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.

- Arnaldo
 
> This benchmark on AMD rome and Intel skylakex shows "bt" is not a good
> option except for very small bitmaps.
> 
> Signed-off-by: Ian Rogers <irog...@google.com>
> ---
>  tools/perf/bench/Build            |   1 +
>  tools/perf/bench/bench.h          |   1 +
>  tools/perf/bench/find-bit-bench.c | 135 ++++++++++++++++++++++++++++++
>  tools/perf/builtin-bench.c        |   1 +
>  4 files changed, 138 insertions(+)
>  create mode 100644 tools/perf/bench/find-bit-bench.c
> 
> diff --git a/tools/perf/bench/Build b/tools/perf/bench/Build
> index 768e408757a0..fb114bca3a8d 100644
> --- a/tools/perf/bench/Build
> +++ b/tools/perf/bench/Build
> @@ -10,6 +10,7 @@ perf-y += epoll-wait.o
>  perf-y += epoll-ctl.o
>  perf-y += synthesize.o
>  perf-y += kallsyms-parse.o
> +perf-y += find-bit-bench.o
>  
>  perf-$(CONFIG_X86_64) += mem-memcpy-x86-64-lib.o
>  perf-$(CONFIG_X86_64) += mem-memcpy-x86-64-asm.o
> diff --git a/tools/perf/bench/bench.h b/tools/perf/bench/bench.h
> index 61cae4966cae..3291b0ddddfe 100644
> --- a/tools/perf/bench/bench.h
> +++ b/tools/perf/bench/bench.h
> @@ -35,6 +35,7 @@ int bench_sched_messaging(int argc, const char **argv);
>  int bench_sched_pipe(int argc, const char **argv);
>  int bench_mem_memcpy(int argc, const char **argv);
>  int bench_mem_memset(int argc, const char **argv);
> +int bench_mem_find_bit(int argc, const char **argv);
>  int bench_futex_hash(int argc, const char **argv);
>  int bench_futex_wake(int argc, const char **argv);
>  int bench_futex_wake_parallel(int argc, const char **argv);
> diff --git a/tools/perf/bench/find-bit-bench.c 
> b/tools/perf/bench/find-bit-bench.c
> new file mode 100644
> index 000000000000..1aadbd9d7e86
> --- /dev/null
> +++ b/tools/perf/bench/find-bit-bench.c
> @@ -0,0 +1,135 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Benchmark find_next_bit and related bit operations.
> + *
> + * Copyright 2020 Google LLC.
> + */
> +#include <stdlib.h>
> +#include "bench.h"
> +#include "../util/stat.h"
> +#include <linux/bitmap.h>
> +#include <linux/bitops.h>
> +#include <linux/time64.h>
> +#include <subcmd/parse-options.h>
> +
> +static unsigned int outer_iterations = 5;
> +static unsigned int inner_iterations = 100000;
> +
> +static const struct option options[] = {
> +     OPT_UINTEGER('i', "outer-iterations", &outer_iterations,
> +             "Number of outerer iterations used"),
> +     OPT_UINTEGER('j', "inner-iterations", &inner_iterations,
> +             "Number of outerer iterations used"),
> +     OPT_END()
> +};
> +
> +static const char *const bench_usage[] = {
> +     "perf bench mem find_bit <options>",
> +     NULL
> +};
> +
> +static unsigned int accumulator;
> +static unsigned int use_of_val;
> +
> +static noinline void workload(int val)
> +{
> +     use_of_val += val;
> +     accumulator++;
> +}
> +
> +#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;
> +}
> +#else
> +#define asm_test_bit test_bit
> +#endif
> +
> +static int do_for_each_set_bit(unsigned int num_bits)
> +{
> +     unsigned long *to_test = bitmap_alloc(num_bits);
> +     struct timeval start, end, diff;
> +     u64 runtime_us;
> +     struct stats fb_time_stats, tb_time_stats;
> +     double time_average, time_stddev;
> +     unsigned int bit, i, j;
> +     unsigned int set_bits, skip;
> +     unsigned int old;
> +
> +     init_stats(&fb_time_stats);
> +     init_stats(&tb_time_stats);
> +
> +     for (set_bits = 1; set_bits <= num_bits; set_bits <<= 1) {
> +             bitmap_zero(to_test, num_bits);
> +             skip = num_bits / set_bits;
> +             for (i = 0; i < num_bits; i += skip)
> +                     set_bit(i, to_test);
> +
> +             for (i = 0; i < outer_iterations; i++) {
> +                     old = accumulator;
> +                     gettimeofday(&start, NULL);
> +                     for (j = 0; j < inner_iterations; j++) {
> +                             for_each_set_bit(bit, to_test, num_bits)
> +                                     workload(bit);
> +                     }
> +                     gettimeofday(&end, NULL);
> +                     assert(old + (inner_iterations * set_bits) == 
> accumulator);
> +                     timersub(&end, &start, &diff);
> +                     runtime_us = diff.tv_sec * USEC_PER_SEC + diff.tv_usec;
> +                     update_stats(&fb_time_stats, runtime_us);
> +
> +                     old = accumulator;
> +                     gettimeofday(&start, NULL);
> +                     for (j = 0; j < inner_iterations; j++) {
> +                             for (bit = 0; bit < num_bits; bit++) {
> +                                     if (asm_test_bit(bit, to_test))
> +                                             workload(bit);
> +                             }
> +                     }
> +                     gettimeofday(&end, NULL);
> +                     assert(old + (inner_iterations * set_bits) == 
> accumulator);
> +                     timersub(&end, &start, &diff);
> +                     runtime_us = diff.tv_sec * USEC_PER_SEC + diff.tv_usec;
> +                     update_stats(&tb_time_stats, runtime_us);
> +             }
> +
> +             printf("%d operations %d bits set of %d bits\n",
> +                     inner_iterations, set_bits, num_bits);
> +             time_average = avg_stats(&fb_time_stats);
> +             time_stddev = stddev_stats(&fb_time_stats);
> +             printf("  Average for_each_set_bit took: %.3f usec (+- %.3f 
> usec)\n",
> +                     time_average, time_stddev);
> +             time_average = avg_stats(&tb_time_stats);
> +             time_stddev = stddev_stats(&tb_time_stats);
> +             printf("  Average test_bit loop took:    %.3f usec (+- %.3f 
> usec)\n",
> +                     time_average, time_stddev);
> +
> +             if (use_of_val == accumulator)  /* Try to avoid compiler 
> tricks. */
> +                     printf("\n");
> +     }
> +     bitmap_free(to_test);
> +     return 0;
> +}
> +
> +int bench_mem_find_bit(int argc, const char **argv)
> +{
> +     int err = 0, i;
> +
> +     argc = parse_options(argc, argv, options, bench_usage, 0);
> +     if (argc) {
> +             usage_with_options(bench_usage, options);
> +             exit(EXIT_FAILURE);
> +     }
> +
> +     for (i = 1; i <= 2048; i <<= 1)
> +             do_for_each_set_bit(i);
> +
> +     return err;
> +}
> diff --git a/tools/perf/builtin-bench.c b/tools/perf/builtin-bench.c
> index cad31b1d3438..690eee1120a7 100644
> --- a/tools/perf/builtin-bench.c
> +++ b/tools/perf/builtin-bench.c
> @@ -52,6 +52,7 @@ static struct bench sched_benchmarks[] = {
>  static struct bench mem_benchmarks[] = {
>       { "memcpy",     "Benchmark for memcpy() functions",             
> bench_mem_memcpy        },
>       { "memset",     "Benchmark for memset() functions",             
> bench_mem_memset        },
> +     { "find_bit",   "Benchmark for find_bit() functions",           
> bench_mem_find_bit      },
>       { "all",        "Run all memory access benchmarks",             NULL    
>                 },
>       { NULL,         NULL,                                           NULL    
>                 }
>  };
> -- 
> 2.28.0.rc0.142.g3c755180ce-goog
> 

-- 

- Arnaldo

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