely) before TEXT_MAIN, ensuring that they are grouped
> > and matched together.
> >
> > 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.
>
> Is there a down-side to using the new order unconditionally?

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Rong Xu]

Writing the doc with all these code-blocks was not fun either.
We are happy to change if there is a better way for this.

-Rong

On Thu, Oct 3, 2024 at 9:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Oct 03, 2024 at 09:11:34AM -0700, Nick Desaulniers wrote:
>
> > > It makes it absolute crap for all of us who 'render' text documents
> > > using less or vi.
> >
> > "It hurts when I punch myself in the face."
>
> Weirdly enough I have a job that entails staring at text documents in
> text editors all day every day :-) sorry for thinking that's a sane
> thing to do.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Mike Rapoport]

On Thu, Oct 03, 2024 at 11:20:17AM -0700, Rong Xu wrote:
> Writing the doc with all these code-blocks was not fun either.
> We are happy to change if there is a better way for this.
> 
> -Rong
> 
> On Thu, Oct 3, 2024 at 9:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Oct 03, 2024 at 09:11:34AM -0700, Nick Desaulniers wrote:
> >
> > > > It makes it absolute crap for all of us who 'render' text documents
> > > > using less or vi.
> > >
> > > "It hurts when I punch myself in the face."
> >
> > Weirdly enough I have a job that entails staring at text documents in
> > text editors all day every day :-) sorry for thinking that's a sane
> > thing to do.

Something like this should do:

> +- For enabling a single file (e.g. foo.o)::
> +
> +        AUTOFDO_PROFILE_foo.o := y


-- 
Sincerely yours,
Mike.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Rong Xu]

On Thu, Oct 3, 2024 at 11:09 PM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Thu, Oct 03, 2024 at 11:20:17AM -0700, Rong Xu wrote:
> > Writing the doc with all these code-blocks was not fun either.
> > We are happy to change if there is a better way for this.
> >
> > -Rong
> >
> > On Thu, Oct 3, 2024 at 9:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Thu, Oct 03, 2024 at 09:11:34AM -0700, Nick Desaulniers wrote:
> > >
> > > > > It makes it absolute crap for all of us who 'render' text documents
> > > > > using less or vi.
> > > >
> > > > "It hurts when I punch myself in the face."
> > >
> > > Weirdly enough I have a job that entails staring at text documents in
> > > text editors all day every day :-) sorry for thinking that's a sane
> > > thing to do.
>
> Something like this should do:
>
> > +- For enabling a single file (e.g. foo.o)::
> > +
> > +        AUTOFDO_PROFILE_foo.o := y

Do you mean we don't use ".. code-block:: " here and just use indented
text separated with blank lines?

-Rong

>
>
> --
> Sincerely yours,
> Mike.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Mike Rapoport]

On Fri, Oct 04, 2024 at 09:28:36AM -0700, Rong Xu wrote:
> On Thu, Oct 3, 2024 at 11:09 PM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > On Thu, Oct 03, 2024 at 11:20:17AM -0700, Rong Xu wrote:
> > > Writing the doc with all these code-blocks was not fun either.
> > > We are happy to change if there is a better way for this.
> > >
> > > -Rong
> > >
> > > On Thu, Oct 3, 2024 at 9:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > >
> > > > On Thu, Oct 03, 2024 at 09:11:34AM -0700, Nick Desaulniers wrote:
> > > >
> > > > > > It makes it absolute crap for all of us who 'render' text documents
> > > > > > using less or vi.
> > > > >
> > > > > "It hurts when I punch myself in the face."
> > > >
> > > > Weirdly enough I have a job that entails staring at text documents in
> > > > text editors all day every day :-) sorry for thinking that's a sane
> > > > thing to do.
> >
> > Something like this should do:
> >
> > > +- For enabling a single file (e.g. foo.o)::
> > > +
> > > +        AUTOFDO_PROFILE_foo.o := y
> 
> Do you mean we don't use ".. code-block:: " here and just use indented
> text separated with blank lines?

The double column (::) in the end of a paragraph means that the next paragraph
is a literal block:

https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#literal-blocks

You'd loose the coloring, but other than that it won't be different from
the ".. code-block::" and easier to read in a text form.
 
> -Rong

-- 
Sincerely yours,
Mike.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Rong Xu]

On Fri, Oct 4, 2024 at 9:38 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Fri, Oct 04, 2024 at 09:28:36AM -0700, Rong Xu wrote:
> > On Thu, Oct 3, 2024 at 11:09 PM Mike Rapoport <rppt@kernel.org> wrote:
> > >
> > > On Thu, Oct 03, 2024 at 11:20:17AM -0700, Rong Xu wrote:
> > > > Writing the doc with all these code-blocks was not fun either.
> > > > We are happy to change if there is a better way for this.
> > > >
> > > > -Rong
> > > >
> > > > On Thu, Oct 3, 2024 at 9:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > > >
> > > > > On Thu, Oct 03, 2024 at 09:11:34AM -0700, Nick Desaulniers wrote:
> > > > >
> > > > > > > It makes it absolute crap for all of us who 'render' text documents
> > > > > > > using less or vi.
> > > > > >
> > > > > > "It hurts when I punch myself in the face."
> > > > >
> > > > > Weirdly enough I have a job that entails staring at text documents in
> > > > > text editors all day every day :-) sorry for thinking that's a sane
> > > > > thing to do.
> > >
> > > Something like this should do:
> > >
> > > > +- For enabling a single file (e.g. foo.o)::
> > > > +
> > > > +        AUTOFDO_PROFILE_foo.o := y
> >
> > Do you mean we don't use ".. code-block:: " here and just use indented
> > text separated with blank lines?
>
> The double column (::) in the end of a paragraph means that the next paragraph
> is a literal block:
>
> https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#literal-blocks
>
> You'd loose the coloring, but other than that it won't be different from
> the ".. code-block::" and easier to read in a text form.

Sorry that I did not notice the (::) in your suggestion.

This is great. I'll change the docs in this patch to use this method.

-Rong
>
> > -Rong
>
> --
> Sincerely yours,
> Mike.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Kees Cook]

On Thu, Oct 03, 2024 at 05:41:43PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 02, 2024 at 04:34:00PM -0700, Rong Xu wrote:
> > +Preparation
> > +===========
> > +
> > +Configure the kernel with:
> > +
> > +   .. code-block:: make
> > +
> > +      CONFIG_AUTOFDO_CLANG=y
> > +
> > +
> [...]
> > +    With a configuration that with LLVM enabled, use the following command:
> > +
> > +      .. code-block:: sh
> > +
> > +         $ scripts/config -e AUTOFDO_CLANG
> [...]
> 
> Can this be done without the endless ... code-block nonsense?

The tradition in kernel .rst is to do this with the trailing "::", e.g.:

+Configure the kernel with::
+
+     CONFIG_AUTOFDO_CLANG=y

This loses the language-specific highlighting when rendered. Perhaps the
"::" extension can be further extended?

+Configure the kernel with::(make)
+
+     CONFIG_AUTOFDO_CLANG=y

Then we could avoid the extra 2 lines but still gain the rendered language
highlights?

-- 
Kees Cook

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Peter Zijlstra]

On Fri, Oct 04, 2024 at 11:10:04AM -0700, Kees Cook wrote:

> +Configure the kernel with::(make)
> +
> +     CONFIG_AUTOFDO_CLANG=y
> 
> Then we could avoid the extra 2 lines but still gain the rendered language
> highlights?

The whole double-colon thing is already a pain to read; you're making it
worse again.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Justin Stitt]

On Fri, Oct 4, 2024 at 11:29 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Oct 04, 2024 at 11:10:04AM -0700, Kees Cook wrote:
>
> > +Configure the kernel with::(make)
> > +
> > +     CONFIG_AUTOFDO_CLANG=y
> >
> > Then we could avoid the extra 2 lines but still gain the rendered language
> > highlights?
>
> The whole double-colon thing is already a pain to read; you're making it
> worse again.

Lots of people read docs on the web and having code blocks with
monospaced fonts (+syntax highlighting) makes them easier to read
there.

Configure the kernel with:

     CONFIG_AUTOFDO_CLANG=y

--versus--

Configure the kernel with::

     CONFIG_AUTOFDO_CLANG=y

This renders better for html through Sphinx and really can't be that
bad to read in vim, can it?

two cents and such,
Justin

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Peter Zijlstra]

On Fri, Oct 04, 2024 at 02:23:45PM -0700, Justin Stitt wrote:
> On Fri, Oct 4, 2024 at 11:29 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, Oct 04, 2024 at 11:10:04AM -0700, Kees Cook wrote:
> >
> > > +Configure the kernel with::(make)
> > > +
> > > +     CONFIG_AUTOFDO_CLANG=y
> > >
> > > Then we could avoid the extra 2 lines but still gain the rendered language
> > > highlights?
> >
> > The whole double-colon thing is already a pain to read; you're making it
> > worse again.
> 
> Lots of people read docs on the web and having code blocks with
> monospaced fonts (+syntax highlighting) makes them easier to read
> there.
> 
> Configure the kernel with:
> 
>      CONFIG_AUTOFDO_CLANG=y
> 
> --versus--
> 
> Configure the kernel with::
> 
>      CONFIG_AUTOFDO_CLANG=y
> 
> This renders better for html through Sphinx and really can't be that
> bad to read in vim, can it?

It's weird; but I've gotten sorta used to it. I was more or less
commenting on Kees' proposal of making it:

  Configure the kernel with::(make)

Which is definitely harder to read since it insta triggers a syntax
error exception in my head.

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Jonathan Corbet]

Kees Cook <kees@kernel.org> writes:

> The tradition in kernel .rst is to do this with the trailing "::", e.g.:
>
> +Configure the kernel with::
> +
> +     CONFIG_AUTOFDO_CLANG=y
>
> This loses the language-specific highlighting when rendered. Perhaps the
> "::" extension can be further extended?
>
> +Configure the kernel with::(make)
> +
> +     CONFIG_AUTOFDO_CLANG=y
>
> Then we could avoid the extra 2 lines but still gain the rendered language
> highlights?

The :: notation is standard Sphinx, not an extension we have done.  So
the proposed syntax would have to be done from the beginning, I'm not
sure how easy or hard that would be, or whether it would be worth it.
But then, I've always seen relatively little value in the highlighting;
others clearly differ.

Thanks,

jon

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Rong Xu]

[-- Attachment #1: Type: text/plain, Size: 1622 bytes --]

I removed the "code-block" directives from the rst files,
and used "::" suggested by Jonathan. The rst files themselves are now
easier to read in vi.  However, the rendered HTML output has some differences:
(1) The text that was previously in code-block no longer indents. It aligns
      with the preceding text, regardless of how many spaces I add.
(2) Previously, "code-block" removed '\' and combined the text into a
single line.
     This is no longer happening -- the '\' is not expanded.

These differences do not seem to be a blocker. I'm attaching the html files to
this email. If there is no objection, I'll change to using the new
method of "::".

-Rong

On Sat, Oct 5, 2024 at 7:43 AM Jonathan Corbet <corbet@lwn.net> wrote:
>
> Kees Cook <kees@kernel.org> writes:
>
> > The tradition in kernel .rst is to do this with the trailing "::", e.g.:
> >
> > +Configure the kernel with::
> > +
> > +     CONFIG_AUTOFDO_CLANG=y
> >
> > This loses the language-specific highlighting when rendered. Perhaps the
> > "::" extension can be further extended?
> >
> > +Configure the kernel with::(make)
> > +
> > +     CONFIG_AUTOFDO_CLANG=y
> >
> > Then we could avoid the extra 2 lines but still gain the rendered language
> > highlights?
>
> The :: notation is standard Sphinx, not an extension we have done.  So
> the proposed syntax would have to be done from the beginning, I'm not
> sure how easy or hard that would be, or whether it would be worth it.
> But then, I've always seen relatively little value in the highlighting;
> others clearly differ.
>
> Thanks,
>
> jon

[-- Attachment #2: autofdo_before.html --]
[-- Type: text/html, Size: 18398 bytes --]

[-- Attachment #3: autofdo_after.html --]
[-- Type: text/html, Size: 18065 bytes --]

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Miguel Ojeda]

On Mon, Oct 7, 2024 at 8:04 PM Rong Xu <xur@google.com> wrote:
>
> I removed the "code-block" directives from the rst files,
> and used "::" suggested by Jonathan. The rst files themselves are now

I think it was Mike.

> (1) The text that was previously in code-block no longer indents. It aligns
>       with the preceding text, regardless of how many spaces I add.

Did you try with a tab? At least in Doc/rust/ all those three ways
(i.e. `::`, `.. code-block::` and a single `:` for indented non-code)
seem to work fine, e.g. the following document uses all of them:

    https://docs.kernel.org/rust/coding-guidelines.html

I hope that helps!

Cheers,
Miguel

Re: [PATCH v2 1/6] Add AutoFDO support for Clang build

[Rong Xu]

On Mon, Oct 7, 2024 at 11:33 AM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Mon, Oct 7, 2024 at 8:04 PM Rong Xu <xur@google.com> wrote:
> >
> > I removed the "code-block" directives from the rst files,
> > and used "::" suggested by Jonathan. The rst files themselves are now
>
> I think it was Mike.

Sorry for mixing up the names.

>
> > (1) The text that was previously in code-block no longer indents. It aligns
> >       with the preceding text, regardless of how many spaces I add.
>
> Did you try with a tab? At least in Doc/rust/ all those three ways
> (i.e. `::`, `.. code-block::` and a single `:` for indented non-code)
> seem to work fine, e.g. the following document uses all of them:
>
>     https://docs.kernel.org/rust/coding-guidelines.html

I looked at run/coding-guidelines.html generated by "make htmldocs".
It has the same issue
of indentation for "::" text. I also tried tab, nothing changed for me.

-Rong

>
> I hope that helps!
>
> Cheers,
> Miguel