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

Nathan Chancellor
2021-01-11T21:05:35Z

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

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
>>> 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 \