On Mon, 14 Jun 2021 at 12:45, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: [...] > I've also been led to believe that the KCOV data format is not in fact > dependent on which toolchain is used. Correct, we use KCOV with both gcc and clang. Both gcc and clang emit the same instrumentation for -fsanitize-coverage. Thus, the user-space portion and interface is indeed identical: https://www.kernel.org/doc/html/latest/dev-tools/kcov.html > > > I'm thinking it might be about time to build _one_ infrastructure for > > > that and define a kernel arc format and call it a day. > > > > > That may be nice, but it's a rather large request. > > Given GCOV just died, perhaps you can look at what KCOV does and see if > that can be extended to do as you want. KCOV is actively used and > we actually tripped over all the fun little noinstr bugs at the time. There might be a subtle mismatch between coverage instrumentation for testing/fuzzing and for profiling. (Disclaimer: I'm not too familiar with Clang-PGO's requirements.) For example, while for testing/fuzzing we may only require information if a code-path has been visited, for profiling the "hotness" might be of interest. Therefore, the user-space exported data format can make several trade-offs in complexity. In theory, I imagine there's a limit to how generic one could make profiling information, because one compiler's optimizations are not another compiler's optimizations. On the other hand, it may be doable to collect unified profiling information for common stuff, but I guess there's little motivation for figuring out the common ground given the producer and consumer of the PGO data is the same compiler by design (unlike coverage info for testing/fuzzing). Therefore, if KCOV's exposed information does not match PGO's requirements today, I'm not sure what realistically can be done without turning KCOV into a monster. Because KCOV is optimized for testing/fuzzing coverage, and I'm not sure how complex we can or want to make it to cater to a new use-case. My intuition is that the simpler design is to have 2 subsystems for instrumentation-based coverage collection: one for testing/fuzzing, and the other for profiling. Alas, there's the problem of GCOV, which should be replaceable by KCOV for most use cases. But it would be good to hear from a GCOV user if there are some. But as we learned GCOV is broken on x86 now, I see these options: 1. Remove GCOV, make KCOV the de-facto test-coverage collection subsystem. Introduce PGO-instrumentation subsystem for profile collection only, and make it _very_ clear that KCOV != PGO data as hinted above. A pre-requisite is that compiler-support for PGO instrumentation adds selective instrumentation support, likely just making attribute no_instrument_function do the right thing. 2. Like (1) but also keep GCOV, given proper support for attribute no_instrument_function would probably fix it (?). 3. Keep GCOV (and KCOV of course). Somehow extract PGO profiles from KCOV. 4. Somehow extract PGO profiles from GCOV, or modify kernel/gcov to do so. Thanks.
