From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6E3B93D7A01; Wed, 14 Oct 2026 21:28:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792013286; cv=none; b=oclnRQAIFlmrOrJKnrHvdJvaIIDGhOQy7Wosf+9x0NSXg3ZtY0CTFD62ZTXYOrwq4W0Ecdgno/ZKhwpj80WcnfGxuUBh0+PSVAibZIRBCi/CXKMX7Sm1RUcnT4XgINPAp8Neq0Qmoduvox26EZC/ho5OLD7brAQdJbPr1VgrofE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792013286; c=relaxed/simple; bh=aQig4N2xfmlRsaa/N0/8aa4FiWF4o63+0/+9xvd7Vks=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kZjTR2GKYUH+YCRQ0rdfJzEfcCmT0uXcCdY875n7hrFtC1P/cRSKnyn+zK+NFV8N2gpQMF9u1ZfFRp6XdkFaVB0P2l06yxcdsyJOdhb/03dnq0BQtBFIRvIZkWHbTV4T6DLLf37PT/thoTCtbz3nLxg5Hd2c7eidlNoNiVqLD7w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XzbjU+HB; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="XzbjU+HB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8833C1F000C4; Wed, 14 Oct 2026 21:28:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792013283; bh=FKZH0us2Zujglnx4VyL4YCvCAGYcpqLAKyu9Q12YHnY=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=XzbjU+HBBlqURUbKwJ80sJ/kifxc61xRosbYCw8uQ9QVFWpy6DGYUQqBAdvg54Ixl PiMEJ5Jpi5fQAVPr/4XioeRL4jukix9bA/QD/2o1T9oijCGEQLDumPH6DIXT0OASgi 3D+o5E04kN3IbtfFBA4+InWVw9jAcaeOeexnXTrDUCxWkuiEvsyCOaZIpcivzOuB6g X6moeo48mXFiM7G6XTSdn7yUGpExFMAdJ6ET7v62PYfNpHSnmIfhpJgsOkV8GW6zj/ jekiePt5HMGqh78hx3DCTYxTtruXgOh3ZFq7TVmQnaBSERhQYbvKn3PwxQBWOHG8k8 IGRwaCtuYNMIQ== Date: Wed, 14 Oct 2026 14:28:02 -0700 From: Namhyung Kim To: Changbin Du Cc: 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, David Laight Subject: Re: [PATCH v4] perf bench: Add atomic operations benchmark Message-ID: References: <20261013085648.976819-1-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=utf-8 Content-Disposition: inline In-Reply-To: <20261013085648.976819-1-changbin.du@gmail.com> On Tue, Oct 13, 2026 at 04:56:48PM +0800, Changbin Du wrote: > 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, Can you please pick a different short option than 'r'? perf bench itself has -r/--repeat option so it would be confusing. It doesn't clash for now, but I think it's better to use perf bench options from 'perf bench atomic ...' later. Otherwise, looks good to me. Thanks, Namhyung > + "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 >