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    SubjectRe: [PATCH v5] pgo: add clang's Profile Guided Optimization infrastructure
    On Sat, Jan 16, 2021 at 9:39 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
    > On Sat, Jan 16, 2021 at 10:44 AM 'Bill Wendling' via Clang Built Linux
    > <clang-built-linux@googlegroups.com> 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>
    > > ---
    > > 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.

    -bw

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