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Date:   Sun, 17 Jan 2021 11:44:40 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v5] pgo: add clang's Profile Guided Optimization infrastructure

On Sat, Jan 16, 2021 at 9:23 PM Bill Wendling <morbo@...gle.com> wrote:
>
> On Sat, Jan 16, 2021 at 9:39 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > On Sat, Jan 16, 2021 at 10:44 AM 'Bill Wendling' via Clang Built Linux
> > <clang-built-linux@...glegroups.com> wrote:
> > >
> > > From: Sami Tolvanen <samitolvanen@...gle.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@...gle.com>
> > > Co-developed-by: Bill Wendling <morbo@...gle.com>
> > > Signed-off-by: Bill Wendling <morbo@...gle.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.
>
> > > +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.
>
> > > +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).
>
> > > +   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.
>
> > 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@...il.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.

- Sedat -

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