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Tue, 13 Oct 2026 00:48:14 -0700 (PDT) Date: Tue, 13 Oct 2026 15:48:04 +0800 From: Changbin Du To: Namhyung Kim Cc: Changbin Du , David Laight , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org Subject: Re: [PATCH v3] perf bench: Add atomic operations benchmark Message-ID: References: <20261008041123.1970315-1-changbin.du@gmail.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Oct 12, 2026 at 12:31:21PM -0700, Namhyung Kim wrote: > On Thu, Oct 08, 2026 at 12:11:23PM +0800, Changbin Du wrote: [...] > > Assisted-by: opencode:DeepSeek-V4-Pro > > Signed-off-by: Changbin Du > > Reviewed-by: Namhyung Kim > > Thanks, > Namhyung > Thanks for the review. As David suggested, the fixed-time run will use a runtime in seconds instead of iteration count. Threads sync on a pthread_barrier, spin on the atomic op while counting their own operations, and stop when the main thread sets a shared done flag after the timer expires. Workers poll it every 1024 ops to keep the hot loop unperturbed. Throughput is total ops over wall time; per-thread counts verify no thread stalled. Please see the V4 patch later for these changes. > > --- > > Changes in v3: > > - Use native word size (unsigned long) for the shared counter instead > > of u64 to avoid libatomic on 32-bit targets. > > - Include explicitly for UINT64_MAX. > > > > Changes in v2: > > - Add an 'atomic inc' subcommand for contended __atomic_fetch_add. > > - Hoist the load out of the CAS retry loop; the failed > > __atomic_compare_exchange_n already returns the current value. > > - Cast the seconds delta to u64 before multiplying by NSEC_PER_SEC to > > avoid 32-bit overflow. > > --- > > tools/perf/Documentation/perf-bench.txt | 32 +++ > > tools/perf/bench/Build | 1 + > > tools/perf/bench/atomic.c | 268 ++++++++++++++++++++++++ > > tools/perf/bench/bench.h | 2 + > > tools/perf/builtin-bench.c | 9 + > > 5 files changed, 312 insertions(+) > > create mode 100644 tools/perf/bench/atomic.c > > > > diff --git a/tools/perf/Documentation/perf-bench.txt b/tools/perf/Documentation/perf-bench.txt > > index c5913cf59c98..871c3aa5fc43 100644 > > --- a/tools/perf/Documentation/perf-bench.txt > > +++ b/tools/perf/Documentation/perf-bench.txt > > @@ -58,6 +58,9 @@ SUBSYSTEM > > 'numa':: > > NUMA scheduling and MM benchmarks. > > > > +'atomic':: > > + Atomic operation benchmarks. > > + > > 'futex':: > > Futex stressing benchmarks. > > > > @@ -283,6 +286,35 @@ SUITES FOR 'numa' > > *mem*:: > > Suite for evaluating NUMA workloads. > > > > +SUITES FOR 'atomic' > > +~~~~~~~~~~~~~~~~~~~ > > +*cas*:: > > +Suite for evaluating compare-and-swap (CAS) atomic operations under > > +multi-threaded contention. > > + > > +*inc*:: > > +Suite for evaluating atomic fetch-add (increment) operations under > > +multi-threaded contention. > > + > > +Options of *cas* and *inc* > > +^^^^^^^^^^^^^^^^^^^^^^^^^^ > > +-t:: > > +--threads=:: > > +Number of threads contending for the shared counter (default: 2). > > + > > +-i:: > > +--iterations=:: > > +Number of iterations per thread (default: 100000000). > > + > > +The per-thread fastest/slowest/avg summary is only printed when > > +running with more than one thread. > > + > > +Note that the cas benchmark counts successful compare-and-swap > > +operations only; retries and the loads between them are additional > > +atomic instructions not included in the ops/sec count, so cas and > > +inc throughputs are not directly comparable at the instruction > > +level. > > + > > SUITES FOR 'futex' > > ~~~~~~~~~~~~~~~~~~ > > *hash*:: > > diff --git a/tools/perf/bench/Build b/tools/perf/bench/Build > > index 67b76fe20ba6..c64c52468d3d 100644 > > --- a/tools/perf/bench/Build > > +++ b/tools/perf/bench/Build > > @@ -3,6 +3,7 @@ perf-bench-y += sched-pipe.o > > perf-bench-y += sched-seccomp-notify.o > > perf-bench-y += syscall.o > > perf-bench-y += mem-functions.o > > +perf-bench-y += atomic.o > > perf-bench-y += futex.o > > perf-bench-y += futex-hash.o > > perf-bench-y += futex-wake.o > > diff --git a/tools/perf/bench/atomic.c b/tools/perf/bench/atomic.c > > new file mode 100644 > > index 000000000000..a7e906cf2ec1 > > --- /dev/null > > +++ b/tools/perf/bench/atomic.c > > @@ -0,0 +1,268 @@ > > +// SPDX-License-Identifier: GPL-2.0 > > +/* > > + * Copyright (C) 2026 Changbin Du > > + * > > + * Benchmarks for atomic read-modify-write operations. > > + * > > + * Measures throughput (ops/sec) of contended atomic operations on a > > + * single shared counter across multiple threads. Each thread performs > > + * a hot loop of the selected operation -- compare-and-swap (cas) or > > + * fetch-add (inc) -- competing with all other threads for the same > > + * cache line. > > + */ > > +#include "bench.h" > > +#include > > +#include "../util/debug.h" > > + > > +#ifndef HAVE_PTHREAD_BARRIER > > +int bench_atomic_cas(int argc __maybe_unused, const char **argv __maybe_unused) > > +{ > > + pr_err("%s: pthread_barrier_t unavailable, disabling this test...\n", __func__); > > + return 0; > > +} > > + > > +int bench_atomic_inc(int argc __maybe_unused, const char **argv __maybe_unused) > > +{ > > + pr_err("%s: pthread_barrier_t unavailable, disabling this test...\n", __func__); > > + return 0; > > +} > > +#else /* HAVE_PTHREAD_BARRIER */ > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include "../util/stat.h" > > + > > +static unsigned int threads = 2; > > +static unsigned int iterations = 100000000; > > + > > +static const struct option options[] = { > > + OPT_UINTEGER('t', "threads", &threads, > > + "Number of threads contending for the counter"), > > + OPT_UINTEGER('i', "iterations", &iterations, > > + "Number of iterations per thread"), > > + OPT_END() > > +}; > > + > > +static const char * const cas_usage[] = { > > + "perf bench atomic cas ", > > + NULL > > +}; > > + > > +static const char * const inc_usage[] = { > > + "perf bench atomic inc ", > > + NULL > > +}; > > + > > +enum atomic_op { > > + OP_CAS, > > + OP_INC, > > +}; > > + > > +static int bench_op = OP_CAS; > > + > > +static unsigned long shared_counter __aligned(64); > > +static pthread_barrier_t start_barrier __aligned(64); > > + > > +struct worker_stats { > > + u64 runtime_ns; > > +}; > > + > > +static void *worker(void *arg) > > +{ > > + struct worker_stats *ws = (struct worker_stats *)arg; > > + struct timespec tstart, tend; > > + u64 i; > > + > > + pthread_barrier_wait(&start_barrier); > > + > > + clock_gettime(CLOCK_MONOTONIC, &tstart); > > + switch (bench_op) { > > + case OP_CAS: > > + for (i = 0; i < iterations; i++) { > > + unsigned long old_val, new_val; > > + > > + old_val = __atomic_load_n(&shared_counter, __ATOMIC_RELAXED); > > + do { > > + new_val = old_val + 1; > > + } while (!__atomic_compare_exchange_n(&shared_counter, > > + &old_val, new_val, > > + false, > > + __ATOMIC_RELAXED, > > + __ATOMIC_RELAXED)); > > + } > > + break; > > + case OP_INC: > > + for (i = 0; i < iterations; i++) > > + __atomic_fetch_add(&shared_counter, 1, __ATOMIC_RELAXED); > > + break; > > + default: > > + break; > > + } > > + clock_gettime(CLOCK_MONOTONIC, &tend); > > + > > + ws->runtime_ns = (u64)(tend.tv_sec - tstart.tv_sec) * NSEC_PER_SEC + > > + (tend.tv_nsec - tstart.tv_nsec); > > + > > + return NULL; > > +} > > + > > +static void print_default_format(struct worker_stats *wstats, > > + struct stats *time_stats) > > +{ > > + double time_avg, time_stddev; > > + > > + printf(" Threads: %u, iterations/thread: %u, repeats: %u (warmup: 1)\n", > > + threads, iterations, bench_repeat); > > + > > + time_avg = avg_stats(time_stats); > > + time_stddev = stddev_stats(time_stats); > > + > > + printf(" Avg wall-clock time: %.3f msec (stddev %.3f msec)\n", > > + time_avg / (double)NSEC_PER_MSEC, > > + time_stddev / (double)NSEC_PER_MSEC); > > + printf(" Total ops: %'" PRIu64 "\n", > > + iterations * (u64)threads); > > + printf(" Throughput total: %'.0f ops/sec\n", > > + (double)(iterations * (u64)threads) / > > + (time_avg / (double)NSEC_PER_SEC)); > > + > > + /* > > + * Per-thread statistics from the last measured repeat show > > + * fairness / imbalance in CPU scheduling. > > + */ > > + if (threads > 1) { > > + u64 min_ns = UINT64_MAX, max_ns = 0, total_ns = 0; > > + double min_thru, max_thru, avg_thru; > > + > > + for (unsigned int t = 0; t < threads; t++) { > > + if (wstats[t].runtime_ns < min_ns) > > + min_ns = wstats[t].runtime_ns; > > + if (wstats[t].runtime_ns > max_ns) > > + max_ns = wstats[t].runtime_ns; > > + total_ns += wstats[t].runtime_ns; > > + } > > + > > + min_thru = (double)iterations * NSEC_PER_SEC / min_ns; > > + max_thru = (double)iterations * NSEC_PER_SEC / max_ns; > > + avg_thru = (double)threads * iterations * NSEC_PER_SEC / total_ns; > > + > > + printf(" Per-thread times and throughput (last repeat):\n"); > > + printf(" fastest: %.3f msec (%.0f ops/sec)\n", > > + min_ns / (double)NSEC_PER_MSEC, min_thru); > > + printf(" slowest: %.3f msec (%.0f ops/sec)\n", > > + max_ns / (double)NSEC_PER_MSEC, max_thru); > > + printf(" avg: %.3f msec (%.0f ops/sec)\n", > > + (total_ns / threads) / (double)NSEC_PER_MSEC, avg_thru); > > + } > > +} > > + > > +static int run_benchmark(void) > > +{ > > + pthread_t *thread_ids; > > + struct worker_stats *wstats; > > + struct stats time_stats; > > + unsigned int r, t; > > + > > + thread_ids = calloc(threads, sizeof(pthread_t)); > > + if (!thread_ids) > > + err(EXIT_FAILURE, "calloc"); > > + > > + wstats = calloc(threads, sizeof(struct worker_stats)); > > + if (!wstats) > > + err(EXIT_FAILURE, "calloc"); > > + > > + init_stats(&time_stats); > > + > > + for (r = 0; r < bench_repeat + 1; r++) { > > + u64 max_runtime_ns = 0; > > + > > + shared_counter = 0; > > + pthread_barrier_init(&start_barrier, NULL, threads); > > + > > + for (t = 0; t < threads; t++) { > > + if (pthread_create(&thread_ids[t], NULL, > > + worker, &wstats[t])) > > + err(EXIT_FAILURE, "pthread_create"); > > + } > > + > > + for (t = 0; t < threads; t++) > > + pthread_join(thread_ids[t], NULL); > > + > > + pthread_barrier_destroy(&start_barrier); > > + > > + for (t = 0; t < threads; t++) { > > + if (wstats[t].runtime_ns > max_runtime_ns) > > + max_runtime_ns = wstats[t].runtime_ns; > > + } > > + > > + /* > > + * Exclude the first repeat (warmup) from statistics so > > + * that cache-cold and lazy-init effects are not counted. > > + */ > > + if (r > 0) > > + update_stats(&time_stats, max_runtime_ns); > > + } > > + > > + switch (bench_format) { > > + case BENCH_FORMAT_DEFAULT: > > + print_default_format(wstats, &time_stats); > > + break; > > + > > + case BENCH_FORMAT_SIMPLE: > > + printf("%.0f\n", > > + (double)(iterations * (u64)threads) / > > + (avg_stats(&time_stats) / (double)NSEC_PER_SEC)); > > + break; > > + > > + default: > > + fprintf(stderr, "Unknown format: %d\n", bench_format); > > + exit(EXIT_FAILURE); > > + } > > + > > + free(thread_ids); > > + free(wstats); > > + > > + return 0; > > +} > > + > > +static int run_atomic_bench(int argc, const char **argv, > > + const char * const *usage) > > +{ > > + if (parse_options(argc, argv, options, usage, 0)) { > > + usage_with_options(usage, options); > > + exit(EXIT_FAILURE); > > + } > > + > > + if (threads < 1) { > > + fprintf(stderr, "Invalid thread count: %u\n", threads); > > + return 1; > > + } > > + > > + if (iterations < 1) { > > + fprintf(stderr, "Invalid iteration count: %u\n", iterations); > > + return 1; > > + } > > + > > + return run_benchmark(); > > +} > > + > > +int bench_atomic_cas(int argc, const char **argv) > > +{ > > + bench_op = OP_CAS; > > + return run_atomic_bench(argc, argv, cas_usage); > > +} > > + > > +int bench_atomic_inc(int argc, const char **argv) > > +{ > > + bench_op = OP_INC; > > + return run_atomic_bench(argc, argv, inc_usage); > > +} > > +#endif /* HAVE_PTHREAD_BARRIER */ > > diff --git a/tools/perf/bench/bench.h b/tools/perf/bench/bench.h > > index 8519eb5a42fa..85e3ac471875 100644 > > --- a/tools/perf/bench/bench.h > > +++ b/tools/perf/bench/bench.h > > @@ -30,6 +30,8 @@ int bench_mem_memcpy(int argc, const char **argv); > > int bench_mem_memset(int argc, const char **argv); > > int bench_mem_mmap(int argc, const char **argv); > > int bench_mem_find_bit(int argc, const char **argv); > > +int bench_atomic_cas(int argc, const char **argv); > > +int bench_atomic_inc(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/builtin-bench.c b/tools/perf/builtin-bench.c > > index 02d47913cc6a..62bebdedd3b8 100644 > > --- a/tools/perf/builtin-bench.c > > +++ b/tools/perf/builtin-bench.c > > @@ -14,6 +14,7 @@ > > * syscall ... System call performance > > * mem ... memory access performance > > * numa ... NUMA scheduling and MM performance > > + * atomic ... Atomic operation performance > > * futex ... Futex performance > > * epoll ... Event poll performance > > */ > > @@ -115,6 +116,13 @@ static const struct bench uprobe_benchmarks[] = { > > { NULL, NULL, NULL }, > > }; > > > > +static const struct bench atomic_benchmarks[] = { > > + { "cas", "Benchmark CAS (compare-and-swap) operations", bench_atomic_cas }, > > + { "inc", "Benchmark atomic fetch-add operations", bench_atomic_inc }, > > + { "all", "Run all atomic benchmarks", NULL }, > > + { NULL, NULL, NULL } > > +}; > > + > > struct collection { > > const char *name; > > const char *summary; > > @@ -128,6 +136,7 @@ static const struct collection collections[] = { > > #ifdef HAVE_LIBNUMA_SUPPORT > > { "numa", "NUMA scheduling and MM benchmarks", numa_benchmarks }, > > #endif > > + { "atomic", "Atomic operation benchmarks", atomic_benchmarks }, > > {"futex", "Futex stressing benchmarks", futex_benchmarks }, > > #ifdef HAVE_EVENTFD_SUPPORT > > {"epoll", "Epoll stressing benchmarks", epoll_benchmarks }, > > -- > > 2.53.0 > > -- Cheers, Changbin Du