linux-kernel - Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant load-tracking support

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Message-ID: <de0feb89-4759-b50c-e44b-a9a51984d396@arm.com>
Date:   Tue, 11 Jul 2017 16:21:06 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant
 load-tracking support

On 11/07/17 07:39, Viresh Kumar wrote:
> On 10-07-17, 14:46, Rafael J. Wysocki wrote:
>> This particular change is about a new feature, so making it in the core is OK
>> in two cases IMO: (a) when you actively want everyone to be affected by it and
> 
> IMO this change should be done for the whole ARM architecture. And if some
> regression happens due to this, then we come back and solve it.
> 
>> (b) when the effect of it on the old systems should not be noticeable.
> 
> I am not sure about the effects of this on performance really.
> 
> @Dietmar: Any inputs for that ?

Like I said in the other email, since for (future)
arm/arm64 fast-switch driver, the return value of
cpufreq_driver->fast_switch() does not give us the information that the
frequency value did actually change, we have to implement
arch_set_freq_scale() in the driver.
This means that we probably only implement this in the subset of drivers
which will be used in platforms on which we want to have
frequency-invariant load-tracking.

A future aperf/mperf like counter FIE solution can give us arch-wide
support when those counters are available.