linux-kernel - Re: [PATCH v5 11/12] sched: replace capacity

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Date:	Thu, 11 Sep 2014 18:15:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	preeti@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-arm-kernel@...ts.infradead.org, riel@...hat.com,
	Morten.Rasmussen@....com, efault@....de, nicolas.pitre@...aro.org,
	linaro-kernel@...ts.linaro.org, daniel.lezcano@...aro.org,
	dietmar.eggemann@....com
Subject: Re: [PATCH v5 11/12] sched: replace capacity_factor by utilization

On Tue, Aug 26, 2014 at 01:06:54PM +0200, Vincent Guittot wrote:
> +static inline int group_has_free_capacity(struct sg_lb_stats *sgs,
> +			struct lb_env *env)
>  {
> +	if ((sgs->group_capacity_orig * 100) >
> +			(sgs->group_utilization * env->sd->imbalance_pct))
> +		return 1;
> +
> +	if (sgs->sum_nr_running < sgs->group_weight)
> +		return 1;
>  
> +	return 0;
> +}
>  
> +static inline int group_is_overloaded(struct sg_lb_stats *sgs,
> +			struct lb_env *env)
> +{
> +	if (sgs->sum_nr_running <= sgs->group_weight)
> +		return 0;
>  
> +	if ((sgs->group_capacity_orig * 100) <
> +			(sgs->group_utilization * env->sd->imbalance_pct))
> +		return 1;
>  
> +	return 0;
>  }

I'm confused about the utilization vs capacity_orig. I see how we should
maybe scale things with the capacity when comparing between CPUs/groups,
but not on the same CPU/group.

I would have expected something simple like:

static inline bool group_has_capacity()
{
	/* Is there a spare cycle? */
	if (sgs->group_utilization < sgs->group_weight * SCHED_LOAD_SCALE)
		return true;

	/* Are there less tasks than logical CPUs? */
	if (sgs->sum_nr_running < sgs->group_weight)
		return true;

	return false;
}

Where group_utilization a pure sum of running_avg.

Now this has a hole when there are RT tasks on the system, in that case
the utilization will never hit 1, but we could fix that another way. I
don't think the capacity_orig thing is right.
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