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

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Message-ID: <20170713144205.sv5wtoadazk65p5n@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Jul 2017 16:42:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "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 Thu, Jul 13, 2017 at 03:04:09PM +0100, Sudeep Holla wrote:

> The question is whether we *need* to know the completion of frequency
> transition. What is the impact of absence of it ? I am considering
> platforms which may take up to a ms or more to do the actual transition
> in the firmware.

So on x86 we can recover from not knowing by means of the APERF/MPERF
thing, which gives us the average effective frequency over the last
period.

If you lack that you need something to update the actual effective
frequency.

Changing the effective frequency at request time might confuse things --
esp. if the request might not be honoured at all or can take a
significant time to complete. Not to mention that _IF_ you rely on the
effective frequency to set other clocks things can come unstuck.

So unless you go the whole distance and do APERF/MPERF like things, I
think it would be very good to have a notification of completion (and
possibly a read-back of the effective frequency that is now set).