linux-kernel - Re: [PATCH v6 03/11] sched/rt: add rt

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Message-ID: <00f701a4-8ea6-82cb-6c76-5597ffcc9f11@arm.com>
Date:   Fri, 15 Jun 2018 13:52:56 +0200
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:     rjw@...ysocki.net, juri.lelli@...hat.com, Morten.Rasmussen@....com,
        viresh.kumar@...aro.org, valentin.schneider@....com,
        patrick.bellasi@....com, joel@...lfernandes.org,
        daniel.lezcano@...aro.org, quentin.perret@....com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v6 03/11] sched/rt: add rt_rq utilization tracking

On 06/08/2018 02:09 PM, Vincent Guittot wrote:
> schedutil governor relies on cfs_rq's util_avg to choose the OPP when cfs
> tasks are running. When the CPU is overloaded by cfs and rt tasks, cfs tasks
> are preempted by rt tasks and in this case util_avg reflects the remaining
> capacity but not what cfs want to use. In such case, schedutil can select a
> lower OPP whereas the CPU is overloaded. In order to have a more accurate
> view of the utilization of the CPU, we track the utilization of rt tasks.
> 
> rt_rq uses rq_clock_task and cfs_rq uses cfs_rq_clock_task but they are
> the same at the root group level, so the PELT windows of the util_sum are
> aligned.
> 
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

[...]

;
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index 4174582..81c0d7e 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -307,3 +307,25 @@ int __update_load_avg_cfs_rq(u64 now, int cpu, struct cfs_rq *cfs_rq)
>   
>   	return 0;
>   }
> +
> +/*
> + * rt_rq:
> + *
> + *   util_sum = \Sum se->avg.util_sum but se->avg.util_sum is not tracked
> + *   util_sum = cpu_scale * load_sum
> + *   runnable_load_sum = load_sum
> + *
> + */
> +
> +int update_rt_rq_load_avg(u64 now, struct rq *rq, int running)
> +{
> +	if (___update_load_sum(now, rq->cpu, &rq->avg_rt,
> +				running,
> +				running,
> +				running)) {

The patch clearly says that this is about utilization but what happens
to load and runnable load for the rt rq part when you call
___update_load_sum() with load=[0,1] and runnable=[0,1]?

It looks like that the math would require 1024 instead of 1 for load and
runnable so that we would see a load_avg or runnable_load_avg != 0.

1594.075128: bprint: update_rt_rq_load_avg: now=1593937228087 cpu=4 running=1
1594.075129: bprint: update_rt_rq_load_avg: delta=3068 cpu=4 load=1 runnable=1 running=1 scale_freq=1024 scale_cpu=1024 periods=2
1594.075130: bprint: update_rt_rq_load_avg: load_sum=23927 +2879 runnable_load_sum=23927 +2879 util_sum=24506165 +2948096
1594.075130: bprint: update_rt_rq_load_avg: load_avg=0 runnable_load_avg=0 util_avg=513

IMHO, the patch should say whether load and runnable load are supported as well or not. 

[...]