rcu: Defer rcu_tasks_kthread() creation till first call_rcu_tasks()

It is expected that many sites will have CONFIG_TASKS_RCU=y, but
will never actually invoke call_rcu_tasks().  For such sites, creating
rcu_tasks_kthread() at boot is wasteful.  This commit therefore defers
creation of this kthread until the time of the first call_rcu_tasks().

This of course means that the first call_rcu_tasks() must be invoked
from process context after the scheduler is fully operational.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 26 insertions, 7 deletions

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 444c8a303963..e1d71741958f 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -375,7 +375,12 @@ DEFINE_SRCU(tasks_rcu_exit_srcu);
 static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;
 module_param(rcu_task_stall_timeout, int, 0644);
 
-/* Post an RCU-tasks callback. */
+static void rcu_spawn_tasks_kthread(void);
+
+/*
+ * Post an RCU-tasks callback.  First call must be from process context
+ * after the scheduler if fully operational.
+ */
 void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
 {
         unsigned long flags;
@@ -388,8 +393,10 @@ void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
         *rcu_tasks_cbs_tail = rhp;
         rcu_tasks_cbs_tail = &rhp->next;
         raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
-        if (needwake)
+        if (needwake) {
+                rcu_spawn_tasks_kthread();
                 wake_up(&rcu_tasks_cbs_wq);
+        }
 }
 EXPORT_SYMBOL_GPL(call_rcu_tasks);
 
@@ -615,15 +622,27 @@ static int __noreturn rcu_tasks_kthread(void *arg)
         }
 }
 
-/* Spawn rcu_tasks_kthread() at boot time. */
-static int __init rcu_spawn_tasks_kthread(void)
+/* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */
+static void rcu_spawn_tasks_kthread(void)
 {
-        struct task_struct __maybe_unused *t;
+        static DEFINE_MUTEX(rcu_tasks_kthread_mutex);
+        static struct task_struct *rcu_tasks_kthread_ptr;
+        struct task_struct *t;
 
+        if (ACCESS_ONCE(rcu_tasks_kthread_ptr)) {
+                smp_mb(); /* Ensure caller sees full kthread. */
+                return;
+        }
+        mutex_lock(&rcu_tasks_kthread_mutex);
+        if (rcu_tasks_kthread_ptr) {
+                mutex_unlock(&rcu_tasks_kthread_mutex);
+                return;
+        }
         t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
         BUG_ON(IS_ERR(t));
-        return 0;
+        smp_mb(); /* Ensure others see full kthread. */
+        ACCESS_ONCE(rcu_tasks_kthread_ptr) = t;
+        mutex_unlock(&rcu_tasks_kthread_mutex);
 }
-early_initcall(rcu_spawn_tasks_kthread);
 
 #endif /* #ifdef CONFIG_TASKS_RCU */