On Sat, Jan 16, 2021 at 9:23 PM Bill Wendling <morbo@xxxxxxxxxx> wrote: > > 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. > 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@xxxxxxxxx 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 -
