Re: sched: how to pick runqueue when checking task hot?

[Peter Zijlstra]

On Fri, Jun 06, 2014 at 02:59:56PM +0800, zhdxzx@xxxxxxxx wrote:
> Hi all
> 
> When computing cache hot, question-1: should we check the number of running tasks
> against the migration target runqueue, as shown by the following diff?
> 
> It looks that we dont migrate task if it is buddy and the target cpu is not idle.
> 
> 
> --- a/kernel/sched/fair.c	Fri Jun  6 12:37:37 2014
> +++ b/kernel/sched/fair.c	Fri Jun  6 13:55:35 2014
> @@ -5051,7 +5050,7 @@ task_hot(struct task_struct *p, u64 now)
>  	/*
>  	 * Buddy candidates are cache hot:
>  	 */
> -	if (sched_feat(CACHE_HOT_BUDDY) && this_rq()->nr_running &&
> +	if (sched_feat(CACHE_HOT_BUDDY) && env->dst_rq->nr_running &&
>  			(&p->se == cfs_rq_of(&p->se)->next ||
>  			 &p->se == cfs_rq_of(&p->se)->last))
>  		return 1;


That is indeed the effective result for normal balancing, seeing how it
pulls to the dst rq, and this_rq would be dst.

This is of course false for a number of cases these days, most obviously
the nohz idle balancing.

> But based on the comment, question-2: should we check running tasks
> against the runqueue of the given task?

> --- a/kernel/sched/fair.c	Fri Jun  6 12:37:37 2014
> +++ b/kernel/sched/fair.c	Fri Jun  6 14:32:34 2014
> @@ -5051,7 +5051,7 @@ task_hot(struct task_struct *p, u64 now)
>  	/*
>  	 * Buddy candidates are cache hot:
>  	 */
> -	if (sched_feat(CACHE_HOT_BUDDY) && this_rq()->nr_running &&
> +	if (sched_feat(CACHE_HOT_BUDDY) && task_rq(p)->nr_running &&
>  			(&p->se == cfs_rq_of(&p->se)->next ||
>  			 &p->se == cfs_rq_of(&p->se)->last))
>  		return 1;

That does appear to make more sense indeed, seeing how buddies are pairs
of tasks, so protecting a lone task doesn't make sense.


Mike, how did you intend this code to work?

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