linux-kernel - Re: [PATCH v6 04/11] cpufreq/schedutil: use rt utilization tracking

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Message-ID: <20180622125433.GA26461@e108498-lin.cambridge.arm.com>
Date:   Fri, 22 Jun 2018 13:54:34 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        Morten.Rasmussen@....com, viresh.kumar@...aro.org,
        valentin.schneider@....com, patrick.bellasi@....com,
        joel@...lfernandes.org, daniel.lezcano@...aro.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v6 04/11] cpufreq/schedutil: use rt utilization tracking

On Friday 22 Jun 2018 at 13:37:13 (+0200), Peter Zijlstra wrote:
> That is true.. So we could limit the scaling to the case where there is
> no idle time, something like:
> 
> 	util = sg_cpu->util_cfs;
> 
> 	cap_cfs = (1024 - (sg_cpu->util_rt + ...));
> 	if (util == cap_cfs)
> 		util = sg_cpu->max;
> 
> That specifically handles the '0% idle -> 100% freq' case, but I don't
> realy like edge behaviour like that. If for some reason it all doesn't
> quite align you're left with bits.
> 
> And the linear scaling is the next simplest thing that avoids the hard
> boundary case.

Right, so maybe we'll get something smoother by just summing the signals
as Vincent is proposing ? You will still request max freq for the
(util == cap_cfs) case you described. By definition, you will have
(util_cfs + util_rt + ...) == 1024 in this case.

cap_cfs is the delta between RT+DL+... and 1024, and the only case where
util_cfs can be equal to cap_cfs is if util_cfs fills that delta
entirely.

I hope that makes sense

Thanks,
Quentin