scx_bpf_cpu_rq() works on an unlocked rq which generally isn't safe.
For the common use-cases scx_bpf_locked_rq() and
scx_bpf_cpu_curr() work, so add a deprecation warning
to scx_bpf_cpu_rq() so it can eventually be removed.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
---
 kernel/sched/ext.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 6ea81b6a6b2d..1a0afdf5fcf5 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -873,6 +873,7 @@ struct scx_sched {
 	struct scx_event_stats __percpu *event_stats_cpu;
 
 	bool			warned_zero_slice;
+	bool			warned_deprecated_rq;
 
 	atomic_t		exit_kind;
 	struct scx_exit_info	*exit_info;
@@ -7424,9 +7425,18 @@ __bpf_kfunc s32 scx_bpf_task_cpu(const struct task_struct *p)
  */
 __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
 {
+	struct scx_sched *sch = scx_root;
+
 	if (!kf_cpu_valid(cpu, NULL))
 		return NULL;
 
+	if (!sch->warned_deprecated_rq) {
+		printk_deferred(KERN_WARNING "sched_ext: %s() is deprecated; "
+				"use scx_bpf_locked_rq() when holding rq lock "
+				"or scx_bpf_cpu_curr() to read remote curr safely.\n", __func__);
+		sch->warned_deprecated_rq = true;
+	}
+
 	return cpu_rq(cpu);
 }
 
-- 
2.34.1