linux-kernel - Re: [PATCH RFC 2/5] sched/fair: Skip frequency update if CPU about to idle

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Date:   Fri, 3 Nov 2017 22:44:25 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Steve Muckle <smuckle@...gle.com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Cc: Srinivas Pandruvada" <srinivas.pandruvada@...ux.intel.com>,
        "Cc: Len Brown" <lenb@...nel.org>,
        "Cc: Juri Lelli" <juri.lelli@....com>,
        "Cc: Patrick Bellasi" <patrick.bellasi@....com>,
        "Cc: Brendan Jackman" <brendan.jackman@....com>,
        "Cc: Chris Redpath" <Chris.Redpath@....com>,
        "Cc: Atish Patra" <atish.patra@...cle.com>,
        "Cc: Dietmar Eggemann" <dietmar.eggemann@....com>,
        "Cc: Vincent Guittot" <vincent.guittot@...aro.org>,
        "Cc: Morten Ramussen" <morten.rasmussen@....com>,
        "Cc: Frederic Weisbecker" <fweisbec@...il.com>,
        "Cc: Thomas Gleixner" <tglx@...utronix.de>,
        "Cc: EAS Dev" <eas-dev@...ts.linaro.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC 2/5] sched/fair: Skip frequency update if CPU about to idle

On Wed, Nov 1, 2017 at 12:35 PM, Steve Muckle <smuckle@...gle.com> wrote:
> On 10/30/2017 12:02 PM, Joel Fernandes wrote:
>>>
>>> Also, this more looks like a policy decision. Will it be better to
>>> put that directly into schedutil? Like this:
>>>
>>>          if (cpu_idle())
>>>                  "Don't change the freq";
>>>
>>> Will something like that work?
>>
>>
>> I thought about this and I think it wont work very well. In the
>> dequeue path we're still running the task being dequeued so the CPU is
>> not yet idle. What is needed here IMO is a notion that the CPU is
>> possibly about to idle and we can get predict that from the dequeue
>> path of the CFS class.
>>
>> Also just looking at whether the CPU is currently idle or not in the
>> governor doesn't help to differentiate between say the dequeue path /
>> tick path. Both of which can occur when the CPU is not idle.
>>
>> Any thoughts about this?
>
>
> Also if it really is the case that this bit of policy is universally
> desirable, I'd think it is better to do this in the scheduler and avoid a
> needless trip through a fn pointer out to schedutil for performance reasons.

I agree.

Peter, what do you think? Are you Ok with the approach of this patch
(preventing of the cpufreq update call during dequeue)?

thanks,

- Joel