On Sat, Jan 16, 2021 at 9:39 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote: > On Sat, Jan 16, 2021 at 10:44 AM 'Bill Wendling' via Clang Built Linux > <clang-built-linux@xxxxxxxxxxxxxxxx> wrote: > > > > From: Sami Tolvanen <samitolvanen@xxxxxxxxxx> > > > > 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@xxxxxxxxxx> > > Co-developed-by: Bill Wendling <morbo@xxxxxxxxxx> > > Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx> > > --- > > 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
