On Sat, Jan 16, 2021 at 1:44 AM Bill Wendling <morbo@xxxxxxxxxx> 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 now be used by the compiler: > > $ make LLVM=1 KCFLAGS=-fprofile-use=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, unlike > the clang 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> > --- > v2: - Added "__llvm_profile_instrument_memop" based on Nathan Chancellor's > testing. > - Corrected documentation, re PGO flags when using LTO, based on Fangrui > Song's comments. > v3: - Added change log section based on Sedat Dilek's comments. > v4: - Remove non-x86 Makfile changes and se "hweight64" instead of using our > own popcount implementation, based on Nick Desaulniers's comment. > v5: - Correct padding calculation, discovered by Nathan Chancellor. Specifically for v5: Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> If anything changes drastically, please drop that and I'll retest it; otherwise for changes to the commit message or docs, feel free to carry it forward. I'll try to provide code review by EOW, assuming we can stop regressing LLVM so I can focus. (Ha!) -- Thanks, ~Nick Desaulniers
