Commit 84a8f446 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 37fe5f0e

kernel/rcu/update.c

+26 −7
Original line number Diff line number Diff line
@@ -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;
@@ -388,9 +393,11 @@ 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 */

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