Messages in this thread | | | From | Ian Rogers <> | Date | Wed, 29 Jul 2020 15:03:43 -0700 | Subject | Re: [PATCH] perf bench: Add benchmark of find_next_bit |
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On Wed, Jul 29, 2020 at 1:44 PM Arnaldo Carvalho de Melo <acme@kernel.org> wrote: > > Em Wed, Jul 29, 2020 at 04:59:18PM -0300, Arnaldo Carvalho de Melo escreveu: > > Em Tue, Jul 28, 2020 at 08:51:52AM -0300, Arnaldo Carvalho de Melo escreveu: > > > Em Fri, Jul 24, 2020 at 12:19:59AM -0700, Ian Rogers escreveu: > > > > for_each_set_bit, or similar functions like for_each_cpu, may be hot > > > > within the kernel. If many bits were set then one could imagine on > > > > Intel a "bt" instruction with every bit may be faster than the function > > > > call and word length find_next_bit logic. Add a benchmark to measure > > > > this. > > > > > > Thanks, applied. > > > > > This benchmark on AMD rome and Intel skylakex shows "bt" is not a good > > > > option except for very small bitmaps. > > > > > > +++ b/tools/perf/bench/find-bit-bench.c > > > > > > +#if defined(__i386__) || defined(__x86_64__) > > > > +static bool asm_test_bit(long nr, const unsigned long *addr) > > > > +{ > > > > + bool oldbit; > > > > + > > > > + asm volatile("bt %2,%1" > > > > + : "=@ccc" (oldbit) > > > > + : "m" (*(unsigned long *)addr), "Ir" (nr) : "memory"); > > > > + > > > > + return oldbit; > > > > Some old clang versions are not liking this: > > Failed with: > > clang version 3.8.0 (tags/RELEASE_380/final) > clang version 3.8.1 (tags/RELEASE_381/final) > clang version 4.0.0 (tags/RELEASE_400/final) > Alpine clang version 8.0.0 (tags/RELEASE_800/final) (based on LLVM 8.0.0) > Alpine clang version 5.0.0 (tags/RELEASE_500/final) (based on LLVM 5.0.0) > Alpine clang version 5.0.1 (tags/RELEASE_501/final) (based on LLVM 5.0.1) > Alpine clang version 5.0.1 (tags/RELEASE_502/final) (based on LLVM 5.0.1) > > Worked with: > > Alpine clang version 9.0.0 (https://git.alpinelinux.org/aports f7f0d2c2b8bcd6a5843401a9a702029556492689) (based on LLVM 9.0.0) > Alpine clang version 10.0.0 (https://gitlab.alpinelinux.org/alpine/aports.git 7445adce501f8473efdb93b17b5eaf2f1445ed4c) > Alpine clang version 10.0.0 (git://git.alpinelinux.org/aports 7445adce501f8473efdb93b17b5eaf2f1445ed4c) > > > Also failed for; > > # grep FAIL dm.log/summary | grep -v alpine > alt:p8: FAIL > clang version 3.8.0 (tags/RELEASE_380/final) > alt:p9: FAIL > clang version 7.0.1 > amazonlinux:1: FAIL > clang version 3.6.2 (tags/RELEASE_362/final) > amazonlinux:2: FAIL > clang version 7.0.1 (Amazon Linux 2 7.0.1-1.amzn2.0.2) > #
Thanks, I added a __GCC_ASM_FLAG_OUTPUTS__ guard: https://lore.kernel.org/lkml/20200729220034.1337168-1-irogers@google.com/T/#u
Ian
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