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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>
This commit is contained in:
Paul E. McKenney 2014-08-04 07:24:21 -07:00
parent 37fe5f0e27
commit 84a8f446ff
1 changed files with 26 additions and 7 deletions
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33
kernel/rcu/update.c
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@ -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 */