[PATCH] rcupdate: reduce sys's overhead when rcu_barrier()s called simultaneous

[Lai Jiangshan <laijs@cn.fujitsu.com>]

rcu_barrier() queues rcu_head on all cpus, it will brings
large overhead for a large system which has a lots cpu.
this fix reduces sys's overhead when rcu_barrier()s called
simultaneous.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index ad63af8..734850b 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -53,6 +53,9 @@ enum rcu_barrier {
 
 static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
 static atomic_t rcu_barrier_cpu_count;
+static unsigned long rcu_barrier_completed;
+static unsigned long rcu_barrier_completed_bh;
+static unsigned long rcu_barrier_completed_sched;
 static DEFINE_MUTEX(rcu_barrier_mutex);
 static struct completion rcu_barrier_completion;
 
@@ -60,7 +63,7 @@ static struct completion rcu_barrier_completion;
  * Awaken the corresponding synchronize_rcu() instance now that a
  * grace period has elapsed.
  */
-void wakeme_after_rcu(struct rcu_head  *head)
+void wakeme_after_rcu(struct rcu_head *head)
 {
 	struct rcu_synchronize *rcu;
 
@@ -113,11 +116,20 @@ static void rcu_barrier_func(void *type)
  * Orchestrate the specified type of RCU barrier, waiting for all
  * RCU callbacks of the specified type to complete.
  */
-static void _rcu_barrier(enum rcu_barrier type)
+static void _rcu_barrier(enum rcu_barrier type, unsigned long *completed)
 {
+	unsigned long batch = ACCESS_ONCE(*completed);
 	BUG_ON(in_interrupt());
 	/* Take cpucontrol mutex to protect against CPU hotplug */
 	mutex_lock(&rcu_barrier_mutex);
+
+	BUG_ON(*completed & 1);
+	if ((*completed - batch) >= 2) {
+		mutex_unlock(&rcu_barrier_mutex);
+		return;
+	}
+	(*completed)++;
+
 	init_completion(&rcu_barrier_completion);
 	/*
 	 * Initialize rcu_barrier_cpu_count to 1, then invoke
@@ -133,6 +145,7 @@ static void _rcu_barrier(enum rcu_barrier type)
 	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
 		complete(&rcu_barrier_completion);
 	wait_for_completion(&rcu_barrier_completion);
+	(*completed)++;
 	mutex_unlock(&rcu_barrier_mutex);
 }
 
@@ -141,7 +154,7 @@ static void _rcu_barrier(enum rcu_barrier type)
  */
 void rcu_barrier(void)
 {
-	_rcu_barrier(RCU_BARRIER_STD);
+	_rcu_barrier(RCU_BARRIER_STD, &rcu_barrier_completed);
 }
 EXPORT_SYMBOL_GPL(rcu_barrier);
 
@@ -150,7 +163,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier);
  */
 void rcu_barrier_bh(void)
 {
-	_rcu_barrier(RCU_BARRIER_BH);
+	_rcu_barrier(RCU_BARRIER_BH, &rcu_barrier_completed_bh);
 }
 EXPORT_SYMBOL_GPL(rcu_barrier_bh);
 
@@ -159,7 +172,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier_bh);
  */
 void rcu_barrier_sched(void)
 {
-	_rcu_barrier(RCU_BARRIER_SCHED);
+	_rcu_barrier(RCU_BARRIER_SCHED, &rcu_barrier_completed_sched);
 }
 EXPORT_SYMBOL_GPL(rcu_barrier_sched);