linux-kernel - Re: [PATCH 3/8] sched/cpufreq: new cfs capacity margin helpers

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Message-ID: <20160315211740.GD6344@twins.programming.kicks-ass.net>
Date:	Tue, 15 Mar 2016 22:17:40 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michael Turquette <mturquette@...libre.com>
Cc:	rjw@...ysocki.net, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, Juri.Lelli@....com,
	steve.muckle@...aro.org, morten.rasmussen@....com,
	dietmar.eggemann@....com, vincent.guittot@...aro.org,
	Michael Turquette <mturquette+renesas@...libre.com>
Subject: Re: [PATCH 3/8] sched/cpufreq: new cfs capacity margin helpers

On Sun, Mar 13, 2016 at 10:22:07PM -0700, Michael Turquette wrote:
> +/**
> + * cpufreq_set_cfs_capacity_margin - Set global cfs enqueue capacity margin
> + * @margin: new capacity margin
> + *
> + * margin is a percentage of capacity that is applied to the current
> + * utilization when selecting a new capacity state or cpu frequency. The value
> + * should be normalized to the range of [0..SCHED_CAPACITY_SCALE], where
> + * SCHED_CAPACITY_SCALE is 100% of the normalized capacity, or equivalent to
> + * multiplying the utilization by one.
> + *
> + * For instance, to add a 25% margin to a utilization, margin should be 1280,
> + * which is 1.25x 1024, the default for SCHED_CAPACITY_SCALE.
> + */
> +void cpufreq_set_cfs_capacity_margin(unsigned long margin)
> +{
> +	cfs_capacity_margin = margin;
> +}
> +EXPORT_SYMBOL_GPL(cpufreq_set_cfs_capacity_margin);

I don't like this as an interface; what's wrong with using percentiles
as per the discussion I had with Rafael last week?

Also, why is this exported?