lib/test_workqueue.c

// SPDX-License-Identifier: GPL-2.0

/*
 * Test module for stress and performance analysis of workqueue.
 *
 * Benchmarks queue_work() throughput on an unbound workqueue to measure
 * pool->lock contention under different affinity scope configurations
 * (e.g., cache vs cache_shard).
 *
 * The affinity scope is changed between runs via the workqueue's sysfs
 * affinity_scope attribute (WQ_SYSFS).
 *
 * Copyright (c) 2026 Meta Platforms, Inc. and affiliates
 * Copyright (c) 2026 Breno Leitao <leitao@debian.org>
 *
 */
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/workqueue.h>
#include <linux/kthread.h>
#include <linux/moduleparam.h>
#include <linux/completion.h>
#include <linux/atomic.h>
#include <linux/slab.h>
#include <linux/ktime.h>
#include <linux/cpumask.h>
#include <linux/sched.h>
#include <linux/sort.h>
#include <linux/fs.h>

#define WQ_NAME "bench_wq"
#define SCOPE_PATH "/sys/bus/workqueue/devices/" WQ_NAME "/affinity_scope"

static int nr_threads;
module_param(nr_threads, int, 0444);
MODULE_PARM_DESC(nr_threads,
		 "Number of threads to spawn (default: 0 = num_online_cpus())");

static int wq_items = 50000;
module_param(wq_items, int, 0444);
MODULE_PARM_DESC(wq_items,
		 "Number of work items each thread queues (default: 50000)");

static struct workqueue_struct *bench_wq;
static atomic_t threads_done;
static DECLARE_COMPLETION(start_comp);
static DECLARE_COMPLETION(all_done_comp);

struct thread_ctx {
	struct completion work_done;
	struct work_struct work;
	u64 *latencies;
	int cpu;
	int items;
};

static void bench_work_fn(struct work_struct *work)
{
	struct thread_ctx *ctx = container_of(work, struct thread_ctx, work);

	complete(&ctx->work_done);
}

static int bench_kthread_fn(void *data)
{
	struct thread_ctx *ctx = data;
	ktime_t t_start, t_end;
	int i;

	/* Wait for all threads to be ready */
	wait_for_completion(&start_comp);

	if (kthread_should_stop())
		return 0;

	for (i = 0; i < ctx->items; i++) {
		reinit_completion(&ctx->work_done);
		INIT_WORK(&ctx->work, bench_work_fn);

		t_start = ktime_get();
		queue_work(bench_wq, &ctx->work);
		t_end = ktime_get();

		ctx->latencies[i] = ktime_to_ns(ktime_sub(t_end, t_start));
		wait_for_completion(&ctx->work_done);
	}

	if (atomic_dec_and_test(&threads_done))
		complete(&all_done_comp);

	/*
	 * Wait for kthread_stop() so the module text isn't freed
	 * while we're still executing.
	 */
	while (!kthread_should_stop())
		schedule();

	return 0;
}

static int cmp_u64(const void *a, const void *b)
{
	u64 va = *(const u64 *)a;
	u64 vb = *(const u64 *)b;

	if (va < vb)
		return -1;
	if (va > vb)
		return 1;
	return 0;
}

static int __init set_affn_scope(const char *scope)
{
	struct file *f;
	loff_t pos = 0;
	ssize_t ret;

	f = filp_open(SCOPE_PATH, O_WRONLY, 0);
	if (IS_ERR(f)) {
		pr_err("test_workqueue: open %s failed: %ld\n",
		       SCOPE_PATH, PTR_ERR(f));
		return PTR_ERR(f);
	}

	ret = kernel_write(f, scope, strlen(scope), &pos);
	filp_close(f, NULL);

	if (ret < 0) {
		pr_err("test_workqueue: write '%s' failed: %zd\n", scope, ret);
		return ret;
	}

	return 0;
}

static int __init run_bench(int n_threads, const char *scope, const char *label)
{
	struct task_struct **tasks;
	unsigned long total_items;
	struct thread_ctx *ctxs;
	u64 *all_latencies;
	ktime_t start, end;
	int cpu, i, j, ret;
	s64 elapsed_us;

	ret = set_affn_scope(scope);
	if (ret)
		return ret;

	ctxs = kcalloc(n_threads, sizeof(*ctxs), GFP_KERNEL);
	if (!ctxs)
		return -ENOMEM;

	tasks = kcalloc(n_threads, sizeof(*tasks), GFP_KERNEL);
	if (!tasks) {
		kfree(ctxs);
		return -ENOMEM;
	}

	total_items = (unsigned long)n_threads * wq_items;
	all_latencies = kvmalloc_array(total_items, sizeof(u64), GFP_KERNEL);
	if (!all_latencies) {
		kfree(tasks);
		kfree(ctxs);
		return -ENOMEM;
	}

	/* Allocate per-thread latency arrays */
	for (i = 0; i < n_threads; i++) {
		ctxs[i].latencies = kvmalloc_array(wq_items, sizeof(u64),
						   GFP_KERNEL);
		if (!ctxs[i].latencies) {
			while (--i >= 0)
				kvfree(ctxs[i].latencies);
			kvfree(all_latencies);
			kfree(tasks);
			kfree(ctxs);
			return -ENOMEM;
		}
	}

	atomic_set(&threads_done, n_threads);
	reinit_completion(&all_done_comp);
	reinit_completion(&start_comp);

	/* Create kthreads, each bound to a different online CPU */
	i = 0;
	for_each_online_cpu(cpu) {
		if (i >= n_threads)
			break;

		ctxs[i].cpu = cpu;
		ctxs[i].items = wq_items;
		init_completion(&ctxs[i].work_done);

		tasks[i] = kthread_create(bench_kthread_fn, &ctxs[i],
					  "wq_bench/%d", cpu);
		if (IS_ERR(tasks[i])) {
			ret = PTR_ERR(tasks[i]);
			pr_err("test_workqueue: failed to create kthread %d: %d\n",
			       i, ret);
			/* Unblock threads waiting on start_comp before stopping them */
			complete_all(&start_comp);
			while (--i >= 0)
				kthread_stop(tasks[i]);
			goto out_free;
		}

		kthread_bind(tasks[i], cpu);
		wake_up_process(tasks[i]);
		i++;
	}

	/* Start timing and release all threads */
	start = ktime_get();
	complete_all(&start_comp);

	/* Wait for all threads to finish the benchmark */
	wait_for_completion(&all_done_comp);

	/* Drain any remaining work */
	flush_workqueue(bench_wq);

	/* Ensure all kthreads have fully exited before module memory is freed */
	for (i = 0; i < n_threads; i++)
		kthread_stop(tasks[i]);

	end = ktime_get();
	elapsed_us = ktime_us_delta(end, start);

	/* Merge all per-thread latencies and sort for percentile calculation */
	j = 0;
	for (i = 0; i < n_threads; i++) {
		memcpy(&all_latencies[j], ctxs[i].latencies,
		       wq_items * sizeof(u64));
		j += wq_items;
	}

	sort(all_latencies, total_items, sizeof(u64), cmp_u64, NULL);

	pr_info("test_workqueue:   %-16s %llu items/sec\tp50=%llu\tp90=%llu\tp95=%llu ns\n",
		label,
		elapsed_us ? div_u64(total_items * 1000000ULL, elapsed_us) : 0,
		all_latencies[total_items * 50 / 100],
		all_latencies[total_items * 90 / 100],
		all_latencies[total_items * 95 / 100]);

	ret = 0;
out_free:
	for (i = 0; i < n_threads; i++)
		kvfree(ctxs[i].latencies);
	kvfree(all_latencies);
	kfree(tasks);
	kfree(ctxs);

	return ret;
}

static const char * const bench_scopes[] = {
	"cpu", "smt", "cache_shard", "cache", "numa", "system",
};

static int __init test_workqueue_init(void)
{
	int n_threads = min(nr_threads ?: num_online_cpus(), num_online_cpus());
	int i;

	if (wq_items <= 0) {
		pr_err("test_workqueue: wq_items must be > 0\n");
		return -EINVAL;
	}

	bench_wq = alloc_workqueue(WQ_NAME, WQ_UNBOUND | WQ_SYSFS, 0);
	if (!bench_wq)
		return -ENOMEM;

	pr_info("test_workqueue: running %d threads, %d items/thread\n",
		n_threads, wq_items);

	for (i = 0; i < ARRAY_SIZE(bench_scopes); i++)
		run_bench(n_threads, bench_scopes[i], bench_scopes[i]);

	destroy_workqueue(bench_wq);

	/* Return -EAGAIN so the module doesn't stay loaded after the benchmark */
	return -EAGAIN;
}

module_init(test_workqueue_init);
MODULE_AUTHOR("Breno Leitao <leitao@debian.org>");
MODULE_DESCRIPTION("Stress/performance benchmark for workqueue subsystem");
MODULE_LICENSE("GPL");