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

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);




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