[PATCH sched_ext/for-6.18] sched_ext: Exit early on hotplug events during attach

[Andrea Righi <arighi@nvidia.com>]

There is no need to complete the entire scx initialization if a
scheduler is failing to be attached due to a hotplug event.

Exit early to avoid unnecessary work and simplify the attach flow.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 9ddf7a145db6b..489462290142a 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4445,7 +4445,7 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops)
 	return ERR_PTR(ret);
 }
 
-static void check_hotplug_seq(struct scx_sched *sch,
+static int check_hotplug_seq(struct scx_sched *sch,
 			      const struct sched_ext_ops *ops)
 {
 	unsigned long long global_hotplug_seq;
@@ -4462,8 +4462,11 @@ static void check_hotplug_seq(struct scx_sched *sch,
 				 SCX_ECODE_ACT_RESTART | SCX_ECODE_RSN_HOTPLUG,
 				 "expected hotplug seq %llu did not match actual %llu",
 				 ops->hotplug_seq, global_hotplug_seq);
+			return -EBUSY;
 		}
 	}
+
+	return 0;
 }
 
 static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
@@ -4560,7 +4563,11 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 		if (((void (**)(void))ops)[i])
 			set_bit(i, sch->has_op);
 
-	check_hotplug_seq(sch, ops);
+	ret = check_hotplug_seq(sch, ops);
+	if (ret) {
+		cpus_read_unlock();
+		goto err_disable;
+	}
 	scx_idle_update_selcpu_topology(ops);
 
 	cpus_read_unlock();
-- 
2.51.0