On Mon, Jan 11, 2021 at 1:18 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > 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. > 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
