selftests/sched_ext: Add test for scx_bpf_select_cpu_and()

Add a selftest to validate the behavior of the built-in idle CPU
selection policy applied to a subset of allowed CPUs, using
scx_bpf_select_cpu_and().

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/sched_ext/Makefile

@@ -173,6 +173,7 @@ auto-test-targets :=			\
 	maybe_null			\
 	minimal				\
 	numa				\
+	allowed_cpus			\
 	prog_run			\
 	reload_loop			\
 	select_cpu_dfl			\

tools/testing/selftests/sched_ext/allowed_cpus.bpf.c

@@ -0,0 +1,121 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A scheduler that validates the behavior of scx_bpf_select_cpu_and() by
+ * selecting idle CPUs strictly within a subset of allowed CPUs.
+ *
+ * Copyright (c) 2025 Andrea Righi <arighi@nvidia.com>
+ */
+
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+UEI_DEFINE(uei);
+
+private(PREF_CPUS) struct bpf_cpumask __kptr * allowed_cpumask;
+
+static void
+validate_idle_cpu(const struct task_struct *p, const struct cpumask *allowed, s32 cpu)
+{
+	if (scx_bpf_test_and_clear_cpu_idle(cpu))
+		scx_bpf_error("CPU %d should be marked as busy", cpu);
+
+	if (bpf_cpumask_subset(allowed, p->cpus_ptr) &&
+	    !bpf_cpumask_test_cpu(cpu, allowed))
+		scx_bpf_error("CPU %d not in the allowed domain for %d (%s)",
+			      cpu, p->pid, p->comm);
+}
+
+s32 BPF_STRUCT_OPS(allowed_cpus_select_cpu,
+		   struct task_struct *p, s32 prev_cpu, u64 wake_flags)
+{
+	const struct cpumask *allowed;
+	s32 cpu;
+
+	allowed = cast_mask(allowed_cpumask);
+	if (!allowed) {
+		scx_bpf_error("allowed domain not initialized");
+		return -EINVAL;
+	}
+
+	/*
+	 * Select an idle CPU strictly within the allowed domain.
+	 */
+	cpu = scx_bpf_select_cpu_and(p, prev_cpu, wake_flags, allowed, 0);
+	if (cpu >= 0) {
+		validate_idle_cpu(p, allowed, cpu);
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
+
+		return cpu;
+	}
+
+	return prev_cpu;
+}
+
+void BPF_STRUCT_OPS(allowed_cpus_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	const struct cpumask *allowed;
+	s32 prev_cpu = scx_bpf_task_cpu(p), cpu;
+
+	scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, 0);
+
+	allowed = cast_mask(allowed_cpumask);
+	if (!allowed) {
+		scx_bpf_error("allowed domain not initialized");
+		return;
+	}
+
+	/*
+	 * Use scx_bpf_select_cpu_and() to proactively kick an idle CPU
+	 * within @allowed_cpumask, usable by @p.
+	 */
+	cpu = scx_bpf_select_cpu_and(p, prev_cpu, 0, allowed, 0);
+	if (cpu >= 0) {
+		validate_idle_cpu(p, allowed, cpu);
+		scx_bpf_kick_cpu(cpu, SCX_KICK_IDLE);
+	}
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(allowed_cpus_init)
+{
+	struct bpf_cpumask *mask;
+
+	mask = bpf_cpumask_create();
+	if (!mask)
+		return -ENOMEM;
+
+	mask = bpf_kptr_xchg(&allowed_cpumask, mask);
+	if (mask)
+		bpf_cpumask_release(mask);
+
+	bpf_rcu_read_lock();
+
+	/*
+	 * Assign the first online CPU to the allowed domain.
+	 */
+	mask = allowed_cpumask;
+	if (mask) {
+		const struct cpumask *online = scx_bpf_get_online_cpumask();
+
+		bpf_cpumask_set_cpu(bpf_cpumask_first(online), mask);
+		scx_bpf_put_cpumask(online);
+	}
+
+	bpf_rcu_read_unlock();
+
+	return 0;
+}
+
+void BPF_STRUCT_OPS(allowed_cpus_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops allowed_cpus_ops = {
+	.select_cpu		= (void *)allowed_cpus_select_cpu,
+	.enqueue		= (void *)allowed_cpus_enqueue,
+	.init			= (void *)allowed_cpus_init,
+	.exit			= (void *)allowed_cpus_exit,
+	.name			= "allowed_cpus",
+};

tools/testing/selftests/sched_ext/allowed_cpus.c

@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2025 Andrea Righi <arighi@nvidia.com>
+ */
+#include <bpf/bpf.h>
+#include <scx/common.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "allowed_cpus.bpf.skel.h"
+#include "scx_test.h"
+
+static enum scx_test_status setup(void **ctx)
+{
+	struct allowed_cpus *skel;
+
+	skel = allowed_cpus__open();
+	SCX_FAIL_IF(!skel, "Failed to open");
+	SCX_ENUM_INIT(skel);
+	SCX_FAIL_IF(allowed_cpus__load(skel), "Failed to load skel");
+
+	*ctx = skel;
+
+	return SCX_TEST_PASS;
+}
+
+static enum scx_test_status run(void *ctx)
+{
+	struct allowed_cpus *skel = ctx;
+	struct bpf_link *link;
+
+	link = bpf_map__attach_struct_ops(skel->maps.allowed_cpus_ops);
+	SCX_FAIL_IF(!link, "Failed to attach scheduler");
+
+	/* Just sleeping is fine, plenty of scheduling events happening */
+	sleep(1);
+
+	SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_NONE));
+	bpf_link__destroy(link);
+
+	return SCX_TEST_PASS;
+}
+
+static void cleanup(void *ctx)
+{
+	struct allowed_cpus *skel = ctx;
+
+	allowed_cpus__destroy(skel);
+}
+
+struct scx_test allowed_cpus = {
+	.name = "allowed_cpus",
+	.description = "Verify scx_bpf_select_cpu_and()",
+	.setup = setup,
+	.run = run,
+	.cleanup = cleanup,
+};
+REGISTER_SCX_TEST(&allowed_cpus)