sched_ext: Build fix on !CONFIG_STACKTRACE[_SUPPORT]

scx_dump_task() uses stack_trace_save_tsk() which is only available when
CONFIG_STACKTRACE. Make CONFIG_SCHED_CLASS_EXT select CONFIG_STACKTRACE if
the support is available and skip capturing stack trace if
!CONFIG_STACKTRACE.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407161844.reewQQrR-lkp@intel.com/Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/Kconfig.preempt

@@ -136,6 +136,7 @@ config SCHED_CORE
 config SCHED_CLASS_EXT
 	bool "Extensible Scheduling Class"
 	depends on BPF_SYSCALL && BPF_JIT && DEBUG_INFO_BTF
+	select STACKTRACE if STACKTRACE_SUPPORT
 	help
 	  This option enables a new scheduler class sched_ext (SCX), which
 	  allows scheduling policies to be implemented as BPF programs to

kernel/sched/ext.c

@@ -4330,7 +4330,7 @@ static void scx_dump_task(struct seq_buf *s, struct scx_dump_ctx *dctx,
 	static unsigned long bt[SCX_EXIT_BT_LEN];
 	char dsq_id_buf[19] = "(n/a)";
 	unsigned long ops_state = atomic_long_read(&p->scx.ops_state);
-	unsigned int bt_len;
+	unsigned int bt_len = 0;
 
 	if (p->scx.dsq)
 		scnprintf(dsq_id_buf, sizeof(dsq_id_buf), "0x%llx",
@@ -4355,7 +4355,9 @@ static void scx_dump_task(struct seq_buf *s, struct scx_dump_ctx *dctx,
 		ops_dump_exit();
 	}
 
+#ifdef CONFIG_STACKTRACE
 	bt_len = stack_trace_save_tsk(p, bt, SCX_EXIT_BT_LEN, 1);
+#endif
 	if (bt_len) {
 		dump_newline(s);
 		dump_stack_trace(s, "    ", bt, bt_len);