selftests/bpf: add bpf task work stress tests

Add stress tests for BPF task-work scheduling kfuncs. The tests spawn
multiple threads that concurrently schedule task_work callbacks against
the same and different map values to exercise the kfuncs under high
contention.
Verify callbacks are reliably enqueued and executed with no drops.

Signed-off-by: Mykyta Yatsenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/task_work_stress.c

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+#include <test_progs.h>
+#include <string.h>
+#include <stdio.h>
+#include "task_work_stress.skel.h"
+#include <linux/bpf.h>
+#include <linux/perf_event.h>
+#include <sys/syscall.h>
+#include <time.h>
+#include <stdlib.h>
+#include <stdatomic.h>
+
+struct test_data {
+	int prog_fd;
+	atomic_int exit;
+};
+
+void *runner(void *test_data)
+{
+	struct test_data *td = test_data;
+	int err = 0;
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+
+	while (!err && !atomic_load(&td->exit))
+		err = bpf_prog_test_run_opts(td->prog_fd, &opts);
+
+	return NULL;
+}
+
+static int get_env_int(const char *str, int def)
+{
+	const char *s = getenv(str);
+	char *end;
+	int retval;
+
+	if (!s || !*s)
+		return def;
+	errno = 0;
+	retval = strtol(s, &end, 10);
+	if (errno || *end || retval < 0)
+		return def;
+	return retval;
+}
+
+static void task_work_run(bool enable_delete)
+{
+	struct task_work_stress *skel;
+	struct bpf_program *scheduler, *deleter;
+	int nthreads = 16;
+	int test_time_s = get_env_int("BPF_TASK_WORK_TEST_TIME", 1);
+	pthread_t tid[nthreads], tid_del;
+	bool started[nthreads], started_del = false;
+	struct test_data td_sched = { .exit = 0 }, td_del = { .exit = 1 };
+	int i, err;
+
+	skel = task_work_stress__open();
+	if (!ASSERT_OK_PTR(skel, "task_work__open"))
+		return;
+
+	scheduler = bpf_object__find_program_by_name(skel->obj, "schedule_task_work");
+	bpf_program__set_autoload(scheduler, true);
+
+	deleter = bpf_object__find_program_by_name(skel->obj, "delete_task_work");
+	bpf_program__set_autoload(deleter, true);
+
+	err = task_work_stress__load(skel);
+	if (!ASSERT_OK(err, "skel_load"))
+		goto cleanup;
+
+	for (i = 0; i < nthreads; ++i)
+		started[i] = false;
+
+	td_sched.prog_fd = bpf_program__fd(scheduler);
+	for (i = 0; i < nthreads; ++i) {
+		if (pthread_create(&tid[i], NULL, runner, &td_sched) != 0) {
+			fprintf(stderr, "could not start thread");
+			goto cancel;
+		}
+		started[i] = true;
+	}
+
+	if (enable_delete)
+		atomic_store(&td_del.exit, 0);
+
+	td_del.prog_fd = bpf_program__fd(deleter);
+	if (pthread_create(&tid_del, NULL, runner, &td_del) != 0) {
+		fprintf(stderr, "could not start thread");
+		goto cancel;
+	}
+	started_del = true;
+
+	/* Run stress test for some time */
+	sleep(test_time_s);
+
+cancel:
+	atomic_store(&td_sched.exit, 1);
+	atomic_store(&td_del.exit, 1);
+	for (i = 0; i < nthreads; ++i) {
+		if (started[i])
+			pthread_join(tid[i], NULL);
+	}
+
+	if (started_del)
+		pthread_join(tid_del, NULL);
+
+	ASSERT_GT(skel->bss->callback_scheduled, 0, "work scheduled");
+	/* Some scheduling attempts should have failed due to contention */
+	ASSERT_GT(skel->bss->schedule_error, 0, "schedule error");
+
+	if (enable_delete) {
+		/* If delete thread is enabled, it has cancelled some callbacks */
+		ASSERT_GT(skel->bss->delete_success, 0, "delete success");
+		ASSERT_LT(skel->bss->callback_success, skel->bss->callback_scheduled, "callbacks");
+	} else {
+		/* Without delete thread number of scheduled callbacks is the same as fired */
+		ASSERT_EQ(skel->bss->callback_success, skel->bss->callback_scheduled, "callbacks");
+	}
+
+cleanup:
+	task_work_stress__destroy(skel);
+}
+
+void test_task_work_stress(void)
+{
+	if (test__start_subtest("no_delete"))
+		task_work_run(false);
+	if (test__start_subtest("with_delete"))
+		task_work_run(true);
+}

tools/testing/selftests/bpf/progs/task_work_stress.c

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <string.h>
+#include <stdbool.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+
+#define ENTRIES 128
+
+char _license[] SEC("license") = "GPL";
+
+__u64 callback_scheduled = 0;
+__u64 callback_success = 0;
+__u64 schedule_error = 0;
+__u64 delete_success = 0;
+
+struct elem {
+	__u32 count;
+	struct bpf_task_work tw;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__uint(max_entries, ENTRIES);
+	__type(key, int);
+	__type(value, struct elem);
+} hmap SEC(".maps");
+
+static int process_work(struct bpf_map *map, void *key, void *value)
+{
+	__sync_fetch_and_add(&callback_success, 1);
+	return 0;
+}
+
+SEC("syscall")
+int schedule_task_work(void *ctx)
+{
+	struct elem empty_work = {.count = 0};
+	struct elem *work;
+	int key = 0, err;
+
+	key = bpf_ktime_get_ns() % ENTRIES;
+	work = bpf_map_lookup_elem(&hmap, &key);
+	if (!work) {
+		bpf_map_update_elem(&hmap, &key, &empty_work, BPF_NOEXIST);
+		work = bpf_map_lookup_elem(&hmap, &key);
+		if (!work)
+			return 0;
+	}
+	err = bpf_task_work_schedule_signal(bpf_get_current_task_btf(), &work->tw, &hmap,
+					    process_work, NULL);
+	if (err)
+		__sync_fetch_and_add(&schedule_error, 1);
+	else
+		__sync_fetch_and_add(&callback_scheduled, 1);
+	return 0;
+}
+
+SEC("syscall")
+int delete_task_work(void *ctx)
+{
+	int key = 0, err;
+
+	key = bpf_get_prandom_u32() % ENTRIES;
+	err = bpf_map_delete_elem(&hmap, &key);
+	if (!err)
+		__sync_fetch_and_add(&delete_success, 1);
+	return 0;
+}