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Date:   Sun, 28 Feb 2021 13:50:58 -0800
From:   Fangrui Song <maskray@google.com>
To:     Bill Wendling <morbo@google.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH v8] pgo: add clang's Profile Guided Optimization
 infrastructure
Message-ID: <20210228215058.r5425p6zidwolhw7@google.com>
References: <20210122101156.3257143-1-morbo@google.com>
 <20210226222030.3718075-1-morbo@google.com>
 <20210228185214.sdmpytoh37nyvwgm@google.com>
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On 2021-02-28, Fangrui Song wrote:
>Reviewed-by: Fangrui Song <maskray@google.com>
>
>Some minor items below:
>
>On 2021-02-26, 'Bill Wendling' via Clang Built Linux wrote:
>>From: Sami Tolvanen <samitolvanen@google.com>
>>
>>Enable the use of clang's Profile-Guided Optimization[1]. To generate a
>>profile, the kernel is instrumented with PGO counters, a representative
>>workload is run, and the raw profile data is collected from
>>/sys/kernel/debug/pgo/profraw.
>>
>>The raw profile data must be processed by clang's "llvm-profdata" tool
>>before it can be used during recompilation:
>>
>> $ cp /sys/kernel/debug/pgo/profraw vmlinux.profraw
>> $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
>>
>>Multiple raw profiles may be merged during this step.
>>
>>The data can now be used by the compiler:
>>
>> $ make LLVM=1 KCFLAGS=-fprofile-use=vmlinux.profdata ...
>>
>>This initial submission is restricted to x86, as that's the platform we
>>know works. This restriction can be lifted once other platforms have
>>been verified to work with PGO.
>>
>>Note that this method of profiling the kernel is clang-native, unlike
>>the clang support in kernel/gcov.
>>
>>[1] https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
>>
>>Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
>>Co-developed-by: Bill Wendling <morbo@google.com>
>>Signed-off-by: Bill Wendling <morbo@google.com>
>>---
>>v8: - Rebased on top-of-tree.
>>v7: - Fix minor build failure reported by Sedat.
>>v6: - Add better documentation about the locking scheme and other things.
>>   - Rename macros to better match the same macros in LLVM's source code.
>>v5: - Correct padding calculation, discovered by Nathan Chancellor.
>>v4: - Remove non-x86 Makfile changes and se "hweight64" instead of using our
>>     own popcount implementation, based on Nick Desaulniers's comment.
>>v3: - Added change log section based on Sedat Dilek's comments.
>>v2: - Added "__llvm_profile_instrument_memop" based on Nathan Chancellor's
>>     testing.
>>   - Corrected documentation, re PGO flags when using LTO, based on Fangrui
>>     Song's comments.
>>---
>>Documentation/dev-tools/index.rst     |   1 +
>>Documentation/dev-tools/pgo.rst       | 127 +++++++++
>>MAINTAINERS                           |   9 +
>>Makefile                              |   3 +
>>arch/Kconfig                          |   1 +
>>arch/x86/Kconfig                      |   1 +
>>arch/x86/boot/Makefile                |   1 +
>>arch/x86/boot/compressed/Makefile     |   1 +
>>arch/x86/crypto/Makefile              |   4 +
>>arch/x86/entry/vdso/Makefile          |   1 +
>>arch/x86/kernel/vmlinux.lds.S         |   2 +
>>arch/x86/platform/efi/Makefile        |   1 +
>>arch/x86/purgatory/Makefile           |   1 +
>>arch/x86/realmode/rm/Makefile         |   1 +
>>arch/x86/um/vdso/Makefile             |   1 +
>>drivers/firmware/efi/libstub/Makefile |   1 +
>>include/asm-generic/vmlinux.lds.h     |  44 +++
>>kernel/Makefile                       |   1 +
>>kernel/pgo/Kconfig                    |  35 +++
>>kernel/pgo/Makefile                   |   5 +
>>kernel/pgo/fs.c                       | 389 ++++++++++++++++++++++++++
>>kernel/pgo/instrument.c               | 189 +++++++++++++
>>kernel/pgo/pgo.h                      | 203 ++++++++++++++
>>scripts/Makefile.lib                  |  10 +
>>24 files changed, 1032 insertions(+)
>>create mode 100644 Documentation/dev-tools/pgo.rst
>>create mode 100644 kernel/pgo/Kconfig
>>create mode 100644 kernel/pgo/Makefile
>>create mode 100644 kernel/pgo/fs.c
>>create mode 100644 kernel/pgo/instrument.c
>>create mode 100644 kernel/pgo/pgo.h
>>
>>diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
>>index f7809c7b1ba9..8d6418e85806 100644
>>--- a/Documentation/dev-tools/index.rst
>>+++ b/Documentation/dev-tools/index.rst
>>@@ -26,6 +26,7 @@ whole; patches welcome!
>>   kgdb
>>   kselftest
>>   kunit/index
>>+   pgo
>>
>>
>>.. only::  subproject and html
>>diff --git a/Documentation/dev-tools/pgo.rst b/Documentation/dev-tools/pgo.rst
>>new file mode 100644
>>index 000000000000..b7f11d8405b7
>>--- /dev/null
>>+++ b/Documentation/dev-tools/pgo.rst
>>@@ -0,0 +1,127 @@
>>+.. SPDX-License-Identifier: GPL-2.0
>>+
>>+===============================
>>+Using PGO with the Linux kernel
>>+===============================
>>+
>>+Clang's profiling kernel support (PGO_) enables profiling of the Linux kernel
>>+when building with Clang. The profiling data is exported via the ``pgo``
>>+debugfs directory.
>>+
>>+.. _PGO: https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
>>+
>>+
>>+Preparation
>>+===========
>>+
>>+Configure the kernel with:
>>+
>>+.. code-block:: make
>>+
>>+   CONFIG_DEBUG_FS=y
>>+   CONFIG_PGO_CLANG=y
>>+
>>+Note that kernels compiled with profiling flags will be significantly larger
>>+and run slower.
>>+
>>+Profiling data will only become accessible once debugfs has been mounted:
>>+
>>+.. code-block:: sh
>>+
>>+   mount -t debugfs none /sys/kernel/debug
>>+
>>+
>>+Customization
>>+=============
>>+
>>+You can enable or disable profiling for individual file and directories by
>>+adding a line similar to the following to the respective kernel Makefile:
>>+
>>+- For a single file (e.g. main.o)
>>+
>>+  .. code-block:: make
>>+
>>+     PGO_PROFILE_main.o := y
>>+
>>+- For all files in one directory
>>+
>>+  .. code-block:: make
>>+
>>+     PGO_PROFILE := y
>>+
>>+To exclude files from being profiled use
>>+
>>+  .. code-block:: make
>>+
>>+     PGO_PROFILE_main.o := n
>>+
>>+and
>>+
>>+  .. code-block:: make
>>+
>>+     PGO_PROFILE := n
>>+
>>+Only files which are linked to the main kernel image or are compiled as kernel
>>+modules are supported by this mechanism.
>>+
>>+
>>+Files
>>+=====
>>+
>>+The PGO kernel support creates the following files in debugfs:
>>+
>>+``/sys/kernel/debug/pgo``
>>+	Parent directory for all PGO-related files.
>>+
>>+``/sys/kernel/debug/pgo/reset``
>>+	Global reset file: resets all coverage data to zero when written to.
>>+
>>+``/sys/kernel/debug/profraw``
>>+	The raw PGO data that must be processed with ``llvm_profdata``.
>>+
>>+
>>+Workflow
>>+========
>>+
>>+The PGO kernel can be run on the host or test machines. The data though should
>>+be analyzed with Clang's tools from the same Clang version as the kernel was
>>+compiled. Clang's tolerant of version skew, but it's easier to use the same
>>+Clang version.
>>+
>>+The profiling data is useful for optimizing the kernel, analyzing coverage,
>>+etc. Clang offers tools to perform these tasks.
>>+
>>+Here is an example workflow for profiling an instrumented kernel with PGO and
>>+using the result to optimize the kernel:
>>+
>>+1) Install the kernel on the TEST machine.
>>+
>>+2) Reset the data counters right before running the load tests
>>+
>>+   .. code-block:: sh
>>+
>>+      $ echo 1 > /sys/kernel/debug/pgo/reset
>>+
>>+3) Run the load tests.
>>+
>>+4) Collect the raw profile data
>>+
>>+   .. code-block:: sh
>>+
>>+      $ cp -a /sys/kernel/debug/pgo/profraw /tmp/vmlinux.profraw
>>+
>>+5) (Optional) Download the raw profile data to the HOST machine.
>>+
>>+6) Process the raw profile data
>>+
>>+   .. code-block:: sh
>>+
>>+      $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
>>+
>>+   Note that multiple raw profile data files can be merged during this step.
>>+
>>+7) Rebuild the kernel using the profile data (PGO disabled)
>>+
>>+   .. code-block:: sh
>>+
>>+      $ make LLVM=1 KCFLAGS=-fprofile-use=vmlinux.profdata ...
>>diff --git a/MAINTAINERS b/MAINTAINERS
>>index c71664ca8bfd..3a6668792bc5 100644
>>--- a/MAINTAINERS
>>+++ b/MAINTAINERS
>>@@ -14019,6 +14019,15 @@ S:	Maintained
>>F:	include/linux/personality.h
>>F:	include/uapi/linux/personality.h
>>
>>+PGO BASED KERNEL PROFILING
>>+M:	Sami Tolvanen <samitolvanen@google.com>
>>+M:	Bill Wendling <wcw@google.com>
>>+R:	Nathan Chancellor <natechancellor@gmail.com>
>>+R:	Nick Desaulniers <ndesaulniers@google.com>
>>+S:	Supported
>>+F:	Documentation/dev-tools/pgo.rst
>>+F:	kernel/pgo
>>+
>>PHOENIX RC FLIGHT CONTROLLER ADAPTER
>>M:	Marcus Folkesson <marcus.folkesson@gmail.com>
>>L:	linux-input@vger.kernel.org
>>diff --git a/Makefile b/Makefile
>>index 6ecd0d22e608..b57d4d44c799 100644
>>--- a/Makefile
>>+++ b/Makefile
>>@@ -657,6 +657,9 @@ endif # KBUILD_EXTMOD
>># Defaults to vmlinux, but the arch makefile usually adds further targets
>>all: vmlinux
>>
>>+CFLAGS_PGO_CLANG := -fprofile-generate
>>+export CFLAGS_PGO_CLANG
>>+
>>CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage \
>>	$(call cc-option,-fno-tree-loop-im) \
>>	$(call cc-disable-warning,maybe-uninitialized,)
>>diff --git a/arch/Kconfig b/arch/Kconfig
>>index 2bb30673d8e6..111e642a2af7 100644
>>--- a/arch/Kconfig
>>+++ b/arch/Kconfig
>>@@ -1192,6 +1192,7 @@ config ARCH_HAS_ELFCORE_COMPAT
>>	bool
>>
>>source "kernel/gcov/Kconfig"
>>+source "kernel/pgo/Kconfig"
>>
>>source "scripts/gcc-plugins/Kconfig"
>>
>>diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>index cd4b9b1204a8..c9808583b528 100644
>>--- a/arch/x86/Kconfig
>>+++ b/arch/x86/Kconfig
>>@@ -99,6 +99,7 @@ config X86
>>	select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP	if NR_CPUS <= 4096
>>	select ARCH_SUPPORTS_LTO_CLANG		if X86_64
>>	select ARCH_SUPPORTS_LTO_CLANG_THIN	if X86_64
>>+	select ARCH_SUPPORTS_PGO_CLANG		if X86_64
>>	select ARCH_USE_BUILTIN_BSWAP
>>	select ARCH_USE_QUEUED_RWLOCKS
>>	select ARCH_USE_QUEUED_SPINLOCKS
>>diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
>>index fe605205b4ce..383853e32f67 100644
>>--- a/arch/x86/boot/Makefile
>>+++ b/arch/x86/boot/Makefile
>>@@ -71,6 +71,7 @@ KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
>>KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>>KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
>>GCOV_PROFILE := n
>>+PGO_PROFILE := n
>>UBSAN_SANITIZE := n
>>
>>$(obj)/bzImage: asflags-y  := $(SVGA_MODE)
>>diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
>>index e0bc3988c3fa..ed12ab65f606 100644
>>--- a/arch/x86/boot/compressed/Makefile
>>+++ b/arch/x86/boot/compressed/Makefile
>>@@ -54,6 +54,7 @@ CFLAGS_sev-es.o += -I$(objtree)/arch/x86/lib/
>>
>>KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
>>GCOV_PROFILE := n
>>+PGO_PROFILE := n
>>UBSAN_SANITIZE :=n
>>
>>KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
>>diff --git a/arch/x86/crypto/Makefile b/arch/x86/crypto/Makefile
>>index b28e36b7c96b..4b2e9620c412 100644
>>--- a/arch/x86/crypto/Makefile
>>+++ b/arch/x86/crypto/Makefile
>>@@ -4,6 +4,10 @@
>>
>>OBJECT_FILES_NON_STANDARD := y
>>
>>+# Disable PGO for curve25519-x86_64. With PGO enabled, clang runs out of
>>+# registers for some of the functions.
>>+PGO_PROFILE_curve25519-x86_64.o := n
>>+
>>obj-$(CONFIG_CRYPTO_TWOFISH_586) += twofish-i586.o
>>twofish-i586-y := twofish-i586-asm_32.o twofish_glue.o
>>obj-$(CONFIG_CRYPTO_TWOFISH_X86_64) += twofish-x86_64.o
>>diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
>>index 05c4abc2fdfd..f7421e44725a 100644
>>--- a/arch/x86/entry/vdso/Makefile
>>+++ b/arch/x86/entry/vdso/Makefile
>>@@ -180,6 +180,7 @@ quiet_cmd_vdso = VDSO    $@
>>VDSO_LDFLAGS = -shared --hash-style=both --build-id=sha1 \
>>	$(call ld-option, --eh-frame-hdr) -Bsymbolic
>>GCOV_PROFILE := n
>>+PGO_PROFILE := n
>>
>>quiet_cmd_vdso_and_check = VDSO    $@
>>      cmd_vdso_and_check = $(cmd_vdso); $(cmd_vdso_check)
>>diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
>>index efd9e9ea17f2..f6cab2316c46 100644
>>--- a/arch/x86/kernel/vmlinux.lds.S
>>+++ b/arch/x86/kernel/vmlinux.lds.S
>>@@ -184,6 +184,8 @@ SECTIONS
>>
>>	BUG_TABLE
>>
>>+	PGO_CLANG_DATA
>>+
>>	ORC_UNWIND_TABLE
>>
>>	. = ALIGN(PAGE_SIZE);
>>diff --git a/arch/x86/platform/efi/Makefile b/arch/x86/platform/efi/Makefile
>>index 84b09c230cbd..5f22b31446ad 100644
>>--- a/arch/x86/platform/efi/Makefile
>>+++ b/arch/x86/platform/efi/Makefile
>>@@ -2,6 +2,7 @@
>>OBJECT_FILES_NON_STANDARD_efi_thunk_$(BITS).o := y
>>KASAN_SANITIZE := n
>>GCOV_PROFILE := n
>>+PGO_PROFILE := n
>>
>>obj-$(CONFIG_EFI) 		+= quirks.o efi.o efi_$(BITS).o efi_stub_$(BITS).o
>>obj-$(CONFIG_EFI_MIXED)		+= efi_thunk_$(BITS).o
>>diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
>>index 95ea17a9d20c..36f20e99da0b 100644
>>--- a/arch/x86/purgatory/Makefile
>>+++ b/arch/x86/purgatory/Makefile
>>@@ -23,6 +23,7 @@ targets += purgatory.ro purgatory.chk
>>
>># Sanitizer, etc. runtimes are unavailable and cannot be linked here.
>>GCOV_PROFILE	:= n
>>+PGO_PROFILE	:= n
>>KASAN_SANITIZE	:= n
>>UBSAN_SANITIZE	:= n
>>KCSAN_SANITIZE	:= n
>>diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
>>index 83f1b6a56449..21797192f958 100644
>>--- a/arch/x86/realmode/rm/Makefile
>>+++ b/arch/x86/realmode/rm/Makefile
>>@@ -76,4 +76,5 @@ KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP -D_WAKEUP \
>>KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
>>KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
>>GCOV_PROFILE := n
>>+PGO_PROFILE := n
>>UBSAN_SANITIZE := n
>>diff --git a/arch/x86/um/vdso/Makefile b/arch/x86/um/vdso/Makefile
>>index 5943387e3f35..54f5768f5853 100644
>>--- a/arch/x86/um/vdso/Makefile
>>+++ b/arch/x86/um/vdso/Makefile
>>@@ -64,6 +64,7 @@ quiet_cmd_vdso = VDSO    $@
>>
>>VDSO_LDFLAGS = -fPIC -shared -Wl,--hash-style=sysv
>>GCOV_PROFILE := n
>>+PGO_PROFILE := n
>>
>>#
>># Install the unstripped copy of vdso*.so listed in $(vdso-install-y).
>>diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
>>index c23466e05e60..724fb389bb9d 100644
>>--- a/drivers/firmware/efi/libstub/Makefile
>>+++ b/drivers/firmware/efi/libstub/Makefile
>>@@ -42,6 +42,7 @@ KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
>>KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))
>>
>>GCOV_PROFILE			:= n
>>+PGO_PROFILE			:= n
>># Sanitizer runtimes are unavailable and cannot be linked here.
>>KASAN_SANITIZE			:= n
>>KCSAN_SANITIZE			:= n
>>diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>>index 6786f8c0182f..4a0c21b840b3 100644
>>--- a/include/asm-generic/vmlinux.lds.h
>>+++ b/include/asm-generic/vmlinux.lds.h
>>@@ -329,6 +329,49 @@
>>#define DTPM_TABLE()
>>#endif
>>
>>+#ifdef CONFIG_PGO_CLANG
>>+#define PGO_CLANG_DATA							\
>>+	__llvm_prf_data : AT(ADDR(__llvm_prf_data) - LOAD_OFFSET) {	\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_start = .;					\
>>+		__llvm_prf_data_start = .;				\
>>+		KEEP(*(__llvm_prf_data))				\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_data_end = .;				\
>>+	}								\
>
>Some minor items on linker script usage. The end of a metadata section
>usually does not need alignment.  Does the . = ALIGN(8) have
>significance? Ditto below.
>
>
>
>This is an item about LD_DEAD_CODE_DATA_ELIMINATION. Feel free to
>postpone after this patch is in tree:
>
>		KEEP(*(__llvm_prf_data))
>
>KEEP should be dropped.
>
>I have been involved in improving GC (my recent interests on such
>metadata sections :)
>https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order)
>
>With LLVM>=13 (https://reviews.llvm.org/D96757), __llvm_prf_* associated
>to non-COMDAT text sections can be GCed as well.  KEEP would
>unnecessarily retain them under LD_DEAD_CODE_DATA_ELIMINATION.
>
>For older releases (at least 10<=LLVM<13), such __llvm_prf_* sections
>are not in zero flag section groups so they usually cannot be discarded.
>So perhaps with KEEP or without KEEP, you won't find many size
>differences.
>
>>+	__llvm_prf_cnts : AT(ADDR(__llvm_prf_cnts) - LOAD_OFFSET) {	\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_cnts_start = .;				\
>>+		KEEP(*(__llvm_prf_cnts))				\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_cnts_end = .;				\
>>+	}								\
>>+	__llvm_prf_names : AT(ADDR(__llvm_prf_names) - LOAD_OFFSET) {	\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_names_start = .;				\
>>+		KEEP(*(__llvm_prf_names))				\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_names_end = .;				\
>>+		. = ALIGN(8);						\
>>+	}								\
>
>__llvm_prf_names does not need alignment.
>It is often 1 in userspace programs.
>
>>+	__llvm_prf_vals : AT(ADDR(__llvm_prf_vals) - LOAD_OFFSET) {	\
>>+		__llvm_prf_vals_start = .;				\
>>+		KEEP(*(__llvm_prf_vals))				\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_vals_end = .;				\
>>+		. = ALIGN(8);						\
>>+	}								\
>>+	__llvm_prf_vnds : AT(ADDR(__llvm_prf_vnds) - LOAD_OFFSET) {	\
>>+		__llvm_prf_vnds_start = .;				\
>>+		KEEP(*(__llvm_prf_vnds))				\
>>+		. = ALIGN(8);						\
>>+		__llvm_prf_vnds_end = .;				\
>>+		__llvm_prf_end = .;					\
>>+	}
>
>In userspace PGO instrumentation, the start is often aligned by 16.
>The end does not need alignment.

Thinking more, my suggestion is to drop explicit alignment annotations
entirely:


	__llvm_prf_vals : AT(ADDR(__llvm_prf_vals) - LOAD_OFFSET) {	\
		__llvm_prf_vals_start = .;				\
		*(__llvm_prf_vals)					\
		__llvm_prf_vals_end = .;				\
	}								\

	__llvm_prf_vnds : AT(ADDR(__llvm_prf_vnds) - LOAD_OFFSET) {	\
		__llvm_prf_vnds_start = .;				\
		*(__llvm_prf_vnds)					\
		__llvm_prf_vnds_end = .;				\
		__llvm_prf_end = .;					\
	}

// _cnts, _names and _data are similar. Just delete all ALIGN.
// I deleted KEEP above to facilitate --gc-sections as well.


Let the linker figure out the alignments in input sections and the
output section alignment
(https://lld.llvm.org/ELF/linker_script.html#output-section-alignment).

Omitting alignment is probably preferable in most cases, unless no input
section is present (either not emitted at all or all discarded by ld
--gc-sections) (very rare event, happened with commit 793f49a87aae
("firmware_loader: align .builtin_fw to 8"), but that case unlikely
happens with PGO).

>

>>+#else
>>+#define PGO_CLANG_DATA
>>+#endif
>>+
>>#define KERNEL_DTB()							\
>>	STRUCT_ALIGN();							\
>>	__dtb_start = .;						\
>>@@ -1105,6 +1148,7 @@
>>		CONSTRUCTORS						\
>>	}								\
>>	BUG_TABLE							\
>>+	PGO_CLANG_DATA
>>
>>#define INIT_TEXT_SECTION(inittext_align)				\
>>	. = ALIGN(inittext_align);					\
>>diff --git a/kernel/Makefile b/kernel/Makefile
>>index 320f1f3941b7..a2a23ef2b12f 100644
>>--- a/kernel/Makefile
>>+++ b/kernel/Makefile
>>@@ -111,6 +111,7 @@ obj-$(CONFIG_BPF) += bpf/
>>obj-$(CONFIG_KCSAN) += kcsan/
>>obj-$(CONFIG_SHADOW_CALL_STACK) += scs.o
>>obj-$(CONFIG_HAVE_STATIC_CALL_INLINE) += static_call.o
>>+obj-$(CONFIG_PGO_CLANG) += pgo/
>>
>>obj-$(CONFIG_PERF_EVENTS) += events/
>>
>>diff --git a/kernel/pgo/Kconfig b/kernel/pgo/Kconfig
>>new file mode 100644
>>index 000000000000..76a640b6cf6e
>>--- /dev/null
>>+++ b/kernel/pgo/Kconfig
>>@@ -0,0 +1,35 @@
>>+# SPDX-License-Identifier: GPL-2.0-only
>>+menu "Profile Guided Optimization (PGO) (EXPERIMENTAL)"
>>+
>>+config ARCH_SUPPORTS_PGO_CLANG
>>+	bool
>>+
>>+config PGO_CLANG
>>+	bool "Enable clang's PGO-based kernel profiling"
>>+	depends on DEBUG_FS
>>+	depends on ARCH_SUPPORTS_PGO_CLANG
>>+	depends on CC_IS_CLANG && CLANG_VERSION >= 120000
>>+	help
>>+	  This option enables clang's PGO (Profile Guided Optimization) based
>>+	  code profiling to better optimize the kernel.
>>+
>>+	  If unsure, say N.
>>+
>>+	  Run a representative workload for your application on a kernel
>>+	  compiled with this option and download the raw profile file from
>>+	  /sys/kernel/debug/pgo/profraw. This file needs to be processed with
>>+	  llvm-profdata. It may be merged with other collected raw profiles.
>>+
>>+	  Copy the resulting profile file into vmlinux.profdata, and enable
>>+	  KCFLAGS=-fprofile-use=vmlinux.profdata to produce an optimized
>>+	  kernel.
>>+
>>+	  Note that a kernel compiled with profiling flags will be
>>+	  significantly larger and run slower. Also be sure to exclude files
>>+	  from profiling which are not linked to the kernel image to prevent
>>+	  linker errors.
>>+
>>+	  Note that the debugfs filesystem has to be mounted to access
>>+	  profiling data.
>>+
>>+endmenu
>>diff --git a/kernel/pgo/Makefile b/kernel/pgo/Makefile
>>new file mode 100644
>>index 000000000000..41e27cefd9a4
>>--- /dev/null
>>+++ b/kernel/pgo/Makefile
>>@@ -0,0 +1,5 @@
>>+# SPDX-License-Identifier: GPL-2.0
>>+GCOV_PROFILE	:= n
>>+PGO_PROFILE	:= n
>>+
>>+obj-y	+= fs.o instrument.o
>>diff --git a/kernel/pgo/fs.c b/kernel/pgo/fs.c
>>new file mode 100644
>>index 000000000000..1678df3b7d64
>>--- /dev/null
>>+++ b/kernel/pgo/fs.c
>>@@ -0,0 +1,389 @@
>>+// SPDX-License-Identifier: GPL-2.0
>>+/*
>>+ * Copyright (C) 2019 Google, Inc.
>>+ *
>>+ * Author:
>>+ *	Sami Tolvanen <samitolvanen@google.com>
>>+ *
>>+ * This software is licensed under the terms of the GNU General Public
>>+ * License version 2, as published by the Free Software Foundation, and
>>+ * may be copied, distributed, and modified under those terms.
>>+ *
>>+ * This program is distributed in the hope that it will be useful,
>>+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>+ * GNU General Public License for more details.
>>+ *
>>+ */
>>+
>>+#define pr_fmt(fmt)	"pgo: " fmt
>>+
>>+#include <linux/kernel.h>
>>+#include <linux/debugfs.h>
>>+#include <linux/fs.h>
>>+#include <linux/module.h>
>>+#include <linux/slab.h>
>>+#include <linux/vmalloc.h>
>>+#include "pgo.h"
>>+
>>+static struct dentry *directory;
>>+
>>+struct prf_private_data {
>>+	void *buffer;
>>+	unsigned long size;
>>+};
>>+
>>+/*
>>+ * Raw profile data format:
>>+ *
>>+ *	- llvm_prf_header
>>+ *	- __llvm_prf_data
>>+ *	- __llvm_prf_cnts
>>+ *	- __llvm_prf_names
>>+ *	- zero padding to 8 bytes
>>+ *	- for each llvm_prf_data in __llvm_prf_data:
>>+ *		- llvm_prf_value_data
>>+ *			- llvm_prf_value_record + site count array
>>+ *				- llvm_prf_value_node_data
>>+ *				...
>>+ *			...
>>+ *		...
>>+ */
>>+
>>+static void prf_fill_header(void **buffer)
>>+{
>>+	struct llvm_prf_header *header = *(struct llvm_prf_header **)buffer;
>>+
>>+#ifdef CONFIG_64BIT
>>+	header->magic = LLVM_INSTR_PROF_RAW_MAGIC_64;
>>+#else
>>+	header->magic = LLVM_INSTR_PROF_RAW_MAGIC_32;
>>+#endif
>>+	header->version = LLVM_VARIANT_MASK_IR_PROF | LLVM_INSTR_PROF_RAW_VERSION;
>>+	header->data_size = prf_data_count();
>>+	header->padding_bytes_before_counters = 0;
>>+	header->counters_size = prf_cnts_count();
>>+	header->padding_bytes_after_counters = 0;
>>+	header->names_size = prf_names_count();
>>+	header->counters_delta = (u64)__llvm_prf_cnts_start;
>>+	header->names_delta = (u64)__llvm_prf_names_start;
>>+	header->value_kind_last = LLVM_INSTR_PROF_IPVK_LAST;
>>+
>>+	*buffer += sizeof(*header);
>>+}
>>+
>>+/*
>>+ * Copy the source into the buffer, incrementing the pointer into buffer in the
>>+ * process.
>>+ */
>>+static void prf_copy_to_buffer(void **buffer, void *src, unsigned long size)
>>+{
>>+	memcpy(*buffer, src, size);
>>+	*buffer += size;
>>+}
>>+
>>+static u32 __prf_get_value_size(struct llvm_prf_data *p, u32 *value_kinds)
>>+{
>>+	struct llvm_prf_value_node **nodes =
>>+		(struct llvm_prf_value_node **)p->values;
>>+	u32 kinds = 0;
>>+	u32 size = 0;
>>+	unsigned int kind;
>>+	unsigned int n;
>>+	unsigned int s = 0;
>>+
>>+	for (kind = 0; kind < ARRAY_SIZE(p->num_value_sites); kind++) {
>>+		unsigned int sites = p->num_value_sites[kind];
>>+
>>+		if (!sites)
>>+			continue;
>>+
>>+		/* Record + site count array */
>>+		size += prf_get_value_record_size(sites);
>>+		kinds++;
>>+
>>+		if (!nodes)
>>+			continue;
>>+
>>+		for (n = 0; n < sites; n++) {
>>+			u32 count = 0;
>>+			struct llvm_prf_value_node *site = nodes[s + n];
>>+
>>+			while (site && ++count <= U8_MAX)
>>+				site = site->next;
>>+
>>+			size += count *
>>+				sizeof(struct llvm_prf_value_node_data);
>>+		}
>>+
>>+		s += sites;
>>+	}
>>+
>>+	if (size)
>>+		size += sizeof(struct llvm_prf_value_data);
>>+
>>+	if (value_kinds)
>>+		*value_kinds = kinds;
>>+
>>+	return size;
>>+}
>>+
>>+static u32 prf_get_value_size(void)
>>+{
>>+	u32 size = 0;
>>+	struct llvm_prf_data *p;
>>+
>>+	for (p = __llvm_prf_data_start; p < __llvm_prf_data_end; p++)
>>+		size += __prf_get_value_size(p, NULL);
>>+
>>+	return size;
>>+}
>>+
>>+/* Serialize the profiling's value. */
>>+static void prf_serialize_value(struct llvm_prf_data *p, void **buffer)
>>+{
>>+	struct llvm_prf_value_data header;
>>+	struct llvm_prf_value_node **nodes =
>>+		(struct llvm_prf_value_node **)p->values;
>>+	unsigned int kind;
>>+	unsigned int n;
>>+	unsigned int s = 0;
>>+
>>+	header.total_size = __prf_get_value_size(p, &header.num_value_kinds);
>>+
>>+	if (!header.num_value_kinds)
>>+		/* Nothing to write. */
>>+		return;
>>+
>>+	prf_copy_to_buffer(buffer, &header, sizeof(header));
>>+
>>+	for (kind = 0; kind < ARRAY_SIZE(p->num_value_sites); kind++) {
>>+		struct llvm_prf_value_record *record;
>>+		u8 *counts;
>>+		unsigned int sites = p->num_value_sites[kind];
>>+
>>+		if (!sites)
>>+			continue;
>>+
>>+		/* Profiling value record. */
>>+		record = *(struct llvm_prf_value_record **)buffer;
>>+		*buffer += prf_get_value_record_header_size();
>>+
>>+		record->kind = kind;
>>+		record->num_value_sites = sites;
>>+
>>+		/* Site count array. */
>>+		counts = *(u8 **)buffer;
>>+		*buffer += prf_get_value_record_site_count_size(sites);
>>+
>>+		/*
>>+		 * If we don't have nodes, we can skip updating the site count
>>+		 * array, because the buffer is zero filled.
>>+		 */
>>+		if (!nodes)
>>+			continue;
>>+
>>+		for (n = 0; n < sites; n++) {
>>+			u32 count = 0;
>>+			struct llvm_prf_value_node *site = nodes[s + n];
>>+
>>+			while (site && ++count <= U8_MAX) {
>>+				prf_copy_to_buffer(buffer, site,
>>+						   sizeof(struct llvm_prf_value_node_data));
>>+				site = site->next;
>>+			}
>>+
>>+			counts[n] = (u8)count;
>>+		}
>>+
>>+		s += sites;
>>+	}
>>+}
>>+
>>+static void prf_serialize_values(void **buffer)
>>+{
>>+	struct llvm_prf_data *p;
>>+
>>+	for (p = __llvm_prf_data_start; p < __llvm_prf_data_end; p++)
>>+		prf_serialize_value(p, buffer);
>>+}
>>+
>>+static inline unsigned long prf_get_padding(unsigned long size)
>>+{
>>+	return 7 & (sizeof(u64) - size % sizeof(u64));
>>+}
>>+
>>+static unsigned long prf_buffer_size(void)
>>+{
>>+	return sizeof(struct llvm_prf_header) +
>>+			prf_data_size()	+
>>+			prf_cnts_size() +
>>+			prf_names_size() +
>>+			prf_get_padding(prf_names_size()) +
>>+			prf_get_value_size();
>>+}
>>+
>>+/*
>>+ * Serialize the profiling data into a format LLVM's tools can understand.
>>+ * Note: caller *must* hold pgo_lock.
>>+ */
>>+static int prf_serialize(struct prf_private_data *p)
>>+{
>>+	int err = 0;
>>+	void *buffer;
>>+
>>+	p->size = prf_buffer_size();
>>+	p->buffer = vzalloc(p->size);
>>+
>>+	if (!p->buffer) {
>>+		err = -ENOMEM;
>>+		goto out;
>>+	}
>>+
>>+	buffer = p->buffer;
>>+
>>+	prf_fill_header(&buffer);
>>+	prf_copy_to_buffer(&buffer, __llvm_prf_data_start,  prf_data_size());
>>+	prf_copy_to_buffer(&buffer, __llvm_prf_cnts_start,  prf_cnts_size());
>>+	prf_copy_to_buffer(&buffer, __llvm_prf_names_start, prf_names_size());
>>+	buffer += prf_get_padding(prf_names_size());
>>+
>>+	prf_serialize_values(&buffer);
>>+
>>+out:
>>+	return err;
>>+}
>>+
>>+/* open() implementation for PGO. Creates a copy of the profiling data set. */
>>+static int prf_open(struct inode *inode, struct file *file)
>>+{
>>+	struct prf_private_data *data;
>>+	unsigned long flags;
>>+	int err;
>>+
>>+	data = kzalloc(sizeof(*data), GFP_KERNEL);
>>+	if (!data) {
>>+		err = -ENOMEM;
>>+		goto out;
>>+	}
>>+
>>+	flags = prf_lock();
>>+
>>+	err = prf_serialize(data);
>>+	if (unlikely(err)) {
>>+		kfree(data);
>>+		goto out_unlock;
>>+	}
>>+
>>+	file->private_data = data;
>>+
>>+out_unlock:
>>+	prf_unlock(flags);
>>+out:
>>+	return err;
>>+}
>>+
>>+/* read() implementation for PGO. */
>>+static ssize_t prf_read(struct file *file, char __user *buf, size_t count,
>>+			loff_t *ppos)
>>+{
>>+	struct prf_private_data *data = file->private_data;
>>+
>>+	BUG_ON(!data);
>>+
>>+	return simple_read_from_buffer(buf, count, ppos, data->buffer,
>>+				       data->size);
>>+}
>>+
>>+/* release() implementation for PGO. Release resources allocated by open(). */
>>+static int prf_release(struct inode *inode, struct file *file)
>>+{
>>+	struct prf_private_data *data = file->private_data;
>>+
>>+	if (data) {
>>+		vfree(data->buffer);
>>+		kfree(data);
>>+	}
>>+
>>+	return 0;
>>+}
>>+
>>+static const struct file_operations prf_fops = {
>>+	.owner		= THIS_MODULE,
>>+	.open		= prf_open,
>>+	.read		= prf_read,
>>+	.llseek		= default_llseek,
>>+	.release	= prf_release
>>+};
>>+
>>+/* write() implementation for resetting PGO's profile data. */
>>+static ssize_t reset_write(struct file *file, const char __user *addr,
>>+			   size_t len, loff_t *pos)
>>+{
>>+	struct llvm_prf_data *data;
>>+
>>+	memset(__llvm_prf_cnts_start, 0, prf_cnts_size());
>>+
>>+	for (data = __llvm_prf_data_start; data < __llvm_prf_data_end; data++) {
>>+		struct llvm_prf_value_node **vnodes;
>>+		u64 current_vsite_count;
>>+		u32 i;
>>+
>>+		if (!data->values)
>>+			continue;
>>+
>>+		current_vsite_count = 0;
>>+		vnodes = (struct llvm_prf_value_node **)data->values;
>>+
>>+		for (i = LLVM_INSTR_PROF_IPVK_FIRST; i <= LLVM_INSTR_PROF_IPVK_LAST; i++)
>>+			current_vsite_count += data->num_value_sites[i];
>>+
>>+		for (i = 0; i < current_vsite_count; i++) {
>>+			struct llvm_prf_value_node *current_vnode = vnodes[i];
>>+
>>+			while (current_vnode) {
>>+				current_vnode->count = 0;
>>+				current_vnode = current_vnode->next;
>>+			}
>>+		}
>>+	}
>>+
>>+	return len;
>>+}
>>+
>>+static const struct file_operations prf_reset_fops = {
>>+	.owner		= THIS_MODULE,
>>+	.write		= reset_write,
>>+	.llseek		= noop_llseek,
>>+};
>>+
>>+/* Create debugfs entries. */
>>+static int __init pgo_init(void)
>>+{
>>+	directory = debugfs_create_dir("pgo", NULL);
>>+	if (!directory)
>>+		goto err_remove;
>>+
>>+	if (!debugfs_create_file("profraw", 0600, directory, NULL,
>>+				 &prf_fops))
>>+		goto err_remove;
>>+
>>+	if (!debugfs_create_file("reset", 0200, directory, NULL,
>>+				 &prf_reset_fops))
>>+		goto err_remove;
>>+
>>+	return 0;
>>+
>>+err_remove:
>>+	pr_err("initialization failed\n");
>>+	return -EIO;
>>+}
>>+
>>+/* Remove debugfs entries. */
>>+static void __exit pgo_exit(void)
>>+{
>>+	debugfs_remove_recursive(directory);
>>+}
>>+
>>+module_init(pgo_init);
>>+module_exit(pgo_exit);
>>diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
>>new file mode 100644
>>index 000000000000..62ff5cfce7b1
>>--- /dev/null
>>+++ b/kernel/pgo/instrument.c
>>@@ -0,0 +1,189 @@
>>+// SPDX-License-Identifier: GPL-2.0
>>+/*
>>+ * Copyright (C) 2019 Google, Inc.
>>+ *
>>+ * Author:
>>+ *	Sami Tolvanen <samitolvanen@google.com>
>>+ *
>>+ * This software is licensed under the terms of the GNU General Public
>>+ * License version 2, as published by the Free Software Foundation, and
>>+ * may be copied, distributed, and modified under those terms.
>>+ *
>>+ * This program is distributed in the hope that it will be useful,
>>+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>+ * GNU General Public License for more details.
>>+ *
>>+ */
>>+
>>+#define pr_fmt(fmt)	"pgo: " fmt
>>+
>>+#include <linux/bitops.h>
>>+#include <linux/kernel.h>
>>+#include <linux/export.h>
>>+#include <linux/spinlock.h>
>>+#include <linux/types.h>
>>+#include "pgo.h"
>>+
>>+/*
>>+ * This lock guards both profile count updating and serialization of the
>>+ * profiling data. Keeping both of these activities separate via locking
>>+ * ensures that we don't try to serialize data that's only partially updated.
>>+ */
>>+static DEFINE_SPINLOCK(pgo_lock);
>>+static int current_node;
>>+
>>+unsigned long prf_lock(void)
>>+{
>>+	unsigned long flags;
>>+
>>+	spin_lock_irqsave(&pgo_lock, flags);
>>+
>>+	return flags;
>>+}
>>+
>>+void prf_unlock(unsigned long flags)
>>+{
>>+	spin_unlock_irqrestore(&pgo_lock, flags);
>>+}
>>+
>>+/*
>>+ * Return a newly allocated profiling value node which contains the tracked
>>+ * value by the value profiler.
>>+ * Note: caller *must* hold pgo_lock.
>>+ */
>>+static struct llvm_prf_value_node *allocate_node(struct llvm_prf_data *p,
>>+						 u32 index, u64 value)
>>+{
>>+	if (&__llvm_prf_vnds_start[current_node + 1] >= __llvm_prf_vnds_end)
>>+		return NULL; /* Out of nodes */
>>+
>>+	current_node++;
>>+
>>+	/* Make sure the node is entirely within the section */
>>+	if (&__llvm_prf_vnds_start[current_node] >= __llvm_prf_vnds_end ||
>>+	    &__llvm_prf_vnds_start[current_node + 1] > __llvm_prf_vnds_end)
>>+		return NULL;
>>+
>>+	return &__llvm_prf_vnds_start[current_node];
>>+}
>>+
>>+/*
>>+ * Counts the number of times a target value is seen.
>>+ *
>>+ * Records the target value for the index if not seen before. Otherwise,
>>+ * increments the counter associated w/ the target value.
>>+ */
>>+void __llvm_profile_instrument_target(u64 target_value, void *data, u32 index);
>>+void __llvm_profile_instrument_target(u64 target_value, void *data, u32 index)
>>+{
>>+	struct llvm_prf_data *p = (struct llvm_prf_data *)data;
>>+	struct llvm_prf_value_node **counters;
>>+	struct llvm_prf_value_node *curr;
>>+	struct llvm_prf_value_node *min = NULL;
>>+	struct llvm_prf_value_node *prev = NULL;
>>+	u64 min_count = U64_MAX;
>>+	u8 values = 0;
>>+	unsigned long flags;
>>+
>>+	if (!p || !p->values)
>>+		return;
>>+
>>+	counters = (struct llvm_prf_value_node **)p->values;
>>+	curr = counters[index];
>>+
>>+	while (curr) {
>>+		if (target_value == curr->value) {
>>+			curr->count++;
>>+			return;
>>+		}
>>+
>>+		if (curr->count < min_count) {
>>+			min_count = curr->count;
>>+			min = curr;
>>+		}
>>+
>>+		prev = curr;
>>+		curr = curr->next;
>>+		values++;
>>+	}
>>+
>>+	if (values >= LLVM_INSTR_PROF_MAX_NUM_VAL_PER_SITE) {
>>+		if (!min->count || !(--min->count)) {
>>+			curr = min;
>>+			curr->value = target_value;
>>+			curr->count++;
>>+		}
>>+		return;
>>+	}
>>+
>>+	/* Lock when updating the value node structure. */
>>+	flags = prf_lock();
>>+
>>+	curr = allocate_node(p, index, target_value);
>>+	if (!curr)
>>+		goto out;
>>+
>>+	curr->value = target_value;
>>+	curr->count++;
>>+
>>+	if (!counters[index])
>>+		counters[index] = curr;
>>+	else if (prev && !prev->next)
>>+		prev->next = curr;
>>+
>>+out:
>>+	prf_unlock(flags);
>>+}
>>+EXPORT_SYMBOL(__llvm_profile_instrument_target);
>>+
>>+/* Counts the number of times a range of targets values are seen. */
>>+void __llvm_profile_instrument_range(u64 target_value, void *data,
>>+				     u32 index, s64 precise_start,
>>+				     s64 precise_last, s64 large_value);
>>+void __llvm_profile_instrument_range(u64 target_value, void *data,
>>+				     u32 index, s64 precise_start,
>>+				     s64 precise_last, s64 large_value)
>>+{
>>+	if (large_value != S64_MIN && (s64)target_value >= large_value)
>>+		target_value = large_value;
>>+	else if ((s64)target_value < precise_start ||
>>+		 (s64)target_value > precise_last)
>>+		target_value = precise_last + 1;
>>+
>>+	__llvm_profile_instrument_target(target_value, data, index);
>>+}
>>+EXPORT_SYMBOL(__llvm_profile_instrument_range);
>>+
>>+static u64 inst_prof_get_range_rep_value(u64 value)
>>+{
>>+	if (value <= 8)
>>+		/* The first ranges are individually tracked, use it as is. */
>>+		return value;
>>+	else if (value >= 513)
>>+		/* The last range is mapped to its lowest value. */
>>+		return 513;
>>+	else if (hweight64(value) == 1)
>>+		/* If it's a power of two, use it as is. */
>>+		return value;
>>+
>>+	/* Otherwise, take to the previous power of two + 1. */
>>+	return (1 << (64 - __builtin_clzll(value) - 1)) + 1;
>>+}

`1 << ...` is another very minor issue.

I sent https://reviews.llvm.org/D97640 to fix the upstream.
The overflow won't happen in practice because the function is only used
by the size parameter of memory operation (e.g. memcpy).

>>+/*
>>+ * The target values are partitioned into multiple ranges. The range spec is
>>+ * defined in compiler-rt/include/profile/InstrProfData.inc.
>>+ */
>>+void __llvm_profile_instrument_memop(u64 target_value, void *data,
>>+				     u32 counter_index);
>>+void __llvm_profile_instrument_memop(u64 target_value, void *data,
>>+				     u32 counter_index)
>>+{
>>+	u64 rep_value;
>>+
>>+	/* Map the target value to the representative value of its range. */
>>+	rep_value = inst_prof_get_range_rep_value(target_value);
>>+	__llvm_profile_instrument_target(rep_value, data, counter_index);
>>+}
>>+EXPORT_SYMBOL(__llvm_profile_instrument_memop);
>>diff --git a/kernel/pgo/pgo.h b/kernel/pgo/pgo.h
>>new file mode 100644
>>index 000000000000..ddc8d3002fe5
>>--- /dev/null
>>+++ b/kernel/pgo/pgo.h
>>@@ -0,0 +1,203 @@
>>+/* SPDX-License-Identifier: GPL-2.0 */
>>+/*
>>+ * Copyright (C) 2019 Google, Inc.
>>+ *
>>+ * Author:
>>+ *	Sami Tolvanen <samitolvanen@google.com>
>>+ *
>>+ * This software is licensed under the terms of the GNU General Public
>>+ * License version 2, as published by the Free Software Foundation, and
>>+ * may be copied, distributed, and modified under those terms.
>>+ *
>>+ * This program is distributed in the hope that it will be useful,
>>+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>+ * GNU General Public License for more details.
>>+ *
>>+ */
>>+
>>+#ifndef _PGO_H
>>+#define _PGO_H
>>+
>>+/*
>>+ * Note: These internal LLVM definitions must match the compiler version.
>>+ * See llvm/include/llvm/ProfileData/InstrProfData.inc in LLVM's source code.
>>+ */
>>+
>>+#define LLVM_INSTR_PROF_RAW_MAGIC_64	\
>>+		((u64)255 << 56 |	\
>>+		 (u64)'l' << 48 |	\
>>+		 (u64)'p' << 40 |	\
>>+		 (u64)'r' << 32 |	\
>>+		 (u64)'o' << 24 |	\
>>+		 (u64)'f' << 16 |	\
>>+		 (u64)'r' << 8  |	\
>>+		 (u64)129)
>>+#define LLVM_INSTR_PROF_RAW_MAGIC_32	\
>>+		((u64)255 << 56 |	\
>>+		 (u64)'l' << 48 |	\
>>+		 (u64)'p' << 40 |	\
>>+		 (u64)'r' << 32 |	\
>>+		 (u64)'o' << 24 |	\
>>+		 (u64)'f' << 16 |	\
>>+		 (u64)'R' << 8  |	\
>>+		 (u64)129)
>>+
>>+#define LLVM_INSTR_PROF_RAW_VERSION		5
>>+#define LLVM_INSTR_PROF_DATA_ALIGNMENT		8
>>+#define LLVM_INSTR_PROF_IPVK_FIRST		0
>>+#define LLVM_INSTR_PROF_IPVK_LAST		1
>>+#define LLVM_INSTR_PROF_MAX_NUM_VAL_PER_SITE	255
>>+
>>+#define LLVM_VARIANT_MASK_IR_PROF	(0x1ULL << 56)
>>+#define LLVM_VARIANT_MASK_CSIR_PROF	(0x1ULL << 57)
>>+
>>+/**
>>+ * struct llvm_prf_header - represents the raw profile header data structure.
>>+ * @magic: the magic token for the file format.
>>+ * @version: the version of the file format.
>>+ * @data_size: the number of entries in the profile data section.
>>+ * @padding_bytes_before_counters: the number of padding bytes before the
>>+ *   counters.
>>+ * @counters_size: the size in bytes of the LLVM profile section containing the
>>+ *   counters.
>>+ * @padding_bytes_after_counters: the number of padding bytes after the
>>+ *   counters.
>>+ * @names_size: the size in bytes of the LLVM profile section containing the
>>+ *   counters' names.
>>+ * @counters_delta: the beginning of the LLMV profile counters section.
>>+ * @names_delta: the beginning of the LLMV profile names section.
>>+ * @value_kind_last: the last profile value kind.
>>+ */
>>+struct llvm_prf_header {
>>+	u64 magic;
>>+	u64 version;
>>+	u64 data_size;
>>+	u64 padding_bytes_before_counters;
>>+	u64 counters_size;
>>+	u64 padding_bytes_after_counters;
>>+	u64 names_size;
>>+	u64 counters_delta;
>>+	u64 names_delta;
>>+	u64 value_kind_last;
>>+};
>>+
>>+/**
>>+ * struct llvm_prf_data - represents the per-function control structure.
>>+ * @name_ref: the reference to the function's name.
>>+ * @func_hash: the hash value of the function.
>>+ * @counter_ptr: a pointer to the profile counter.
>>+ * @function_ptr: a pointer to the function.
>>+ * @values: the profiling values associated with this function.
>>+ * @num_counters: the number of counters in the function.
>>+ * @num_value_sites: the number of value profile sites.
>>+ */
>>+struct llvm_prf_data {
>>+	const u64 name_ref;
>>+	const u64 func_hash;
>>+	const void *counter_ptr;
>>+	const void *function_ptr;
>>+	void *values;
>>+	const u32 num_counters;
>>+	const u16 num_value_sites[LLVM_INSTR_PROF_IPVK_LAST + 1];
>>+} __aligned(LLVM_INSTR_PROF_DATA_ALIGNMENT);
>>+
>>+/**
>>+ * structure llvm_prf_value_node_data - represents the data part of the struct
>>+ *   llvm_prf_value_node data structure.
>>+ * @value: the value counters.
>>+ * @count: the counters' count.
>>+ */
>>+struct llvm_prf_value_node_data {
>>+	u64 value;
>>+	u64 count;
>>+};
>>+
>>+/**
>>+ * struct llvm_prf_value_node - represents an internal data structure used by
>>+ *   the value profiler.
>>+ * @value: the value counters.
>>+ * @count: the counters' count.
>>+ * @next: the next value node.
>>+ */
>>+struct llvm_prf_value_node {
>>+	u64 value;
>>+	u64 count;
>>+	struct llvm_prf_value_node *next;
>>+};
>>+
>>+/**
>>+ * struct llvm_prf_value_data - represents the value profiling data in indexed
>>+ *   format.
>>+ * @total_size: the total size in bytes including this field.
>>+ * @num_value_kinds: the number of value profile kinds that has value profile
>>+ *   data.
>>+ */
>>+struct llvm_prf_value_data {
>>+	u32 total_size;
>>+	u32 num_value_kinds;
>>+};
>>+
>>+/**
>>+ * struct llvm_prf_value_record - represents the on-disk layout of the value
>>+ *   profile data of a particular kind for one function.
>>+ * @kind: the kind of the value profile record.
>>+ * @num_value_sites: the number of value profile sites.
>>+ * @site_count_array: the first element of the array that stores the number
>>+ *   of profiled values for each value site.
>>+ */
>>+struct llvm_prf_value_record {
>>+	u32 kind;
>>+	u32 num_value_sites;
>>+	u8 site_count_array[];
>>+};
>>+
>>+#define prf_get_value_record_header_size()		\
>>+	offsetof(struct llvm_prf_value_record, site_count_array)
>>+#define prf_get_value_record_site_count_size(sites)	\
>>+	roundup((sites), 8)
>>+#define prf_get_value_record_size(sites)		\
>>+	(prf_get_value_record_header_size() +		\
>>+	 prf_get_value_record_site_count_size((sites)))
>>+
>>+/* Data sections */
>>+extern struct llvm_prf_data __llvm_prf_data_start[];
>>+extern struct llvm_prf_data __llvm_prf_data_end[];
>>+
>>+extern u64 __llvm_prf_cnts_start[];
>>+extern u64 __llvm_prf_cnts_end[];
>>+
>>+extern char __llvm_prf_names_start[];
>>+extern char __llvm_prf_names_end[];
>>+
>>+extern struct llvm_prf_value_node __llvm_prf_vnds_start[];
>>+extern struct llvm_prf_value_node __llvm_prf_vnds_end[];
>>+
>>+/* Locking for vnodes */
>>+extern unsigned long prf_lock(void);
>>+extern void prf_unlock(unsigned long flags);
>>+
>>+#define __DEFINE_PRF_SIZE(s) \
>>+	static inline unsigned long prf_ ## s ## _size(void)		\
>>+	{								\
>>+		unsigned long start =					\
>>+			(unsigned long)__llvm_prf_ ## s ## _start;	\
>>+		unsigned long end =					\
>>+			(unsigned long)__llvm_prf_ ## s ## _end;	\
>>+		return roundup(end - start,				\
>>+				sizeof(__llvm_prf_ ## s ## _start[0]));	\
>>+	}								\
>>+	static inline unsigned long prf_ ## s ## _count(void)		\
>>+	{								\
>>+		return prf_ ## s ## _size() /				\
>>+			sizeof(__llvm_prf_ ## s ## _start[0]);		\
>>+	}
>>+
>>+__DEFINE_PRF_SIZE(data);
>>+__DEFINE_PRF_SIZE(cnts);
>>+__DEFINE_PRF_SIZE(names);
>>+__DEFINE_PRF_SIZE(vnds);
>>+
>>+#undef __DEFINE_PRF_SIZE
>>+
>>+#endif /* _PGO_H */
>>diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
>>index eee59184de64..48a65d092c5b 100644
>>--- a/scripts/Makefile.lib
>>+++ b/scripts/Makefile.lib
>>@@ -139,6 +139,16 @@ _c_flags += $(if $(patsubst n%,, \
>>		$(CFLAGS_GCOV))
>>endif
>>
>>+#
>>+# Enable clang's PGO profiling flags for a file or directory depending on
>>+# variables PGO_PROFILE_obj.o and PGO_PROFILE.
>>+#
>>+ifeq ($(CONFIG_PGO_CLANG),y)
>>+_c_flags += $(if $(patsubst n%,, \
>>+		$(PGO_PROFILE_$(basetarget).o)$(PGO_PROFILE)y), \
>>+		$(CFLAGS_PGO_CLANG))
>>+endif
>>+
>>#
>># Enable address sanitizer flags for kernel except some files or directories
>># we don't want to check (depends on variables KASAN_SANITIZE_obj.o, KASAN_SANITIZE)
>>-- 
>>2.30.1.766.gb4fecdf3b7-goog
>>
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