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

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Date:	Thu, 16 Feb 2012 22:32:46 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] sched: Avoid unnecessary work in reweight_entity

Hi, All

I have correct the issue about invoke update_curr too many times, it caused by
the context in dequeu_entity, the se will be cfs_rq->curr and se->on_rq is 0.

I suppose this patch can avoid some work like:

1. reduce times to check "se->on" from 2 to 1.
2. reduce times to check "parent_entity(se)" from 2 to 1.
3. reduce times to check "entity_is_task(se)" from 2 to 1.
4. reduce times to set "cfs_rq->load.inv_weight" from 4 to 2.
5. reduce times to set "rq_of(cfs_rq)->load.inv_weight" from 2 to 1.
6. no need to use "list_add(&se->group_node, &cfs_rq->tasks);" any more.
7. no need to use "list_del_init(&se->group_node);" any more.
8. no need to do "cfs_rq->nr_running++" and "cfs_rq->nr_running--" any more.

Please tell me if you found in some conditions it will not work or even reduce 
the performance, any comments are appreciated :)

Best Regards.
Michael Wang

On 02/16/2012 10:14 PM, 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.
> 
> v2:
> 	Add more description and revise wrong code.
> 
> v3:
> 	Mark add_cfs_task_weight and sub_add_cfs_task_weight as inline. 
> 
> v4:
> 	Now we invoke update_curr when really necessary. 
> 
> 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;
> +}
>  #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);
> +		}
> +		if (entity_is_task(se)) {
> +			sub_add_cfs_task_weight(cfs_rq, se->load.weight, weight);
> +		}
>  	}
> -
>  	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;
> +}
> +
>  static inline void update_load_set(struct load_weight *lw, unsigned long w)
>  {
>  	lw->weight = w;


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