linux-kernel - Re: [PATCH] cpufreq: schedutil: add up/down frequency transition rate limits

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Message-ID: <20161121141728.GF3092@twins.programming.kicks-ass.net>
Date:   Mon, 21 Nov 2016 15:17:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <Juri.Lelli@....com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Robin Randhawa <robin.randhawa@....com>,
        Steve Muckle <smuckle.linux@...il.com>, tkjos@...gle.com,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH] cpufreq: schedutil: add up/down frequency transition
 rate limits

On Mon, Nov 21, 2016 at 01:53:08PM +0000, Juri Lelli wrote:
> On 21/11/16 13:26, Peter Zijlstra wrote:

> > So the limited decay would be the dominant factor in ramp-up time,
> > leaving the regular PELT period the dominant factor for ramp-down.
> > 
> 
> Hmmm, AFAIU the limited decay will help not forgetting completely the
> contribution of tasks that sleep for a long time, but it won't modify
> the actual ramp-up of the signal. So, for new tasks we will need to play
> with a sensible initial value (trading off perf and power as usual).

Oh, you mean ramp-up for bright spanking new tasks? I forgot the
details, but I think we can fudge the 'history' such that those too ramp
up quickly.

> > (Note that the decay limit would only be applied on the per-task signal,
> > not the accumulated signal.)
> > 
> 
> Right, and since schedutil consumes the latter, we could still suffer
> from too frequent frequency switch events I guess (this is where the
> down threshold thing came as a quick and dirty fix). Maybe we can think
> of some smoothing applied to the accumulated signal, or make it decay
> slower (don't really know what this means in practice, though :) ?

Not sure I follow. So by limiting decay to the task value, the moment we
add it back to the accumulated signal (wakeup), the accumulated signal
jumps up quickly and ramp-up is achieved.