[PATCH 2/2] selftests/sched_ext: Add cyclic SCX_KICK_WAIT stress test

Tejun Heo
2026-03-29T00:18:59Z

Add a test that creates a 3-CPU kick_wait cycle (A->B->C->A). A BPF
scheduler kicks the next CPU in the ring with SCX_KICK_WAIT on every
enqueue while userspace workers generate continuous scheduling churn via
sched_yield(). Without the preceding fix, this hangs the machine within seconds.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
tools/testing/selftests/sched_ext/Makefile | 1 +
.../sched_ext/cyclic_kick_wait.bpf.c | 68 ++++++
.../selftests/sched_ext/cyclic_kick_wait.c | 194 ++++++++++++++++++
3 files changed, 263 insertions(+)
create mode 100644 tools/testing/selftests/sched_ext/cyclic_kick_wait.bpf.c
create mode 100644 tools/testing/selftests/sched_ext/cyclic_kick_wait.c

diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
index 006300ac6dff..1c9ca328cca1 100644
--- a/tools/testing/selftests/sched_ext/Makefile
+++ b/tools/testing/selftests/sched_ext/Makefile
@@ -188,6 +188,7 @@ auto-test-targets := \
rt_stall \
test_example \
total_bw \
+ cyclic_kick_wait \

testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets)))

diff --git a/tools/testing/selftests/sched_ext/cyclic_kick_wait.bpf.c b/tools/testing/selftests/sched_ext/cyclic_kick_wait.bpf.c
new file mode 100644
index 000000000000..cb34d3335917
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/cyclic_kick_wait.bpf.c
@@ -0,0 +1,68 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Stress concurrent SCX_KICK_WAIT calls to reproduce wait-cycle deadlock.
+ *
+ * Three CPUs are designated from userspace. Every enqueue from one of the
+ * three CPUs kicks the next CPU in the ring with SCX_KICK_WAIT, creating a
+ * persistent A -> B -> C -> A wait cycle pressure.
+ */
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+const volatile s32 test_cpu_a;
+const volatile s32 test_cpu_b;
+const volatile s32 test_cpu_c;
+
+u64 nr_enqueues;
+u64 nr_wait_kicks;
+
+UEI_DEFINE(uei);
+
+static s32 target_cpu(s32 cpu)
+{
+ if (cpu == test_cpu_a)
+ return test_cpu_b;
+ if (cpu == test_cpu_b)
+ return test_cpu_c;
+ if (cpu == test_cpu_c)
+ return test_cpu_a;
+ return -1;
+}
+
+void BPF_STRUCT_OPS(cyclic_kick_wait_enqueue, struct task_struct *p,
+ u64 enq_flags)
+{
+ s32 this_cpu = bpf_get_smp_processor_id();
+ s32 tgt;
+
+ __sync_fetch_and_add(&nr_enqueues, 1);
+
+ if (p->flags & PF_KTHREAD) {
+ scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_INF,
+ enq_flags | SCX_ENQ_PREEMPT);
+ return;
+ }
+
+ scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags);
+
+ tgt = target_cpu(this_cpu);
+ if (tgt < 0 || tgt == this_cpu)
+ return;
+
+ __sync_fetch_and_add(&nr_wait_kicks, 1);
+ scx_bpf_kick_cpu(tgt, SCX_KICK_WAIT);
+}
+
+void BPF_STRUCT_OPS(cyclic_kick_wait_exit, struct scx_exit_info *ei)
+{
+ UEI_RECORD(uei, ei);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops cyclic_kick_wait_ops = {
+ .enqueue = cyclic_kick_wait_enqueue,
+ .exit = cyclic_kick_wait_exit,
+ .name = "cyclic_kick_wait",
+ .timeout_ms = 1000U,
+};
diff --git a/tools/testing/selftests/sched_ext/cyclic_kick_wait.c b/tools/testing/selftests/sched_ext/cyclic_kick_wait.c
new file mode 100644
index 000000000000..c2e5aa9de715
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/cyclic_kick_wait.c
@@ -0,0 +1,194 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Test SCX_KICK_WAIT forward progress under cyclic wait pressure.
+ *
+ * SCX_KICK_WAIT busy-waits until the target CPU enters the scheduling path.
+ * If multiple CPUs form a wait cycle (A waits for B, B waits for C, C waits
+ * for A), all CPUs deadlock unless the implementation breaks the cycle.
+ *
+ * This test creates that scenario: three CPUs are arranged in a ring. The BPF
+ * scheduler's ops.enqueue() kicks the next CPU in the ring with SCX_KICK_WAIT
+ * on every enqueue. Userspace pins 4 worker threads per CPU that loop calling
+ * sched_yield(), generating a steady stream of enqueues and thus sustained
+ * A->B->C->A kick_wait cycle pressure. The test passes if the system remains
+ * responsive for 5 seconds without the scheduler being killed by the watchdog.
+ */
+#define _GNU_SOURCE
+
+#include <bpf/bpf.h>
+#include <errno.h>
+#include <pthread.h>
+#include <sched.h>
+#include <scx/common.h>
+#include <stdint.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+
+#include "scx_test.h"
+#include "cyclic_kick_wait.bpf.skel.h"
+
+#define WORKERS_PER_CPU 4
+#define NR_TEST_CPUS 3
+#define NR_WORKERS (NR_TEST_CPUS * WORKERS_PER_CPU)
+
+struct worker_ctx {
+ pthread_t tid;
+ int cpu;
+ volatile bool stop;
+ volatile __u64 iters;
+ bool started;
+};
+
+static void *worker_fn(void *arg)
+{
+ struct worker_ctx *worker = arg;
+ cpu_set_t mask;
+
+ CPU_ZERO(&mask);
+ CPU_SET(worker->cpu, &mask);
+
+ if (sched_setaffinity(0, sizeof(mask), &mask))
+ return (void *)(uintptr_t)errno;
+
+ while (!worker->stop) {
+ sched_yield();
+ worker->iters++;
+ }
+
+ return NULL;
+}
+
+static int join_worker(struct worker_ctx *worker)
+{
+ void *ret;
+ struct timespec ts;
+ int err;
+
+ if (!worker->started)
+ return 0;
+
+ if (clock_gettime(CLOCK_REALTIME, &ts))
+ return -errno;
+
+ ts.tv_sec += 2;
+ err = pthread_timedjoin_np(worker->tid, &ret, &ts);
+ if (err == ETIMEDOUT)
+ pthread_detach(worker->tid);
+ if (err)
+ return -err;
+
+ if ((uintptr_t)ret)
+ return -(int)(uintptr_t)ret;
+
+ return 0;
+}
+
+static enum scx_test_status setup(void **ctx)
+{
+ struct cyclic_kick_wait *skel;
+
+ skel = cyclic_kick_wait__open();
+ SCX_FAIL_IF(!skel, "Failed to open skel");
+ SCX_ENUM_INIT(skel);
+
+ *ctx = skel;
+ return SCX_TEST_PASS;
+}
+
+static enum scx_test_status run(void *ctx)
+{
+ struct cyclic_kick_wait *skel = ctx;
+ struct worker_ctx workers[NR_WORKERS] = {};
+ struct bpf_link *link = NULL;
+ enum scx_test_status status = SCX_TEST_PASS;
+ int test_cpus[NR_TEST_CPUS];
+ int nr_cpus = 0;
+ cpu_set_t mask;
+ int ret, i;
+
+ if (sched_getaffinity(0, sizeof(mask), &mask)) {
+ SCX_ERR("Failed to get affinity (%d)", errno);
+ return SCX_TEST_FAIL;
+ }
+
+ for (i = 0; i < CPU_SETSIZE; i++) {
+ if (CPU_ISSET(i, &mask))
+ test_cpus[nr_cpus++] = i;
+ if (nr_cpus == NR_TEST_CPUS)
+ break;
+ }
+
+ if (nr_cpus < NR_TEST_CPUS)
+ return SCX_TEST_SKIP;
+
+ skel->rodata->test_cpu_a = test_cpus[0];
+ skel->rodata->test_cpu_b = test_cpus[1];
+ skel->rodata->test_cpu_c = test_cpus[2];
+
+ if (cyclic_kick_wait__load(skel)) {
+ SCX_ERR("Failed to load skel");
+ return SCX_TEST_FAIL;
+ }
+
+ link = bpf_map__attach_struct_ops(skel->maps.cyclic_kick_wait_ops);
+ if (!link) {
+ SCX_ERR("Failed to attach scheduler");
+ return SCX_TEST_FAIL;
+ }
+
+ for (i = 0; i < NR_WORKERS; i++)
+ workers[i].cpu = test_cpus[i / WORKERS_PER_CPU];
+
+ for (i = 0; i < NR_WORKERS; i++) {
+ ret = pthread_create(&workers[i].tid, NULL, worker_fn, &workers[i]);
+ if (ret) {
+ SCX_ERR("Failed to create worker thread %d (%d)", i, ret);
+ status = SCX_TEST_FAIL;
+ goto out;
+ }
+ workers[i].started = true;
+ }
+
+ sleep(5);
+
+ if (skel->data->uei.kind != EXIT_KIND(SCX_EXIT_NONE)) {
+ SCX_ERR("Scheduler exited unexpectedly (kind=%llu code=%lld)",
+ (unsigned long long)skel->data->uei.kind,
+ (long long)skel->data->uei.exit_code);
+ status = SCX_TEST_FAIL;
+ }
+
+out:
+ for (i = 0; i < NR_WORKERS; i++)
+ workers[i].stop = true;
+
+ for (i = 0; i < NR_WORKERS; i++) {
+ ret = join_worker(&workers[i]);
+ if (ret && status == SCX_TEST_PASS) {
+ SCX_ERR("Failed to join worker thread %d (%d)", i, ret);
+ status = SCX_TEST_FAIL;
+ }
+ }
+
+ if (link)
+ bpf_link__destroy(link);
+
+ return status;
+}
+
+static void cleanup(void *ctx)
+{
+ struct cyclic_kick_wait *skel = ctx;
+
+ cyclic_kick_wait__destroy(skel);
+}
+
+struct scx_test cyclic_kick_wait = {
+ .name = "cyclic_kick_wait",
+ .description = "Verify SCX_KICK_WAIT forward progress under a 3-CPU wait cycle",
+ .setup = setup,
+ .run = run,
+ .cleanup = cleanup,
+};
+REGISTER_SCX_TEST(&cyclic_kick_wait)
--
2.53.0