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

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Message-ID: <d2b121f1-1fd4-2842-f5bd-110a76997b65@arm.com>
Date:   Thu, 13 Jul 2017 13:40:39 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        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 16:21, Dietmar Eggemann wrote:
> 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

I was under the impression that we strictly don't care about that
information when I started exploring the fast_switch with the standard
firmware interface on ARM platforms(until if and when ARM provides an
instruction to achieve that).

If f/w failed to change the frequency, will that be not corrected in the
next sample or instance. I would like to know the impact of absence of
such notifications.

> 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.
> 

Agreed.

-- 
Regards,
Sudeep