[PATCH] sched_ext: idle: honor built-in idle disablement in node kfuncs

[Joseph Salisbury]

The node-aware idle kfunc helpers validate per-node idle tracking, but they
don't check whether built-in idle tracking itself is enabled.

As a result, when ops.update_idle() disables built-in idle tracking, the
node helpers can still read per-node idle masks and attempt idle CPU
selection.  This violates the documented behavior and can expose stale
idle state to BPF schedulers.

Fix this by checking check_builtin_idle_enabled() in the node mask getters
and in scx_bpf_pick_idle_cpu_node(), matching the behavior of the non-node
helpers.

scx_bpf_pick_any_cpu_node() is different by: when built-in idle
tracking is disabled, it should skip idle selection and fall back directly
to the any-CPU path.  Make it do so and match scx_bpf_pick_any_cpu().

Fixes: 01059219b0cf ("sched_ext: idle: Introduce node-aware idle cpu kfunc helpers")
Cc: stable@xxxxxxxxxxxxxxx # v6.15+
Assisted-by: Codex:GPT-5
Signed-off-by: Joseph Salisbury <joseph.salisbury@xxxxxxxxxx>
---
 kernel/sched/ext_idle.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index ba298ac3ce6c..948f6b4f8ab5 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -1082,6 +1082,9 @@ __bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask_node(int node)
 	if (node < 0)
 		return cpu_none_mask;
 
+	if (!check_builtin_idle_enabled(sch))
+		return cpu_none_mask;
+
 	return idle_cpumask(node)->cpu;
 }
 
@@ -1137,6 +1140,9 @@ __bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask_node(int node)
 	if (node < 0)
 		return cpu_none_mask;
 
+	if (!check_builtin_idle_enabled(sch))
+		return cpu_none_mask;
+
 	if (sched_smt_active())
 		return idle_cpumask(node)->smt;
 	else
@@ -1253,6 +1259,9 @@ __bpf_kfunc s32 scx_bpf_pick_idle_cpu_node(const struct cpumask *cpus_allowed,
 	if (node < 0)
 		return node;
 
+	if (!check_builtin_idle_enabled(sch))
+		return -EBUSY;
+
 	return scx_pick_idle_cpu(cpus_allowed, node, flags);
 }
 
@@ -1337,9 +1346,11 @@ __bpf_kfunc s32 scx_bpf_pick_any_cpu_node(const struct cpumask *cpus_allowed,
 	if (node < 0)
 		return node;
 
-	cpu = scx_pick_idle_cpu(cpus_allowed, node, flags);
-	if (cpu >= 0)
-		return cpu;
+	if (static_branch_likely(&scx_builtin_idle_enabled)) {
+		cpu = scx_pick_idle_cpu(cpus_allowed, node, flags);
+		if (cpu >= 0)
+			return cpu;
+	}
 
 	if (flags & SCX_PICK_IDLE_IN_NODE)
 		cpu = cpumask_any_and_distribute(cpumask_of_node(node), cpus_allowed);
-- 
2.47.3