Re: [PATCH] sched: Accelerate "pick_next_entity" under special condition

[Michael Wang <wangyun@linux.vnet.ibm.com>]

On 01/16/2012 05:37 PM, Michael Wang wrote:

> From: Michael Wang <wangyun@linux.vnet.ibm.com>
> 
> We can avoid some useless operation in some special condition.
> 
> For example:
> If we have "cfs_rq->next" and it can be use, we just return it directly.
> 


Please tell me if I got wrong understanding on these code, I think the change can 
maintain the old logic and suppose to be a little quick in some condition.
 

> Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
> ---
>  kernel/sched/fair.c |   28 +++++++++++++++++-----------
>  1 files changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 84adb2d..9fc2c3c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1295,6 +1295,8 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  static int
>  wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
>  
> +#define ENTITY_PREEMPT_ALLOWED(prev,next)	(wakeup_preempt_entity(prev, next) < 1)


I want to use a name which can make it more easy to be understand, please tell me 
if it is a bad idea...

Regards,
Michael Wang

> +
>  /*
>   * Pick the next process, keeping these things in mind, in this order:
>   * 1) keep things fair between processes/task groups
> @@ -1308,29 +1310,33 @@ static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
>  	struct sched_entity *left = se;
>  
>  	/*
> -	 * Avoid running the skip buddy, if running something else can
> -	 * be done without getting too unfair.
> +	 * Someone really wants this to run. If it's not unfair, run it.
>  	 */
> -	if (cfs_rq->skip == se) {
> -		struct sched_entity *second = __pick_next_entity(se);
> -		if (second && wakeup_preempt_entity(second, left) < 1)
> -			se = second;
> +	if (cfs_rq->next && ENTITY_PREEMPT_ALLOWED(cfs_rq->next, left)) {
> +		se = cfs_rq->next;
> +		goto out;
>  	}
>  
>  	/*
>  	 * Prefer last buddy, try to return the CPU to a preempted task.
>  	 */
> -	if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
> +	if (cfs_rq->last && ENTITY_PREEMPT_ALLOWED(cfs_rq->last, left)) {
>  		se = cfs_rq->last;
> +		goto out;
> +	}
>  
>  	/*
> -	 * Someone really wants this to run. If it's not unfair, run it.
> +	 * Avoid running the skip buddy, if running something else can
> +	 * be done without getting too unfair.
>  	 */
> -	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
> -		se = cfs_rq->next;
> +	if (cfs_rq->skip == se) {
> +		struct sched_entity *second = __pick_next_entity(se);
> +		if (second && ENTITY_PREEMPT_ALLOWED(second, left))
> +			se = second;
> +	}
>  
> +out:
>  	clear_buddies(cfs_rq, se);
> -
>  	return se;
>  }
>