[PATCH v4 04/11] context_tracking: allow runtime per-CPU user tracking enable/disable

ct_cpu_track_user() and the context_tracking_key static key are currently
restricted to boot-time use: the key is __ro_after_init and the function
is __init with __initdata state.  This prevents enabling nohz_full context
tracking for CPUs isolated at runtime via cpuset partitions.

Split ct_cpu_track_user() into three functions:

  ct_cpu_track_user(cpu)      - sets per_cpu(context_tracking.active) and
                                increments context_tracking_key; callable
                                at runtime with the CPU offline.

  ct_cpu_untrack_user(cpu)    - reverses the above; for de-isolation.

  ct_cpu_track_user_init(cpu) - __init wrapper; calls ct_cpu_track_user()
                                and handles TIF_NOHZ / tasklist setup.

Change context_tracking_key from DEFINE_STATIC_KEY_FALSE_RO to
DEFINE_STATIC_KEY_FALSE so that static_branch_inc/dec() can be called
after the __ro_after_init window closes.

Update tick_nohz_init() to call ct_cpu_track_user_init() so boot
behaviour is unchanged.

This is a prerequisite for DHM (Dynamic Housekeeping Management) runtime
CPU noise isolation without boot parameters.

Co-developed-by: Qiliang Yuan <[email protected]>
Signed-off-by: Qiliang Yuan <[email protected]>
Signed-off-by: Jing Wu <[email protected]>
---
 include/linux/context_tracking.h |  2 ++
 kernel/context_tracking.c        | 38 ++++++++++++++++++++++++++++++++++----
 kernel/time/tick-sched.c         |  2 +-
 3 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index af9fe87a09225..735d353d87560 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -12,6 +12,8 @@
 
 #ifdef CONFIG_CONTEXT_TRACKING_USER
 extern void ct_cpu_track_user(int cpu);
+extern void ct_cpu_untrack_user(int cpu);
+extern void __init ct_cpu_track_user_init(int cpu);
 
 /* Called with interrupts disabled.  */
 extern void __ct_user_enter(enum ctx_state state);
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index a743e7ffa6c00..a81d9f8b85eed 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -411,7 +411,7 @@ static __always_inline void ct_kernel_enter(bool user, int offset) { }
 #define CREATE_TRACE_POINTS
 #include <trace/events/context_tracking.h>
 
-DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key);
+DEFINE_STATIC_KEY_FALSE(context_tracking_key);
 EXPORT_SYMBOL_GPL(context_tracking_key);
 
 static noinstr bool context_tracking_recursion_enter(void)
@@ -674,14 +674,44 @@ void user_exit_callable(void)
 }
 NOKPROBE_SYMBOL(user_exit_callable);
 
-void __init ct_cpu_track_user(int cpu)
+/**
+ * ct_cpu_track_user - enable context tracking for a CPU
+ * @cpu: target CPU (must be offline when called at runtime)
+ *
+ * Marks @cpu as actively tracking user/kernel transitions and increments
+ * the context_tracking_key refcount.  Safe to call at runtime provided
+ * the CPU is offline so no context-tracking readers are active on it.
+ */
+void ct_cpu_track_user(int cpu)
 {
-	static __initdata bool initialized = false;
-
 	if (!per_cpu(context_tracking.active, cpu)) {
 		per_cpu(context_tracking.active, cpu) = true;
 		static_branch_inc(&context_tracking_key);
 	}
+}
+EXPORT_SYMBOL_GPL(ct_cpu_track_user);
+
+/**
+ * ct_cpu_untrack_user - disable context tracking for a CPU
+ * @cpu: target CPU (must be offline when called)
+ *
+ * Reverses ct_cpu_track_user().  The CPU must be offline so that no
+ * context-tracking readers are active on it.
+ */
+void ct_cpu_untrack_user(int cpu)
+{
+	if (per_cpu(context_tracking.active, cpu)) {
+		per_cpu(context_tracking.active, cpu) = false;
+		static_branch_dec(&context_tracking_key);
+	}
+}
+EXPORT_SYMBOL_GPL(ct_cpu_untrack_user);
+
+void __init ct_cpu_track_user_init(int cpu)
+{
+	static __initdata bool initialized = false;
+
+	ct_cpu_track_user(cpu);
 
 	if (initialized)
 		return;
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index cbbb87a0c6e7c..ba7adc671c580 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -677,7 +677,7 @@ void __init tick_nohz_init(void)
 	}
 
 	for_each_cpu(cpu, tick_nohz_full_mask)
-		ct_cpu_track_user(cpu);
+		ct_cpu_track_user_init(cpu);
 
 	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
 					"kernel/nohz:predown", NULL,

-- 
2.43.0