[PATCH RFC 02/12] sched/isolation: Introduce reconfiguration notifier chain

[Qiliang Yuan <realwujing@gmail.com>]

Introduce a blocking notifier chain for housekeeping to support dynamic
reconfiguration of masks. Subsystems like RCU, Timers, and IRQ
management need to be notified when the isolation state of a CPU
changes at runtime.

1. Define 'struct housekeeping_update' and 'HK_UPDATE_MASK' in
   isolation.h to pass update event details (type of housekeeping and
   the new mask).
2. Implement housekeeping_register_notifier() and
   housekeeping_unregister_notifier() to allow subsystems to subscribe
   to updates.
3. Provide an internal housekeeping_update_notify() helper to trigger
   the chain.

This signaling mechanism enables other kernel subsystems to adapt their
internal state (e.g., migrating kthreads or timers) dynamically when
housekeeping masks are modified via upcoming sysfs interfaces.

Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
---
 include/linux/sched/isolation.h | 20 ++++++++++++++++++++
 kernel/sched/isolation.c        | 23 +++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
index cecb80b09120..5c07e3e9b8d1 100644
--- a/include/linux/sched/isolation.h
+++ b/include/linux/sched/isolation.h
@@ -5,6 +5,7 @@
 #include <linux/cpuset.h>
 #include <linux/init.h>
 #include <linux/tick.h>
+#include <linux/notifier.h>
 
 enum hk_type {
 	HK_TYPE_DOMAIN,
@@ -24,6 +25,13 @@ enum hk_type {
 	HK_TYPE_KTHREAD = HK_TYPE_KERNEL_NOISE
 };
 
+struct housekeeping_update {
+	enum hk_type type;
+	const struct cpumask *new_mask;
+};
+
+#define HK_UPDATE_MASK	0x01
+
 #ifdef CONFIG_CPU_ISOLATION
 DECLARE_STATIC_KEY_FALSE(housekeeping_overridden);
 extern int housekeeping_any_cpu(enum hk_type type);
@@ -32,6 +40,8 @@ extern bool housekeeping_enabled(enum hk_type type);
 extern void housekeeping_affine(struct task_struct *t, enum hk_type type);
 extern bool housekeeping_test_cpu(int cpu, enum hk_type type);
 extern void housekeeping_init(void);
+extern int housekeeping_register_notifier(struct notifier_block *nb);
+extern int housekeeping_unregister_notifier(struct notifier_block *nb);
 
 #else
 
@@ -59,6 +69,16 @@ static inline bool housekeeping_test_cpu(int cpu, enum hk_type type)
 }
 
 static inline void housekeeping_init(void) { }
+
+static inline int housekeeping_register_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
+
+static inline int housekeeping_unregister_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
 #endif /* CONFIG_CPU_ISOLATION */
 
 static inline bool housekeeping_cpu(int cpu, enum hk_type type)
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 811bf6cbe68d..97cc41626a33 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -18,6 +18,7 @@ enum hk_flags {
 };
 
 static DEFINE_MUTEX(housekeeping_mutex);
+static BLOCKING_NOTIFIER_HEAD(housekeeping_notifier_list);
 DEFINE_STATIC_KEY_FALSE(housekeeping_overridden);
 EXPORT_SYMBOL_GPL(housekeeping_overridden);
 
@@ -86,6 +87,28 @@ bool housekeeping_test_cpu(int cpu, enum hk_type type)
 }
 EXPORT_SYMBOL_GPL(housekeeping_test_cpu);
 
+int housekeeping_register_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&housekeeping_notifier_list, nb);
+}
+EXPORT_SYMBOL_GPL(housekeeping_register_notifier);
+
+int housekeeping_unregister_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&housekeeping_notifier_list, nb);
+}
+EXPORT_SYMBOL_GPL(housekeeping_unregister_notifier);
+
+static int __maybe_unused housekeeping_update_notify(enum hk_type type, const struct cpumask *new_mask)
+{
+	struct housekeeping_update update = {
+		.type = type,
+		.new_mask = new_mask,
+	};
+
+	return blocking_notifier_call_chain(&housekeeping_notifier_list, HK_UPDATE_MASK, &update);
+}
+
 void housekeeping_init(void)
 {
 	enum hk_type type;

-- 
2.51.0