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

[Nick Desaulniers <ndesaulniers@google.com>]

On Mon, Jan 11, 2021 at 1:04 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Mon, Jan 11, 2021 at 12:18:21AM -0800, 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 be used either by the compiler if LTO isn't enabled:
> >
> >     ... -fprofile-use=vmlinux.profdata ...
> >
> > or by LLD if LTO is enabled:
> >
> >     ... -lto-cs-profile-file=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 and isn't
> > compatible with clang's gcov 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>
>
> I took this for a spin against x86_64_defconfig and ran into two issues:
>
> 1. https://github.com/ClangBuiltLinux/linux/issues/1252

"Cannot split an edge from a CallBrInst"
Looks like that should be fixed first, then we should gate this
feature on clang-12.

>
>    There is also one in drivers/gpu/drm/i915/i915_query.c. For the time
>    being, I added PGO_PROFILE_... := n for those two files.
>
> 2. After doing that, I run into an undefined function error with ld.lld.
>
> How I tested:
>
> $ make -skj"$(nproc)" LLVM=1 defconfig
>
> $ scripts/config -e PGO_CLANG
>
> $ make -skj"$(nproc)" LLVM=1 olddefconfig vmlinux all
> ld.lld: error: undefined symbol: __llvm_profile_instrument_memop

Err...that seems like it should be implemented in
kernel/pgo/instrument.c in this patch in a v2?

> >>> referenced by head64.c
> >>>               arch/x86/kernel/head64.o:(__early_make_pgtable)
> >>> referenced by head64.c
> >>>               arch/x86/kernel/head64.o:(x86_64_start_kernel)
> >>> referenced by head64.c
> >>>               arch/x86/kernel/head64.o:(copy_bootdata)
> >>> referenced 2259 more times
>
> Local diff:
>
> diff --git a/drivers/char/Makefile b/drivers/char/Makefile
> index ffce287ef415..4b2f238770b5 100644
> --- a/drivers/char/Makefile
> +++ b/drivers/char/Makefile
> @@ -4,6 +4,7 @@
>  #
>
>  obj-y                          += mem.o random.o
> +PGO_PROFILE_random.o           := n
>  obj-$(CONFIG_TTY_PRINTK)       += ttyprintk.o
>  obj-y                          += misc.o
>  obj-$(CONFIG_ATARI_DSP56K)     += dsp56k.o
> diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> index e5574e506a5c..d83cacc79b1a 100644
> --- a/drivers/gpu/drm/i915/Makefile
> +++ b/drivers/gpu/drm/i915/Makefile
> @@ -168,6 +168,7 @@ i915-y += \
>           i915_vma.o \
>           intel_region_lmem.o \
>           intel_wopcm.o
> +PGO_PROFILE_i915_query.o := n
>
>  # general-purpose microcontroller (GuC) support
>  i915-y += gt/uc/intel_uc.o \

I'd rather have these both sorted out before landing with PGO disabled
on these files.

-- 
Thanks,
~Nick Desaulniers