linux-kernel - Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT

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Date:   Thu, 29 Nov 2018 13:53:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        pkondeti@...eaurora.org, Quentin Perret <quentin.perret@....com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT

On Wed, Nov 28, 2018 at 11:53:36AM +0000, Patrick Bellasi wrote:

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ac855b2f4774..93e0cf5d8a76 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3661,6 +3661,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
>  	if (!task_sleep)
>  		return;
>  
> +	/* Skip samples which do not represent an actual utilization */
> +	if (unlikely(task_util(p) > capacity_of(task_cpu(p))))
> +		return;
> +
>  	/*
>  	 * If the PELT values haven't changed since enqueue time,
>  	 * skip the util_est update.

Would you not want something like:

	min(task_util(p), capacity_of(task_cpu(p)))

And is this the only place where we need this?

OTOH, if the task is always running, it will be always running
irrespective of where it runs.

Not storing these samples seems weird though; this is the exact
condition you want to record -- the task is very active, if we skip
these, we'll come back at a low frequency on the next wakeup.