linux-kernel - Re: [PATCH v5 10/12] sched: get CPU's utilization statistic

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Date:	Thu, 11 Sep 2014 14:34:12 +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 10/12] sched: get CPU's utilization statistic

On Tue, Aug 26, 2014 at 01:06:53PM +0200, Vincent Guittot wrote:
> Monitor the utilization level of each group of each sched_domain level. The
> utilization is the amount of cpu_capacity that is currently used on a CPU or
> group of CPUs. We use the usage_load_avg to evaluate this utilization level.
> In the special use case where the CPU is fully loaded by more than 1 task,
> the activity level is set above the cpu_capacity in order to reflect the overload
> of the CPU
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1fd2131..2f95d1c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4126,6 +4126,11 @@ static unsigned long capacity_of(int cpu)
>  	return cpu_rq(cpu)->cpu_capacity;
>  }
>  
> +static unsigned long capacity_orig_of(int cpu)
> +{
> +	return cpu_rq(cpu)->cpu_capacity_orig;
> +}
> +
>  static unsigned long cpu_avg_load_per_task(int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);

This hunk should probably go into patch 6.

> @@ -4514,6 +4519,17 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  	return target;
>  }
>  
> +static int get_cpu_utilization(int cpu)
> +{
> +	unsigned long usage = cpu_rq(cpu)->cfs.usage_load_avg;
> +	unsigned long capacity = capacity_of(cpu);
> +
> +	if (usage >= SCHED_LOAD_SCALE)
> +		return capacity + 1;
> +
> +	return (usage * capacity) >> SCHED_LOAD_SHIFT;
> +}

So if I understood patch 9 correct, your changelog is iffy.
usage_load_avg should never get > 1 (of whatever unit), no matter how
many tasks are on the rq. You can only maximally run all the time.

Therefore I can only interpret the if (usage >= SCHED_LOAD_SCALE) as
numerical error handling, nothing more.

Also I'm not entirely sure I like the usage, utilization names/metrics.
I would suggest to reverse them. Call the pure running number
'utilization' and this scaled with capacity 'usage' or so.

> @@ -6188,7 +6206,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  		/* Now, start updating sd_lb_stats */
>  		sds->total_load += sgs->group_load;
>  		sds->total_capacity += sgs->group_capacity;
> -
>  		sg = sg->next;
>  	} while (sg != env->sd->groups);
>  

I like that extra line of whitespace, it separates the body from the
loop itself.
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