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

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Date:   Thu, 13 Jul 2017 13:54:49 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        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 12/07/17 10:27, Viresh Kumar wrote:
> On 12-07-17, 10:31, Peter Zijlstra wrote:
>> So the problem with the thread is two-fold; one the one hand we like the
>> scheduler to directly set frequency, but then we need to schedule a task
>> to change the frequency, which will change the frequency and around we
>> go.
>>
>> On the other hand, there's very nasty issues with PI. This thread would
>> have very high priority (otherwise the SCHED_DEADLINE stuff won't work)
>> but that then means this thread needs to boost the owner of the i2c
>> mutex. And that then creates a massive bandwidth accounting hole.
>>
>>
>> The advantage of using an interrupt driven state machine is that all
>> those issues go away.
>>
>> But yes, whichever way around you turn things, its crap. But given the
>> hardware its the best we can do.
> 
> Thanks for the explanation Peter.
> 
> IIUC, it will take more time to change the frequency eventually with
> the interrupt-driven state machine as there may be multiple bottom
> halves involved here, for supply, clk, etc, which would run at normal
> priorities now. And those were boosted currently due to the high
> priority sugov thread. And we are fine with that (from performance
> point of view) ?
> 
> Coming back to where we started from (where should we call
> arch_set_freq_scale() from ?).
> 
> I think we would still need some kind of synchronization between
> cpufreq core and the cpufreq drivers to make sure we don't start
> another freq change before the previous one is complete. Otherwise
> the cpufreq drivers would be required to have similar support with
> proper locking in place.
> 

Good point, but with firmware interface we are considering fro
fast-switch, the firmware can override the previous request if it's not
yet started. So I assume that's fine and expected ?

> And if the core is going to get notified about successful freq changes
> (which it should IMHO),

Is that mandatory for even fast-switching ?

-- 
Regards,
Sudeep