linux-kernel - Re: [RFC PATCH v3 4/6] sched/cpufreq: Introduce sugov_cpu_ramp

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Message-ID: <65bfb1eb-2348-e36c-9ba1-31e59a9afc96@arm.com>
Date:   Mon, 14 Oct 2019 16:32:11 +0100
From:   Douglas Raillard <douglas.raillard@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, rjw@...ysocki.net, viresh.kumar@...aro.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, qperret@...rret.net,
        patrick.bellasi@...bug.net, dh.han@...sung.com
Subject: Re: [RFC PATCH v3 4/6] sched/cpufreq: Introduce sugov_cpu_ramp_boost

Hi Peter,

On 10/14/19 3:33 PM, Peter Zijlstra wrote:
> On Fri, Oct 11, 2019 at 02:44:58PM +0100, Douglas RAILLARD wrote:
>> Use the utilization signals dynamic to detect when the utilization of a
>> set of tasks starts increasing because of a change in tasks' behavior.
>> This allows detecting when spending extra power for faster frequency
>> ramp up response would be beneficial to the reactivity of the system.
>>
>> This ramp boost is computed as the difference
>> util_avg-util_est_enqueued. This number somehow represents a lower bound
> 
> That reads funny, maybe 'as the difference between util_avg and
> util_est_enqueued' ?

Indeed, it was not clear that it was a formula. Talking about formulas, I remember laying down
the relations between the various flavors of util signals in the v2 thread. This could be
turned rather easily into a doc page for PELT, along with a signal-processing-friendly
accurate description of how the PELT signals are built. Would such a patch be of any
interest the kernel tree ?

>> of how much extra utilization this tasks is actually using, compared to
>> our best current stable knowledge of it (which is util_est_enqueued).
>>
>> When the set of runnable tasks changes, the boost is disabled as the
>> impact of blocked utilization on util_avg will make the delta with
>> util_est_enqueued not very informative.
> 
>> @@ -561,6 +604,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
>>   		}
>>   	}
>>   
>> +
>>   	return get_next_freq(sg_policy, util, max);
>>   }
> 
> Surely we can do without this extra whitespace? :-)
> 
woops ...

Cheers,
Douglas