Add wait_kick_cycle, a test stressing an SCX_KICK_WAIT cycle between three CPUs by calling SCX_KICK_WAIT between them to test if sched_ext prevents a deadlock.
Note: hangs on unfixed kernels Signed-off-by: Christian Loehle <[email protected]> --- tools/testing/selftests/sched_ext/Makefile | 1 + .../selftests/sched_ext/wait_kick_cycle.bpf.c | 70 ++++++ .../selftests/sched_ext/wait_kick_cycle.c | 223 ++++++++++++++++++ 3 files changed, 294 insertions(+) create mode 100644 tools/testing/selftests/sched_ext/wait_kick_cycle.bpf.c create mode 100644 tools/testing/selftests/sched_ext/wait_kick_cycle.c diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile index 006300ac6dff..0b5b527265f7 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 \ + wait_kick_cycle \ testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets))) diff --git a/tools/testing/selftests/sched_ext/wait_kick_cycle.bpf.c b/tools/testing/selftests/sched_ext/wait_kick_cycle.bpf.c new file mode 100644 index 000000000000..c53cda86ec75 --- /dev/null +++ b/tools/testing/selftests/sched_ext/wait_kick_cycle.bpf.c @@ -0,0 +1,70 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright (c) 2026 Christian Loehle <[email protected]> + * + * Stress concurrent SCX_KICK_WAIT calls to validate forward progress. + * + * 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(wait_kick_cycle_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(wait_kick_cycle_exit, struct scx_exit_info *ei) +{ + UEI_RECORD(uei, ei); +} + +SEC(".struct_ops.link") +struct sched_ext_ops wait_kick_cycle_ops = { + .enqueue = wait_kick_cycle_enqueue, + .exit = wait_kick_cycle_exit, + .name = "wait_kick_cycle", + .timeout_ms = 1000U, +}; diff --git a/tools/testing/selftests/sched_ext/wait_kick_cycle.c b/tools/testing/selftests/sched_ext/wait_kick_cycle.c new file mode 100644 index 000000000000..3889e7a9a0a7 --- /dev/null +++ b/tools/testing/selftests/sched_ext/wait_kick_cycle.c @@ -0,0 +1,223 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright (c) 2026 Christian Loehle <[email protected]> + */ +#define _GNU_SOURCE + +#ifndef ARRAY_SIZE +#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0])) +#endif + +#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 "wait_kick_cycle.bpf.skel.h" + +/* + * Multiple workers per test CPU. Packing several runnable threads onto each + * CPU causes frequent context switching and back-to-back enqueue() calls, which + * maximizes the chance that all three test CPUs fire enqueue() concurrently + * and enter the SCX_KICK_WAIT cycle simultaneously. + */ +#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 int pick_test_cpus(int *cpu_a, int *cpu_b, int *cpu_c) +{ + cpu_set_t mask; + int cpus[4]; + int nr = 0; + int cpu; + + if (sched_getaffinity(0, sizeof(mask), &mask)) + return -errno; + + for (cpu = 0; cpu < CPU_SETSIZE && nr < ARRAY_SIZE(cpus); cpu++) { + if (!CPU_ISSET(cpu, &mask)) + continue; + cpus[nr++] = cpu; + } + + if (nr < 3) + return -EOPNOTSUPP; + + /* Leave one CPU unused when possible so one CPU remains uncongested. */ + if (nr >= 4) { + *cpu_a = cpus[1]; + *cpu_b = cpus[2]; + *cpu_c = cpus[3]; + } else { + *cpu_a = cpus[0]; + *cpu_b = cpus[1]; + *cpu_c = cpus[2]; + } + return 0; +} + +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; + + /* + * Tight yield loop — no sleep. Keeping the CPU continuously busy + * with rapid context switches ensures enqueue() fires at the highest + * possible rate on each test CPU. + */ + 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 wait_kick_cycle *skel; + + skel = wait_kick_cycle__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 wait_kick_cycle *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] = { -1, -1, -1 }; + int ret; + int i; + + ret = pick_test_cpus(&test_cpus[0], &test_cpus[1], &test_cpus[2]); + if (ret == -EOPNOTSUPP) + return SCX_TEST_SKIP; + if (ret) { + SCX_ERR("Failed to pick test cpus (%d)", ret); + return SCX_TEST_FAIL; + } + + 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 (wait_kick_cycle__load(skel)) { + SCX_ERR("Failed to load skel"); + return SCX_TEST_FAIL; + } + + link = bpf_map__attach_struct_ops(skel->maps.wait_kick_cycle_ops); + if (!link) { + SCX_ERR("Failed to attach scheduler"); + return SCX_TEST_FAIL; + } + + /* WORKERS_PER_CPU threads per test CPU, all in tight yield loops. */ + 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(3); + + 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 wait_kick_cycle *skel = ctx; + + wait_kick_cycle__destroy(skel); +} + +struct scx_test wait_kick_cycle = { + .name = "wait_kick_cycle", + .description = "Verify SCX_KICK_WAIT forward progress under a 3-CPU wait cycle", + .setup = setup, + .run = run, + .cleanup = cleanup, +}; +REGISTER_SCX_TEST(&wait_kick_cycle) -- 2.34.1
