On Mon, Jan 11, 2021 at 12:39 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote: > > On Mon, Jan 11, 2021 at 9:18 AM 'Bill Wendling' via Clang Built Linux > <clang-built-linux@xxxxxxxxxxxxxxxx> 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 > > > > Hi Bill and Sami, > > I have seen the pull-request in the CBL issue tracker and had some > questions in mind. > > Good you send this. > > First of all, I like to fetch any development stuff easily from a Git > repository. The version in the pull-request in the CBL issue tracker is roughly the same as this patch. (There are some changes, but they aren't functionality changes.) > Can you offer this, please? > What is the base for your work? > I hope this is (fresh released) Linux v5.11-rc3. > This patch (and the PR on the CBL issue tracker) are from top-of-tree Linux. > I myself had some experiences with a PGO + ThinLTO optimized LLVM > toolchain built with the help of tc-build. > Here it takes very long to build it. > > This means I have some profile-data archived. > Can I use it? > LLVM is more tolerant of "stale" profile data than gcov, so it's possible that your archived profile data would still work, but I can't guarantee that it will be better than using new profile data. > Is an own PGO + ThinLTO optimized LLVM toolchain pre-requirement for > this or not? > That is one of my important questions. > Do you mean that the LLVM tools (clang, llc, etc.) are compiled with PGO + ThinLTO? -bw
