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 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 \
