selftests/sched_ext: Fix exit selftest hang on UP

On single-CPU systems, ops.select_cpu() is never called, causing the
EXIT_SELECT_CPU test case to wait indefinitely.

Avoid the stall by skipping this specific sub-test when only one CPU is
available.

Reported-by: Phil Auld <pauld@redhat.com>
Fixes: a5db781 ("sched_ext: Add selftests")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Phil Auld <pauld@redhat.com>
Tested-by: Phil Auld <pauld@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: arighi

tools/testing/selftests/sched_ext/exit.c

@@ -22,6 +22,14 @@ static enum scx_test_status run(void *ctx)
 		struct bpf_link *link;
 		char buf[16];
 
+		/*
+		 * On single-CPU systems, ops.select_cpu() is never
+		 * invoked, so skip this test to avoid getting stuck
+		 * indefinitely.
+		 */
+		if (tc == EXIT_SELECT_CPU && libbpf_num_possible_cpus() == 1)
+			continue;
+
 		skel = exit__open();
 		SCX_ENUM_INIT(skel);
 		skel->rodata->exit_point = tc;