On Mon, Jan 11, 2021 at 1:04 PM Nathan Chancellor <natechancellor@xxxxxxxxx> wrote: > > On Mon, Jan 11, 2021 at 12:18:21AM -0800, Bill Wendling 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 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@xxxxxxxxxx> > > Co-developed-by: Bill Wendling <morbo@xxxxxxxxxx> > > Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx> > > 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
