Hi Bill, On Tue, Jan 12, 2021 at 10:19:58PM -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 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> > Change-Id: Ic78e69c682286d3a44c4549a0138578c98138b77 Small nit: This should be removed. I applied this patch on top of v5.11-rc3, built it with LLVM 12 (f1d5cbbdee5526bc86eac0a5652b115d9bc158e5 + D94470) with Microsoft's WSL 5.4 config [1] + CONFIG_PGO_CLANG=y, and ran it on WSL2. $ zgrep PGO /proc/config.gz # Profile Guided Optimization (PGO) (EXPERIMENTAL) CONFIG_ARCH_SUPPORTS_PGO_CLANG=y CONFIG_PGO_CLANG=y # end of Profile Guided Optimization (PGO) (EXPERIMENTAL) However, I see an issue with actually using the data: $ sudo -s # mount -t debugfs none /sys/kernel/debug # cp -a /sys/kernel/debug/pgo/profraw vmlinux.profraw # chown nathan:nathan vmlinux.profraw # exit $ tc-build/build/llvm/stage1/bin/llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw warning: vmlinux.profraw: Invalid instrumentation profile data (bad magic) error: No profiles could be merged. Am I holding it wrong? :) Note, this is virtualized, I do not have any "real" x86 hardware that I can afford to test on right now. [1]: https://github.com/microsoft/WSL2-Linux-Kernel/raw/linux-msft-wsl-5.4.y/Microsoft/config-wsl Cheers, Nathan
