linux-kernel - Re: [PATCH v10 07/11] sched: get CPU's usage statistic

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Date:	Tue, 03 Mar 2015 12:47:27 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"kamalesh@...ux.vnet.ibm.com" <kamalesh@...ux.vnet.ibm.com>
CC:	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v10 07/11] sched: get CPU's usage statistic

On 27/02/15 15:54, Vincent Guittot wrote:
> Monitor the usage level of each group of each sched_domain level. The usage is
> the portion of cpu_capacity_orig 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_load_avg only takes into account the running time of the CFS
> tasks on a CPU with a maximum value of SCHED_LOAD_SCALE when the CPU is fully
> utilized. Nevertheless, we must cap utilization_load_avg which can be temporaly

s/temporaly/temporally

> greater than SCHED_LOAD_SCALE after the migration of a task on this CPU and
> until the metrics are stabilized.
> 
> The utilization_load_avg is in the range [0..SCHED_LOAD_SCALE] to reflect the
> running load on the CPU whereas the available capacity for the CFS task is in
> the range [0..cpu_capacity_orig]. In order to test if a CPU is fully utilized
> by CFS tasks, we have to scale the utilization in the cpu_capacity_orig range
> of the CPU to get the usage of the latter. The usage can then be compared with
> the available capacity (ie cpu_capacity) to deduct the usage level of a CPU.
> 
> The frequency scaling invariance of the usage is not taken into account in this
> patch, it will be solved in another patch which will deal with frequency
> scaling invariance on the running_load_avg.

The use of underscores in running_load_avg implies to me that this is a
data member of struct sched_avg or something similar. But there is no
running_load_avg in the current code. However, I can see that
sched_avg::*running_avg_sum* (and therefore
cfs_rq::*utilization_load_avg*) are frequency scale invariant.

> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> Acked-by: Morten Rasmussen <morten.rasmussen@....com>
> ---
>  kernel/sched/fair.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 10f84c3..faf61a2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4781,6 +4781,33 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  done:
>  	return target;
>  }
> +/*
> + * get_cpu_usage returns the amount of capacity of a CPU that is used by CFS
> + * tasks. The unit of the return value must capacity so we can compare the

s/must capacity/must be the one of capacity

> + * usage with the capacity of the CPU that is available for CFS task (ie
> + * cpu_capacity).
> + * cfs.utilization_load_avg is the sum of running time of runnable tasks on a
> + * CPU. It represents the amount of utilization of a CPU in the range
> + * [0..SCHED_LOAD_SCALE].  The usage of a CPU can't be higher than the full
> + * capacity of the CPU because it's about the running time on this CPU.
> + * Nevertheless, cfs.utilization_load_avg can be higher than SCHED_LOAD_SCALE
> + * because of unfortunate rounding in avg_period and running_load_avg or just
> + * after migrating tasks until the average stabilizes with the new running
> + * time. So we need to check that the usage stays into the range
> + * [0..cpu_capacity_orig] and cap if necessary.
> + * Without capping the usage, a group could be seen as overloaded (CPU0 usage
> + * at 121% + CPU1 usage at 80%) whereas CPU1 has 20% of available capacity/

s/capacity\//capacity.

[...]

-- Dietmar

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