[PATCH v3 tip/core/rcu 8/9] rcu: Improve RCU-tasks energy efficiency

Paul E. McKenney
2014-07-31T21:57:04Z

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

The current RCU-tasks implementation uses strict polling to detect
callback arrivals. This works quite well, but is not so good for
energy efficiency. This commit therefore replaces the strict polling
with a wait queue.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/update.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index e940b86af4e8..f14a79d0d6de 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -371,6 +371,7 @@ static LIST_HEAD(rcu_tasks_holdouts);
/* Global list of callbacks and associated lock. */
static struct rcu_head *rcu_tasks_cbs_head;
static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
+static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);

/* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
@@ -381,13 +382,17 @@ module_param(rcu_task_stall_timeout, int, 0644);
void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
{
unsigned long flags;
+ bool needwake;

rhp->next = NULL;
rhp->func = func;
raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
+ needwake = !rcu_tasks_cbs_head;
*rcu_tasks_cbs_tail = rhp;
rcu_tasks_cbs_tail = &rhp->next;
raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
+ if (needwake)
+ wake_up(&rcu_tasks_cbs_wq);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks);

@@ -498,8 +503,12 @@ static int __noreturn rcu_tasks_kthread(void *arg)

/* If there were none, wait a bit and start over. */
if (!list) {
- schedule_timeout_interruptible(HZ);
- flush_signals(current);
+ wait_event_interruptible(rcu_tasks_cbs_wq,
+ rcu_tasks_cbs_head);
+ if (!rcu_tasks_cbs_head) {
+ flush_signals(current);
+ schedule_timeout_interruptible(HZ/10);
+ }
continue;
}

@@ -591,6 +600,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
list = next;
cond_resched();
}
+ schedule_timeout_uninterruptible(HZ/10);
}
}

--
1.8.1.5