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

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

Paul E. McKenney wrote:
> On Fri, Oct 24, 2008 at 01:38:13PM +0800, Lai Jiangshan wrote:
>> 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.
> 
> Hello, Jiangshan,
> 
> If we were to have problems with many concurrent rcu_barrier()
> calls stacking up, this patch looks like it would be a reasonable
> was of addressing those problems.
> 
> But do we really have problems with this at the moment?
> 
> 							Thanx, Paul

Hi, Paul,

	Thanks, we do not have problems with this at the moment.

	I suddenly had an association of ideas to synchronize_srcu(),
so I made use of the ideas in synchronize_srcu() and this patch was made.

							Thanx, Lai.


> 
>> 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);
>>
>>
>>
> 
> 
>