linux-kernel - Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote callback for shared policies

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Message-ID: <597901EA.6080702@codeaurora.org>
Date:   Wed, 26 Jul 2017 13:56:10 -0700
From:   Saravana Kannan <skannan@...eaurora.org>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, smuckle.linux@...il.com,
        eas-dev@...ts.linaro.org
Subject: Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote callback
 for shared policies

On 07/20/2017 05:22 AM, Peter Zijlstra wrote:
> So the typical implementation of fast switching we're thinking of is the
> CPU writing the DVFS request into a machine register. Now machine
> registers are typically per logical CPU.

Writing to a memory addressable register. AFAIK, ARM has no support for 
a machine register for DVFS request. So, even if any ARM licensee wants 
to add one, it won't be possible.

Also, even if we have an ARM CPU with a machine register, rejecting a 
valid frequency switch just because it happened to come on a different 
CPU seem silly (you can have a huge performance hit due to that). A much 
better solution is to just make an IPI to the right CPU and execute the 
machine register write on the right CPU.

-Saravana

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