linux-kernel - Re: [PATCH v4] sched: Avoid unnecessary work in reweight

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Date:	Fri, 17 Feb 2012 10:28:30 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] sched: Avoid unnecessary work in reweight_entity

Hi, Peter

Thanks for your reply.

On 02/16/2012 10:29 PM, Peter Zijlstra wrote:

> On Thu, 2012-02-16 at 22:14 +0800, Michael Wang wrote:
>> From: Michael Wang <wangyun@...ux.vnet.ibm.com>
>>
>> In original code, using account_entity_dequeue and account_entity_enqueue
>> as a pair will do some work unnecessary, such like remove node from list
>> and add it back again immediately.
>>
>> And because there are no really enqueue and dequeue happen here, it is not
>> suitable to use account_entity_dequeue and account_entity_enqueue here.
>>
>> This patch combine the work of them in order to save time.
> 
> And yet no numbers on how much it saves :-(
> 


That's right...
Actually I don't know how to prove the performance improvement, please 
give me some suggestion and I will do it :)

>> Signed-off-by: Michael Wang <wangyun@...ux.vnet.ibm.com>
>> ---
>>  kernel/sched/fair.c  |   27 +++++++++++++++++++++------
>>  kernel/sched/sched.h |    8 ++++++++
>>  2 files changed, 29 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 7c6414f..fda63ec 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -777,16 +777,28 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>   */
>>  
>>  #if defined CONFIG_SMP && defined CONFIG_FAIR_GROUP_SCHED
>> -static void
>> +static inline void
>>  add_cfs_task_weight(struct cfs_rq *cfs_rq, unsigned long weight)
>>  {
>>  	cfs_rq->task_weight += weight;
>>  }
>> +
>> +static inline void
>> +sub_add_cfs_task_weight(struct cfs_rq *cfs_rq, unsigned long sub, unsigned long add)
>> +{
>> +	cfs_rq->task_weight -= sub;
>> +	cfs_rq->task_weight += add;
>> +}
> 
> This is pointless, just use: add_cfs_task_weight(cfs_rq, add - sub);
> 


Sorry for the silly code, v5 will correct it and all the issues mentioned below.

Regards,
Michael Wang

>>  #else
>>  static inline void
>>  add_cfs_task_weight(struct cfs_rq *cfs_rq, unsigned long weight)
>>  {
>>  }
>> +
>> +static inline void
>> +sub_add_cfs_task_weight(struct cfs_rq *cfs_rq, unsigned long sub, unsigned long add)
>> +{
>> +}
>>  #endif
>>  
>>  static void
>> @@ -938,6 +950,7 @@ static inline void update_entity_shares_tick(struct cfs_rq *cfs_rq)
>>  {
>>  }
>>  # endif /* CONFIG_SMP */
>> +
>>  static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>>  			    unsigned long weight)
>>  {
>> @@ -945,13 +958,15 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>>  		/* commit outstanding execution time */
>>  		if (cfs_rq->curr == se)
>>  			update_curr(cfs_rq);
>> -		account_entity_dequeue(cfs_rq, se);
>> +		update_load_sub_add(&cfs_rq->load, se->load.weight, weight);
>> +		if (!parent_entity(se)) {
>> +			update_load_sub_add(&rq_of(cfs_rq)->load, se->load.weight, weight);
> 
> same here:
> 	update_load_add(&rq_of(cfs_rq)->load, weight - se->load.weight);
> 
> Also superfluous use of curly-braces here.
> 
>> +		}
>> +		if (entity_is_task(se)) {
>> +			sub_add_cfs_task_weight(cfs_rq, se->load.weight, weight);
>> +		}
> 
> idem.
> 
>>  	}
>> -
>>  	update_load_set(&se->load, weight);
>> -
>> -	if (se->on_rq)
>> -		account_entity_enqueue(cfs_rq, se);
>>  }
>>  
>>  static void update_cfs_shares(struct cfs_rq *cfs_rq)
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 98c0c26..ec4430f 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -779,6 +779,14 @@ static inline void update_load_sub(struct load_weight *lw, unsigned long dec)
>>  	lw->inv_weight = 0;
>>  }
>>  
>> +static inline void update_load_sub_add(struct load_weight *lw, unsigned long dec,
>> +					unsigned long inc)
>> +{
>> +	lw->weight -= dec;
>> +	lw->weight += inc;
>> +	lw->inv_weight = 0;
>> +}
> 
> again, pointless stuff.
> 
>>  static inline void update_load_set(struct load_weight *lw, unsigned long w)
>>  {
>>  	lw->weight = w;
> 


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