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

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Date:   Fri, 30 Jun 2017 06:53:06 +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 Fri, Jun 30, 2017 at 09:04:25AM +0530, Viresh Kumar wrote:
> On 29-06-17, 20:01, Dominik Brodowski wrote:
> > 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,
> 
> Well, I assumed so. I am not sure the hardware would break though.
> Overheating ?
> 
> > 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[*],
> 
> We must by doing dynamic freq scaling even without this patch. I don't
> see why you say the above then.
> 
> All we do here is that we get rid of the limit on how soon we can
> change the freq again.

Well, as I understand it, first generation "speedstep" was designed more or
less to switch frequencies only when AC power was lost or restored.

The Linux implementation merely said: "no on-the-fly changes", but switch
frequencies whenever a user explicitly requested such a change (presumably
only every once in an unspecified while).

This same reasoning may be present in other drivers using CPUFREQ_ETERNAL.

> > 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).
> 
> Honestly I am not sure if any hardware can break or not, just because
> of this commit.

I am not *sure* either, I am just worried of the consequences of doing
things out-of-spec...

Best
	Dominik

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