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

[Bill Wendling <morbo@google.com>]

On Mon, Jan 11, 2021 at 1:18 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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.
>
Weird. I'll investigate.

> >
> >    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?
>
Yes. I'll submit a new V2 with this and other feedback integrated.

> > >>> 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.
>
Agreed.

-bw