Add proper support for auto-detach in sched_ext

kernel/sched/ext.c

@@ -3555,6 +3555,23 @@ static int bpf_scx_init(struct btf *btf)
 	return 0;
 }
 
+static int bpf_scx_update(void *kdata, void *old_kdata)
+{
+	/*
+	 * sched_ext does not support updating the actively-loaded BPF
+	 * scheduler, as registering a BPF scheduler can always fail if the
+	 * scheduler returns an error code for e.g. ops.init(),
+	 * ops.prep_enable(), etc. Similarly, we can always race with
+	 * unregistration happening elsewhere, such as with sysrq.
+	 */
+	return -EOPNOTSUPP;
+}
+
+static int bpf_scx_validate(void *kdata)
+{
+	return 0;
+}
+
 /* "extern" to avoid sparse warning, only used in this file */
 extern struct bpf_struct_ops bpf_sched_ext_ops;
 
@@ -3565,6 +3582,8 @@ struct bpf_struct_ops bpf_sched_ext_ops = {
 	.check_member = bpf_scx_check_member,
 	.init_member = bpf_scx_init_member,
 	.init = bpf_scx_init,
+	.update = bpf_scx_update,
+	.validate = bpf_scx_validate,
 	.name = "sched_ext_ops",
 };
 

tools/sched_ext/atropos/src/bpf/atropos.bpf.c

@@ -723,7 +723,7 @@ void BPF_STRUCT_OPS(atropos_exit, struct scx_exit_info *ei)
 	exit_type = ei->type;
 }
 
-SEC(".struct_ops")
+SEC(".struct_ops.link")
 struct sched_ext_ops atropos = {
 	.select_cpu = (void *)atropos_select_cpu,
 	.enqueue = (void *)atropos_enqueue,

tools/sched_ext/scx_example_central.bpf.c

@@ -314,7 +314,7 @@ void BPF_STRUCT_OPS(central_exit, struct scx_exit_info *ei)
 	uei_record(&uei, ei);
 }
 
-SEC(".struct_ops")
+SEC(".struct_ops.link")
 struct sched_ext_ops central_ops = {
 	/*
 	 * We are offloading all scheduling decisions to the central CPU and

tools/sched_ext/scx_example_flatcg.bpf.c

@@ -853,7 +853,7 @@ void BPF_STRUCT_OPS(fcg_exit, struct scx_exit_info *ei)
 	uei_record(&uei, ei);
 }
 
-SEC(".struct_ops")
+SEC(".struct_ops.link")
 struct sched_ext_ops flatcg_ops = {
 	.enqueue		= (void *)fcg_enqueue,
 	.dispatch		= (void *)fcg_dispatch,

tools/sched_ext/scx_example_pair.bpf.c

@@ -613,7 +613,7 @@ void BPF_STRUCT_OPS(pair_exit, struct scx_exit_info *ei)
 	uei_record(&uei, ei);
 }
 
-SEC(".struct_ops")
+SEC(".struct_ops.link")
 struct sched_ext_ops pair_ops = {
 	.enqueue		= (void *)pair_enqueue,
 	.dispatch		= (void *)pair_dispatch,

tools/sched_ext/scx_example_qmap.bpf.c

@@ -384,7 +384,7 @@ void BPF_STRUCT_OPS(qmap_exit, struct scx_exit_info *ei)
 	uei_record(&uei, ei);
 }
 
-SEC(".struct_ops")
+SEC(".struct_ops.link")
 struct sched_ext_ops qmap_ops = {
 	.select_cpu		= (void *)qmap_select_cpu,
 	.enqueue		= (void *)qmap_enqueue,

tools/sched_ext/scx_example_simple.bpf.c

@@ -117,7 +117,7 @@ void BPF_STRUCT_OPS(simple_exit, struct scx_exit_info *ei)
 	uei_record(&uei, ei);
 }
 
-SEC(".struct_ops")
+SEC(".struct_ops.link")
 struct sched_ext_ops simple_ops = {
 	.enqueue		= (void *)simple_enqueue,
 	.running		= (void *)simple_running,

tools/sched_ext/scx_example_userland.bpf.c

@@ -249,7 +249,7 @@ void BPF_STRUCT_OPS(userland_exit, struct scx_exit_info *ei)
 	uei_record(&uei, ei);
 }
 
-SEC(".struct_ops")
+SEC(".struct_ops.link")
 struct sched_ext_ops userland_ops = {
 	.select_cpu		= (void *)userland_select_cpu,
 	.enqueue		= (void *)userland_enqueue,