On Sun, Jan 17, 2021 at 11:53 AM Sedat Dilek [off-list ref] wrote:

On Sun, Jan 17, 2021 at 11:44 AM Sedat Dilek [off-list ref] wrote:

quoted

On Sat, Jan 16, 2021 at 9:23 PM Bill Wendling [off-list ref] wrote:

quoted

On Sat, Jan 16, 2021 at 9:39 AM Sedat Dilek [off-list ref] wrote:

quoted

On Sat, Jan 16, 2021 at 10:44 AM 'Bill Wendling' via Clang Built Linux
[off-list ref] wrote:

quoted

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>
---
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.
v3: - Added change log section based on Sedat Dilek's comments.
v4: - Remove non-x86 Makfile changes and se "hweight64" instead of using our
      own popcount implementation, based on Nick Desaulniers's comment.
v5: - Correct padding calculation, discovered by Nathan Chancellor.
---
 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              |   2 +
 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                       | 382 ++++++++++++++++++++++++++
 kernel/pgo/instrument.c               | 185 +++++++++++++
 kernel/pgo/pgo.h                      | 206 ++++++++++++++
 scripts/Makefile.lib                  |  10 +
 24 files changed, 1022 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 f7809c7b1ba9e..8d6418e858062 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 0000000000000..b7f11d8405b73
--- /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
+

I do not get this...

# mount | grep debugfs
debugfs on /sys/kernel/debug type debugfs (rw,nosuid,nodev,noexec,relatime)

After the load-test...?

echo 0 > /sys/kernel/debug/pgo/reset

Writing anything to /sys/kernel/debug/pgo/reset will cause it to reset
the profiling counters. I picked 1 (one) semi-randomly, but it could
be any number, letter, your favorite short story, etc. You don't want
to reset it before collecting the profiling data from your load tests
though.

quoted

quoted

+3) Run the load tests.
+
+4) Collect the raw profile data
+
+   .. code-block:: sh
+
+      $ cp -a /sys/kernel/debug/pgo/profraw /tmp/vmlinux.profraw
+

This is only 4,9M small and seen from the date 5mins before I run the
echo-1 line.

# ll /sys/kernel/debug/pgo
insgesamt 0
drwxr-xr-x  2 root root 0 16. Jan 17:29 .
drwx------ 41 root root 0 16. Jan 17:29 ..
-rw-------  1 root root 0 16. Jan 17:29 profraw
--w-------  1 root root 0 16. Jan 18:19 reset

# cp -a /sys/kernel/debug/pgo/profraw /tmp/vmlinux.profraw

# ll /tmp/vmlinux.profraw
-rw------- 1 root root 4,9M 16. Jan 17:29 /tmp/vmlinux.profraw

For me there was no prof-data collected from my defconfig kernel-build.

The /sys/kernel/debug/pgo/profraw file is read-only. Nothing writes to
it, not even the kernel. All it does is serialize the profiling
counters from a memory location in the kernel into a format that
LLVM's tools can understand.

quoted

quoted

+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
+

Is that executed in /path/to/linux/git?

The llvm-profdata tool is not in the linux source tree. You need to
grab it from a clang distribution (or built from clang's git repo).

quoted

quoted

+   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 ...

How big is vmlinux.profdata (make defconfig)?

I don't have numbers for this, but from what you listed here, it's ~5M
in size. The size is proportional to the number of counters
instrumented in the kernel.

quoted

Do I need to do a full defconfig build or can I stop the build after
let me say 10mins?

You should do a full rebuild. Make sure that PGO is disabled during the rebuild.

Thanks Bill for all the information.

And sorry if I am so pedantic.

I have installed my Debian system with Legacy-BIOS enabled.

When I rebuild with KCFLAGS=-fprofile-use=vmlinux.profdata (LLVM=1 I
have as a default) my system hangs on reboot.

[ diffconfig ]
$ scripts/diffconfig /boot/config-5.11.0-rc3-8-amd64-clang12-pgo
/boot/config-5.11.0-rc3-9-amd64-clang12-pgo
BUILD_SALT "5.11.0-rc3-8-amd64-clang12-pgo" -> "5.11.0-rc3-9-amd64-clang12-pgo"
PGO_CLANG y -> n

[ my make line ]
$ cat ../start-build_5.11.0-rc3-9-amd64-clang12-pgo.txt
dileks     63120   63095  0 06:47 pts/2    00:00:00 /usr/bin/perf_5.10
stat make V=1 -j4 HOSTCC=clang HOSTCXX=clang++ HOSTLD=ld.lld CC=clang
LD=ld.lld LLVM=1 LLVM_IAS=1 PAHOLE=/opt/pahole/bin/pahole
LOCALVERSION=-9-amd64-clang12-pgo KBUILD_VERBOSE=1
KBUILD_BUILD_HOST=iniza KBUILD_BUILD_USER=sedat.dilek@gmail.com
KBUILD_BUILD_TIMESTAMP=2021-01-17 bindeb-pkg
KDEB_PKGVERSION=5.11.0~rc3-9~bullseye+dileks1
KCFLAGS=-fprofile-use=vmlinux.profdata

( Yes, 06:47 a.m. in the morning :-). )

When I boot with the rebuild Linux-kernel I see:

Wrong EFI loader signature
...
Decompressing
Parsing EFI
Performing Relocations done.
Booting the Kernel.

*** SYSTEM HANGS ***
( I waited for approx 1 min )

I tried to turn UEFI support ON and OFF.
No success.

Does Clang-PGO support Legacy-BIOS or is something different wrong?

Thanks.

My bootloader is GRUB.

In UEFI-BIOS settings there is no secure-boot disable option.
Just simple "Use UEFI BIOS" enabled|disabled.

Installed Debian packages:

ii grub-common 2.04-12
ii grub-pc 2.04-12
ii grub-pc-bin 2.04-12
ii grub2-common 2.04-12

I found in the below link to do in grub-shell:

set check_signatures=no

But this is when grub-efi is installed.

- Sedat -

Link: https://unix.stackexchange.com/questions/126286/grub-efi-disable-signature-check

Forget about that "Wrong EFI bootloader" - I see this with all other
kernels (all boot fine).

I tried in QEMU with and without KASLR:

[ run_qemu.sh ]
KPATH=$(pwd)

APPEND="root=/dev/ram0 console=ttyS0 hung_task_panic=1 earlyprintk=ttyS0,115200"
APPEND="$APPEND nokaslr"

qemu-system-x86_64 -enable-kvm -M pc -kernel $KPATH/bzImage -initrd
$KPATH/initrd.img -m 512 -net none -serial stdio -append "${APPEND}"
[ /run_qemu.sh ]

$ ./run_qemu.sh
Probing EDD (edd=off to disable)... ok
Wrong EFI loader signature.
early console in extract_kernel
input_data: 0x000000000289940d
input_len: 0x000000000069804a
output: 0x0000000001000000
output_len: 0x0000000001ef2010
kernel_total_size: 0x0000000001c2c000
needed_size: 0x0000000002000000
trampoline_32bit: 0x000000000009d000


KASLR disabled: 'nokaslr' on cmdline.


Decompressing Linux... Parsing ELF... No relocation needed... done.
Booting the kernel.

QEMU run stops, too.

- Sedat