linux-kernel - Re: [PATCH 3/6] cpufreq: governor: Drop min_sampling

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Message-ID: <20170629180123.GA2443@light.dominikbrodowski.net>
Date:   Thu, 29 Jun 2017 20:01:23 +0200
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] cpufreq: governor: Drop min_sampling_rate

On Thu, Jun 29, 2017 at 04:29:06PM +0530, Viresh Kumar wrote:
> The cpufreq core and governors aren't supposed to set a limit on how
> fast we want to try changing the frequency. This is currently done for
> the legacy governors with help of min_sampling_rate.
> 
> At worst, we may end up setting the sampling rate to a value lower than
> the rate at which frequency can be changed and then one of the CPUs in
> the policy will be only changing frequency for ever.

Is it safe to issue requests to change the CPU frequency so frequently, even
on historic hardware such as speedstep-{ich,smi,centrino}? In the past,
these checks more or less disallowed the running of dynamic frequency
scaling at least on speedstep-smi[*], but maybe on a few other platforms as
well. That's why I am curious on whether this may break systems potentially
on a hardware level if the hardware was not designed to do dynamic frequency
scaling (and not just frequency switches on battery/AC).

Best,
	Dominik