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 4C98B3CAA55; Mon, 12 Oct 2026 19:31:23 +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=1791833484; cv=none; b=nMdk7x+vjSQgPSm2qRqvY4lPmixMi8yfw7BQRGXev89uRYTWE39zmc4Zr2H28+cXlZO/rdPe8D/a+SE0ZeO9GarM1DDg0dAB7+oV79tKJYJ/nBtbpE+UwsyKNA2v+JUksT3xbmUbsArOxq8QB/LgtxD4xaDgFJh1WoSKozWuBFM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791833484; c=relaxed/simple; bh=OrQ1YY4DuoZLuAGX+lVhjHZRvOjKZdU580NnKoeD0w0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=D/XD5sIu4jzPWydXhS3JohslWV0TVFMheuQWzU0KHb53+/WAP0TxZOtEOQQNhx2/IbRXmlnEQQxfvDUyvnYGi863e7HXGiQknbG1jkap/DwyhwsFbWi3oRiIK03bLwfdQNIVzZIsVss5cPYz8l0z5C1V6W1HuO510KqL/Yaij4M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=S6eWxkca; 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="S6eWxkca" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AB19F1F0089B; Mon, 12 Oct 2026 19:31:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791833483; bh=RGtllKCd8fsk4pyX+3nLVPSM5ErTPJMjld7HACxueTg=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=S6eWxkcajw/hDLS/OZTMmXHQ7mFN9T1DqZcUetQ5/S1WPmFdDRWqrxAgP0NbNwkQI LoCQL4s/lM7F/5XRn26R1Fbey3Pk+bjLpaWDeMre3dmh3volfveyo5mvMMs8n1gjB5 Pet7h7SnK2Q66rFkB0SpG/6JtAvA8TeaNdCu/6WVCBzgJR6RMAPqBn6/6OGNNULo0v ZUBDJM/iut736HTi7rHo1rLgMNVGA5cX/55i3bhlTdTRpKC/vj9ph1WdsE6VwjA9yP gWAdpdXQm/nHmJ7r1ZqWq+MMeclfQRlBNubxiLrhiW0GS0bTrcQp7J7aHkLzC6j0o6 Pz9RG+Rg3Qglw== Date: Mon, 12 Oct 2026 12:31:21 -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 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=utf-8 Content-Disposition: inline In-Reply-To: <20261008041123.1970315-1-changbin.du@gmail.com> On Thu, Oct 08, 2026 at 12:11:23PM +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 configurable thread count and iteration count 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: > - 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, > incrementing it from 0 to iterations. > - The shared counter is cache-line aligned (64 bytes) to isolate > contention to the target cache line and avoid false sharing. > - The wall-clock time is measured as the max of all per-thread > runtimes (the time for the slowest thread to finish). > - 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, iterations/thread: 100000000, repeats: 10 (warmup: 1) > Avg wall-clock time: 3286.297 msec (stddev 6.820 msec) > Total ops: 200,000,000 > Throughput total: 60,858,769 ops/sec > Per-thread times and throughput (last repeat): > fastest: 3304.312 msec (30263489 ops/sec) > slowest: 3325.518 msec (30070504 ops/sec) > avg: 3314.915 msec (30166688 ops/sec) > > # Running atomic/inc benchmark... > Threads: 2, iterations/thread: 100000000, repeats: 10 (warmup: 1) > Avg wall-clock time: 2170.716 msec (stddev 7.306 msec) > Total ops: 200,000,000 > Throughput total: 92,135,503 ops/sec > Per-thread times and throughput (last repeat): > fastest: 2196.458 msec (45527849 ops/sec) > slowest: 2208.764 msec (45274198 ops/sec) > avg: 2202.611 msec (45400669 ops/sec) > > Output fields explained: > - Threads: number of contending threads > - iterations/thread: atomic operations each thread performs > - repeats: number of test runs (first is warmup) > - Avg wall-clock time: mean time for all threads to complete > - stddev: standard deviation across repeats > - Total ops: threads x iterations/thread > - Throughput total: aggregate ops/sec across all threads > - Per-thread times: fastest/slowest/avg thread completion time > - Per-thread throughput: per-thread ops/sec (shows scheduling imbalance) > > Assisted-by: opencode:DeepSeek-V4-Pro > Signed-off-by: Changbin Du Reviewed-by: Namhyung Kim Thanks, Namhyung > --- > 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 >