futex-wake.c

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// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2013  Davidlohr Bueso <davidlohr@hp.com>
 *
 * futex-wake: Block a bunch of threads on a futex and wake'em up, N at a time.
 *
 * This program is particularly useful to measure the latency of nthread wakeups
 * in non-error situations:  all waiters are queued and all wake calls wakeup
 * one or more tasks, and thus the waitqueue is never empty.
 */

/* For the CLR_() macros */
#include <string.h>
#include <pthread.h>

#include <signal.h>
#include "../util/stat.h"
#include <subcmd/parse-options.h>
#include <linux/compiler.h>
#include <linux/kernel.h>
#include <linux/time64.h>
#include <errno.h>
#include "bench.h"
#include "futex.h"
#include "cpumap.h"

#include <err.h>
#include <stdlib.h>
#include <sys/time.h>

/* all threads will block on the same futex */
static u_int32_t futex1 = 0;

/*
 * How many wakeups to do at a time.
 * Default to 1 in order to make the kernel work more.
 */
static unsigned int nwakes = 1;

pthread_t *worker;
static bool done = false, silent = false, fshared = false;
static pthread_mutex_t thread_lock;
static pthread_cond_t thread_parent, thread_worker;
static struct stats waketime_stats, wakeup_stats;
static unsigned int ncpus, threads_starting, nthreads = 0;
static int futex_flag = 0;

static const struct option options[] = {
    OPT_UINTEGER('t', "threads", &nthreads, "Specify amount of threads"),
    OPT_UINTEGER('w', "nwakes",  &nwakes,   "Specify amount of threads to wake at once"),
    OPT_BOOLEAN( 's', "silent",  &silent,   "Silent mode: do not display data/details"),
    OPT_BOOLEAN( 'S', "shared",  &fshared,  "Use shared futexes instead of private ones"),
    OPT_END()
};

static const char * const bench_futex_wake_usage[] = {
    "perf bench futex wake <options>",
    NULL
};

static void *workerfn(void *arg __maybe_unused)
{
    pthread_mutex_lock(&thread_lock);
    threads_starting--;
    if (!threads_starting)
        pthread_cond_signal(&thread_parent);
    pthread_cond_wait(&thread_worker, &thread_lock);
    pthread_mutex_unlock(&thread_lock);

    while (1) {
        if (futex_wait(&futex1, 0, NULL, futex_flag) != EINTR)
            break;
    }

    pthread_exit(NULL);
    return NULL;
}

static void print_summary(void)
{
    double waketime_avg = avg_stats(&waketime_stats);
    double waketime_stddev = stddev_stats(&waketime_stats);
    unsigned int wakeup_avg = avg_stats(&wakeup_stats);

    printf("Wokeup %d of %d threads in %.4f ms (+-%.2f%%)\n",
           wakeup_avg,
           nthreads,
           waketime_avg / USEC_PER_MSEC,
           rel_stddev_stats(waketime_stddev, waketime_avg));
}

static void block_threads(pthread_t *w,
              pthread_attr_t thread_attr, struct cpu_map *cpu)
{
    cpu_set_t cpuset;
    unsigned int i;

    threads_starting = nthreads;

    /* create and block all threads */
    for (i = 0; i < nthreads; i++) {
        CPU_ZERO(&cpuset);
        CPU_SET(cpu->map[i % cpu->nr], &cpuset);

        if (pthread_attr_setaffinity_np(&thread_attr, sizeof(cpu_set_t), &cpuset))
            err(EXIT_FAILURE, "pthread_attr_setaffinity_np");

        if (pthread_create(&w[i], &thread_attr, workerfn, NULL))
            err(EXIT_FAILURE, "pthread_create");
    }
}

static void toggle_done(int sig __maybe_unused,
            siginfo_t *info __maybe_unused,
            void *uc __maybe_unused)
{
    done = true;
}

int bench_futex_wake(int argc, const char **argv)
{
    int ret = 0;
    unsigned int i, j;
    struct sigaction act;
    pthread_attr_t thread_attr;
    struct cpu_map *cpu;

    argc = parse_options(argc, argv, options, bench_futex_wake_usage, 0);
    if (argc) {
        usage_with_options(bench_futex_wake_usage, options);
        exit(EXIT_FAILURE);
    }

    cpu = cpu_map__new(NULL);
    if (!cpu)
        err(EXIT_FAILURE, "calloc");

    sigfillset(&act.sa_mask);
    act.sa_sigaction = toggle_done;
    sigaction(SIGINT, &act, NULL);

    if (!nthreads)
        nthreads = ncpus;

    worker = calloc(nthreads, sizeof(*worker));
    if (!worker)
        err(EXIT_FAILURE, "calloc");

    if (!fshared)
        futex_flag = FUTEX_PRIVATE_FLAG;

    printf("Run summary [PID %d]: blocking on %d threads (at [%s] futex %p), "
           "waking up %d at a time.\n\n",
           getpid(), nthreads, fshared ? "shared":"private",  &futex1, nwakes);

    init_stats(&wakeup_stats);
    init_stats(&waketime_stats);
    pthread_attr_init(&thread_attr);
    pthread_mutex_init(&thread_lock, NULL);
    pthread_cond_init(&thread_parent, NULL);
    pthread_cond_init(&thread_worker, NULL);

    for (j = 0; j < bench_repeat && !done; j++) {
        unsigned int nwoken = 0;
        struct timeval start, end, runtime;

        /* create, launch & block all threads */
        block_threads(worker, thread_attr, cpu);

        /* make sure all threads are already blocked */
        pthread_mutex_lock(&thread_lock);
        while (threads_starting)
            pthread_cond_wait(&thread_parent, &thread_lock);
        pthread_cond_broadcast(&thread_worker);
        pthread_mutex_unlock(&thread_lock);

        usleep(100000);

        /* Ok, all threads are patiently blocked, start waking folks up */
        gettimeofday(&start, NULL);
        while (nwoken != nthreads)
            nwoken += futex_wake(&futex1, nwakes, futex_flag);
        gettimeofday(&end, NULL);
        timersub(&end, &start, &runtime);

        update_stats(&wakeup_stats, nwoken);
        update_stats(&waketime_stats, runtime.tv_usec);

        if (!silent) {
            printf("[Run %d]: Wokeup %d of %d threads in %.4f ms\n",
                   j + 1, nwoken, nthreads, runtime.tv_usec / (double)USEC_PER_MSEC);
        }

        for (i = 0; i < nthreads; i++) {
            ret = pthread_join(worker[i], NULL);
            if (ret)
                err(EXIT_FAILURE, "pthread_join");
        }

    }

    /* cleanup & report results */
    pthread_cond_destroy(&thread_parent);
    pthread_cond_destroy(&thread_worker);
    pthread_mutex_destroy(&thread_lock);
    pthread_attr_destroy(&thread_attr);

    print_summary();

    free(worker);
    return ret;
}