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Tue, 13 Oct 2026 01:57:02 -0700 (PDT) Received: from morefine ([5.34.221.10]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-cd3da03f449sm5230806a12.28.2026.10.13.01.56.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 13 Oct 2026 01:57:01 -0700 (PDT) From: Changbin Du To: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim Cc: Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, David Laight , Changbin Du Subject: [PATCH v4] perf bench: Add atomic operations benchmark Date: Tue, 13 Oct 2026 16:56:48 +0800 Message-ID: <20261013085648.976819-1-changbin.du@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Add a new 'atomic' collection to perf bench for benchmarking atomic read-modify-write operations with multi-threaded contention testing. It provides two subcommands: - atomic cas: contended compare-and-swap operations (__atomic_compare_exchange_n) - atomic inc: contended fetch-add operations (__atomic_fetch_add) Both support a configurable thread count and runtime to measure atomic contention effects. Why this benchmark is needed: - Atomic read-modify-write operations are fundamental to lock-free algorithms and data structures. Understanding their performance characteristics under contention is critical for designing high-performance concurrent applications. - The benchmark helps identify atomic operation latency and scalability issues across different thread counts, revealing contention patterns that are not visible in single-threaded tests. - Useful for evaluating atomic implementation quality on different architectures and for regression testing after changes to atomic primitives or memory ordering. Measurement methodology: - By default two threads contend for the counter, enough to expose the contention effect; use -t to scale the count. - Each thread starts a private timer (clock_gettime CLOCK_MONOTONIC) after synchronizing on a pthread_barrier, ensuring all threads begin simultaneously. - Each thread performs a hot loop of the selected atomic operation (compare-and-swap or fetch-add) on a shared unsigned long counter for a fixed runtime. It counts its own completed operations in a local variable, and the loop runs in batches so the termination flag is only polled between batches. - The main thread sleeps for the runtime and then sets the flag; the throughput of a repeat is the total operation count divided by the measured wall time (the max per-thread runtime). - The shared counter is cache-line aligned (64 bytes) to isolate contention to the target cache line and avoid false sharing. - The per-thread operation counts of the last repeat show whether any thread failed to make progress under contention. - The first repeat is excluded from statistics as a warmup phase to avoid cache-cold effects. Example usage: $ perf bench atomic all # Running atomic/cas benchmark... Threads: 2, runtime: 2 sec, repeats: 10 (warmup: 1) Avg throughput: 68,599,954 ops/sec (+- 0.13%) Per-thread ops (last repeat): fastest: 74,842,112 ops slowest: 63,176,704 ops avg: 69,009,408 ops # Running atomic/inc benchmark... Threads: 2, runtime: 2 sec, repeats: 10 (warmup: 1) Avg throughput: 128,428,772 ops/sec (+- 0.54%) Per-thread ops (last repeat): fastest: 146,230,272 ops slowest: 112,825,344 ops avg: 129,527,808 ops Output fields explained: - Threads: number of contending threads - runtime: seconds each thread runs per repeat - repeats: number of test runs (first is warmup) - Avg throughput: mean aggregate ops/sec across repeats - +- %: relative standard error of the mean throughput across repeats - Per-thread ops: fastest/slowest/avg operations per thread in the last repeat (shows scheduling imbalance and lack of progress) Assisted-by: opencode:DeepSeek-V4-Pro Signed-off-by: Changbin Du --- Changes in v4: - Default to 2 threads (1 on a single-CPU machine). - Replace the iteration count with a fixed runtime per repeat. - Report per-thread operation counts to show lack of progress. 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 | 38 +++ tools/perf/bench/Build | 1 + tools/perf/bench/atomic.c | 295 ++++++++++++++++++++++++ tools/perf/bench/bench.h | 2 + tools/perf/builtin-bench.c | 9 + 5 files changed, 345 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..2a1206fea0c9 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 +286,41 @@ 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, or +1 on a single-CPU machine). + +-r:: +--runtime=:: +Runtime of each repeat in seconds (default: 2). + +The per-thread fastest/slowest/avg ops summary is only printed when +running with more than one thread. + +The average throughput is annotated with its relative deviation +("+- x%"), i.e. the standard error of the mean over the measured +repeats. A small value means the average is reproducible; it does not +reflect per-thread fairness, which the per-thread ops summary shows. + +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..2da74bf078bc --- /dev/null +++ b/tools/perf/bench/atomic.c @@ -0,0 +1,295 @@ +// 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. Threads run for a fixed runtime, so a system that + * cannot make progress under heavy contention reports a low (or + * zero) throughput instead of hanging. + */ +#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 +#include +#include +#include "../util/stat.h" + +static unsigned int threads; /* 0: default to min(2, online CPUs) */ +static unsigned int runtime = 2; /* seconds per repeat */ + +static const struct option options[] = { + OPT_UINTEGER('t', "threads", &threads, + "Number of contending threads (default: 2, or 1 on single CPU)"), + OPT_UINTEGER('r', "runtime", &runtime, + "Runtime of each repeat in seconds (default: 2)"), + 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; + +/* + * The termination flag is polled between batches only, so that the + * check does not perturb the measured loop. + */ +#define BATCH_OPS 1024 + +static unsigned long shared_counter __aligned(64); +static pthread_barrier_t start_barrier __aligned(64); +static bool done; + +struct worker_stats { + u64 runtime_ns; + u64 ops; +}; + +static void cas_batch(void) +{ + unsigned long i; + + for (i = 0; i < BATCH_OPS; 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)); + } +} + +static void inc_batch(void) +{ + unsigned long i; + + for (i = 0; i < BATCH_OPS; i++) + __atomic_fetch_add(&shared_counter, 1, __ATOMIC_RELAXED); +} + +static void *worker(void *arg) +{ + struct worker_stats *ws = (struct worker_stats *)arg; + struct timespec tstart, tend; + u64 ops = 0; + + pthread_barrier_wait(&start_barrier); + + clock_gettime(CLOCK_MONOTONIC, &tstart); + while (!__atomic_load_n(&done, __ATOMIC_RELAXED)) { + switch (bench_op) { + case OP_CAS: + cas_batch(); + break; + case OP_INC: + inc_batch(); + break; + default: + BUG_ON(1); + break; + } + ops += BATCH_OPS; + } + clock_gettime(CLOCK_MONOTONIC, &tend); + + ws->runtime_ns = (u64)(tend.tv_sec - tstart.tv_sec) * NSEC_PER_SEC + + (tend.tv_nsec - tstart.tv_nsec); + ws->ops = ops; + + return NULL; +} + +static void print_default_format(struct worker_stats *wstats, + struct stats *throughput_stats) +{ + double thru_avg, thru_stddev; + + printf(" Threads: %u, runtime: %u sec, repeats: %u (warmup: 1)\n", + threads, runtime, bench_repeat); + + thru_avg = avg_stats(throughput_stats); + thru_stddev = stddev_stats(throughput_stats); + + printf(" Avg throughput: %'.0f ops/sec (+- %.2f%%)\n", + thru_avg, rel_stddev_stats(thru_stddev, thru_avg)); + + /* + * Per-thread operation counts from the last measured repeat show + * whether any thread failed to make progress under contention. + */ + if (threads > 1) { + u64 min_ops = UINT64_MAX, max_ops = 0, total_ops = 0; + + for (unsigned int t = 0; t < threads; t++) { + if (wstats[t].ops < min_ops) + min_ops = wstats[t].ops; + if (wstats[t].ops > max_ops) + max_ops = wstats[t].ops; + total_ops += wstats[t].ops; + } + + printf(" Per-thread ops (last repeat):\n"); + printf(" fastest: %'" PRIu64 " ops\n", max_ops); + printf(" slowest: %'" PRIu64 " ops\n", min_ops); + printf(" avg: %'" PRIu64 " ops\n", total_ops / threads); + } +} + +static int run_benchmark(void) +{ + pthread_t *thread_ids; + struct worker_stats *wstats; + struct stats throughput_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(&throughput_stats); + + for (r = 0; r < bench_repeat + 1; r++) { + u64 max_runtime_ns = 0, total_ops = 0; + double throughput; + + shared_counter = 0; + __atomic_store_n(&done, false, __ATOMIC_RELAXED); + 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"); + } + + sleep(runtime); + __atomic_store_n(&done, true, __ATOMIC_RELAXED); + + for (t = 0; t < threads; t++) + pthread_join(thread_ids[t], NULL); + + pthread_barrier_destroy(&start_barrier); + + for (t = 0; t < threads; t++) { + total_ops += wstats[t].ops; + 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 && max_runtime_ns) { + throughput = (double)total_ops / + ((double)max_runtime_ns / NSEC_PER_SEC); + update_stats(&throughput_stats, (u64)throughput); + } + } + + switch (bench_format) { + case BENCH_FORMAT_DEFAULT: + print_default_format(wstats, &throughput_stats); + break; + + case BENCH_FORMAT_SIMPLE: + printf("%.0f\n", avg_stats(&throughput_stats)); + 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) +{ + struct perf_cpu_map *cpu; + + if (parse_options(argc, argv, options, usage, 0)) { + usage_with_options(usage, options); + exit(EXIT_FAILURE); + } + + if (!threads) { /* default to 2 threads (1 on a single-CPU machine) */ + cpu = perf_cpu_map__new_online_cpus(); + threads = (cpu && perf_cpu_map__nr(cpu) > 1) ? 2 : 1; + perf_cpu_map__put(cpu); + } + + if (runtime < 1) { + fprintf(stderr, "Invalid runtime: %u\n", runtime); + 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