linux-kernel - Re: [PATCH 4/6 v5] sched: propagate load during synchronous attach/detach

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Date:   Fri, 21 Oct 2016 13:19:01 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org
Cc:     yuyang.du@...el.com, Morten.Rasmussen@....com,
        linaro-kernel@...ts.linaro.org, pjt@...gle.com, bsegall@...gle.com,
        kernellwp@...il.com
Subject: Re: [PATCH 4/6 v5] sched: propagate load during synchronous
 attach/detach

On 10/17/2016 10:14 AM, Vincent Guittot wrote:
> When a task moves from/to a cfs_rq, we set a flag which is then used to
> propagate the change at parent level (sched_entity and cfs_rq) during
> next update. If the cfs_rq is throttled, the flag will stay pending until
> the cfs_rw is unthrottled.

minor nit:

s/cfs_rw/cfs_rq

[...]

> @@ -8704,6 +8867,22 @@ static void detach_task_cfs_rq(struct task_struct *p)
>  	update_load_avg(se, 0);
>  	detach_entity_load_avg(cfs_rq, se);
>  	update_tg_load_avg(cfs_rq, false);
> +
> +#ifdef CONFIG_FAIR_GROUP_SCHED
> +	/*
> +	 * Propagate the detach across the tg tree to make it visible to the
> +	 * root
> +	 */
> +	se = se->parent;
> +	for_each_sched_entity(se) {
> +		cfs_rq = cfs_rq_of(se);
> +
> +		if (cfs_rq_throttled(cfs_rq))
> +			break;
> +
> +		update_load_avg(se, UPDATE_TG);
> +	}
> +#endif
>  }
>
>  static void attach_entity_cfs_rq(struct sched_entity *se)
> @@ -8722,6 +8901,22 @@ static void attach_entity_cfs_rq(struct sched_entity *se)
>  	update_load_avg(se, sched_feat(ATTACH_AGE_LOAD) ? 0 : SKIP_AGE_LOAD);
>  	attach_entity_load_avg(cfs_rq, se);
>  	update_tg_load_avg(cfs_rq, false);
> +
> +#ifdef CONFIG_FAIR_GROUP_SCHED
> +	/*
> +	 * Propagate the attach across the tg tree to make it visible to the
> +	 * root
> +	 */
> +	se = se->parent;
> +	for_each_sched_entity(se) {
> +		cfs_rq = cfs_rq_of(se);
> +
> +		if (cfs_rq_throttled(cfs_rq))
> +			break;
> +
> +		update_load_avg(se, UPDATE_TG);
> +	}
> +#endif
>  }

The 'detach across' and 'attach across' in detach_task_cfs_rq() and 
attach_entity_cfs_rq() do the same so couldn't you not create a function 
propagate_foo() for it? This would avoid this ifdef as well.

You could further create in your '[PATCH 1/6 v5] sched: factorize attach 
entity':

detach_entity_cfs_rq() {
   update_load_avg()
   detach_entity_load_avg()
   update_tg_load_avg()
   propagate_load_avg()
}

and then we would have:

attach_task_cfs_rq() -> attach_entity_cfs_rq() -> propagate_foo()
detach_task_cfs_rq() -> detach_entity_cfs_rq() -> propagate_foo()

I guess you didn't because it would be only called one time but this 
symmetric approaches are easier to remember (at least for me).

[...]