Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

Peter Zijlstra
2021-06-12T16:59:49Z

On Wed, Apr 07, 2021 at 02:17:04PM -0700, Bill Wendling 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.

*sigh*, and not a single x86 person on Cc, how nice :-/

> 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

Also, and I don't see this answered *anywhere*, why are you not using
perf for this? Your link even mentions Sampling Profilers (and I happen
to know there's been significant effort to make perf output work as
input for the PGO passes of the various compilers).

> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Co-developed-by: Bill Wendling <morbo@google.com>
> Signed-off-by: Bill Wendling <morbo@google.com>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Fangrui Song <maskray@google.com>
> ---
> 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 | 4 +
> 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 | 34 +++
> kernel/Makefile | 1 +
> kernel/pgo/Kconfig | 35 +++
> kernel/pgo/Makefile | 5 +
> kernel/pgo/fs.c | 389 ++++++++++++++++++++++++++
> kernel/pgo/instrument.c | 189 +++++++++++++
> kernel/pgo/pgo.h | 203 ++++++++++++++
> 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

> --- a/Makefile
> +++ b/Makefile
> @@ -660,6 +660,9 @@ endif # KBUILD_EXTMOD
> # Defaults to vmlinux, but the arch makefile usually adds further targets
> all: vmlinux
>
> +CFLAGS_PGO_CLANG := -fprofile-generate
> +export CFLAGS_PGO_CLANG
> +
> CFLAGS_GCOV := -fprofile-arcs -ftest-coverage \
> $(call cc-option,-fno-tree-loop-im) \
> $(call cc-disable-warning,maybe-uninitialized,)

And which of the many flags in noinstr disables this?

Basically I would like to NAK this whole thing until someone can
adequately explain the interaction with noinstr and why we need those
many lines of kernel code and can't simply use perf for this.