On Mon, Jun 14, 2021 at 3:45 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Mon, Jun 14, 2021 at 02:39:41AM -0700, Bill Wendling wrote: > > On Mon, Jun 14, 2021 at 2:01 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > Because having GCOV, KCOV and PGO all do essentially the same thing > > > differently, makes heaps of sense? > > > > > It does when you're dealing with one toolchain without access to another. > > Here's a sekrit, don't tell anyone, but you can get a free copy of GCC > right here: > > https://gcc.gnu.org/ > > We also have this linux-toolchains list (Cc'ed now) that contains folks > from both sides. > Your sarcasm is not useful. > > > I understand that the compilers actually generates radically different > > > instrumentation for the various cases, but essentially they're all > > > collecting (function/branch) arcs. > > > > > That's true, but there's no one format for profiling data that's > > usable between all compilers. I'm not even sure there's a good way to > > translate between, say, gcov and llvm's format. To make matters more > > complicated, each compiler's format is tightly coupled to a specific > > version of that compiler. And depending on *how* the data is collected > > (e.g. sampling or instrumentation), it may not give us the full > > benefit of FDO/PGO. > > I'm thinking that something simple like: > > struct arc { > u64 from; > u64 to; > u64 nr; > u64 cntrs[0]; > }; > > goes a very long way. Stick a header on that says how large cntrs[] is, > and some other data (like load offset and whatnot) and you should be > good. > > Combine that with the executable image (say /proc/kcore) to recover > what's @from (call, jmp or conditional branch) and I'm thinking one > ought to be able to construct lots of useful data. > > I've also been led to believe that the KCOV data format is not in fact > dependent on which toolchain is used. > > > > 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.
