linux-kernel - Re: [PATCH v7 5/7] sched: get CPU's usage statistic

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Message-ID: <20141009113614.GP4750@worktop.programming.kicks-ass.net>
Date:	Thu, 9 Oct 2014 13:36:14 +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, Morten.Rasmussen@....com,
	kamalesh@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-arm-kernel@...ts.infradead.org, riel@...hat.com,
	efault@....de, nicolas.pitre@...aro.org,
	linaro-kernel@...ts.linaro.org, daniel.lezcano@...aro.org,
	dietmar.eggemann@....com, pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH v7 5/7] sched: get CPU's usage statistic

On Tue, Oct 07, 2014 at 02:13:35PM +0200, Vincent Guittot wrote:
> Monitor the usage level of each group of each sched_domain level. The usage is
> the amount of cpu_capacity that is currently used on a CPU or group of CPUs.
> We use the utilization_load_avg to evaluate the usage level of each group.
> 
> The utilization_avg_contrib only takes into account the running time but not
> the uArch so the utilization_load_avg is in the range [0..SCHED_LOAD_SCALE]
> to reflect the running load on the CPU. We have to scale the utilization with
> the capacity of the CPU to get the usage of the latter. The usage can then be
> compared with the available capacity.

You say cpu_capacity, but in actual fact you use capacity_orig and fail
to justify/clarify this.

> The frequency scaling invariance is not taken into account in this patchset,
> it will be solved in another patchset

Maybe explain what the specific invariance issue is that is skipped over
for now.

> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d3e9067..7364ed4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4551,6 +4551,17 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  	return target;
>  }
>  
> +static int get_cpu_usage(int cpu)
> +{
> +	unsigned long usage = cpu_rq(cpu)->cfs.utilization_load_avg;
> +	unsigned long capacity = capacity_orig_of(cpu);
> +
> +	if (usage >= SCHED_LOAD_SCALE)
> +		return capacity + 1;

Like Morten I'm confused by that +1 thing.

> +
> +	return (usage * capacity) >> SCHED_LOAD_SHIFT;
> +}

A comment with that function that it returns capacity units might
clarify shift confusion Morten raised the other day.
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