selftests/sched_ext: Add test for scx_bpf_select_cpu_and() via test_run

Update the allowed_cpus selftest to include a check to validate the
behavior of scx_bpf_select_cpu_and() when invoked via a BPF test_run
call.

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

Author: arighi

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

@@ -111,6 +111,29 @@ void BPF_STRUCT_OPS(allowed_cpus_exit, struct scx_exit_info *ei)
 	UEI_RECORD(uei, ei);
 }
 
+struct task_cpu_arg {
+	pid_t pid;
+};
+
+SEC("syscall")
+int select_cpu_from_user(struct task_cpu_arg *input)
+{
+	struct task_struct *p;
+	int cpu;
+
+	p = bpf_task_from_pid(input->pid);
+	if (!p)
+		return -EINVAL;
+
+	bpf_rcu_read_lock();
+	cpu = scx_bpf_select_cpu_and(p, bpf_get_smp_processor_id(), 0, p->cpus_ptr, 0);
+	bpf_rcu_read_unlock();
+
+	bpf_task_release(p);
+
+	return cpu;
+}
+
 SEC(".struct_ops.link")
 struct sched_ext_ops allowed_cpus_ops = {
 	.select_cpu		= (void *)allowed_cpus_select_cpu,

tools/testing/selftests/sched_ext/allowed_cpus.c

@@ -23,6 +23,30 @@ static enum scx_test_status setup(void **ctx)
 	return SCX_TEST_PASS;
 }
 
+static int test_select_cpu_from_user(const struct allowed_cpus *skel)
+{
+	int fd, ret;
+	__u64 args[1];
+
+	LIBBPF_OPTS(bpf_test_run_opts, attr,
+		.ctx_in = args,
+		.ctx_size_in = sizeof(args),
+	);
+
+	args[0] = getpid();
+	fd = bpf_program__fd(skel->progs.select_cpu_from_user);
+	if (fd < 0)
+		return fd;
+
+	ret = bpf_prog_test_run_opts(fd, &attr);
+	if (ret < 0)
+		return ret;
+
+	fprintf(stderr, "%s: CPU %d\n", __func__, attr.retval);
+
+	return 0;
+}
+
 static enum scx_test_status run(void *ctx)
 {
 	struct allowed_cpus *skel = ctx;
@@ -31,6 +55,9 @@ static enum scx_test_status run(void *ctx)
 	link = bpf_map__attach_struct_ops(skel->maps.allowed_cpus_ops);
 	SCX_FAIL_IF(!link, "Failed to attach scheduler");
 
+	/* Pick an idle CPU from user-space */
+	SCX_FAIL_IF(test_select_cpu_from_user(skel), "Failed to pick idle CPU");
+
 	/* Just sleeping is fine, plenty of scheduling events happening */
 	sleep(1);