linux-kernel - Re: [RFC v3 5/5] sched/{core,cpufreq_schedutil}: add capacity clamping for RT/DL tasks

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Date:   Thu, 16 Mar 2017 11:16:44 +0000
From:   Juri Lelli <juri.lelli@....com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        "Joel Fernandes (Google)" <joel.opensrc@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Andres Oportus <andresoportus@...gle.com>
Subject: Re: [RFC v3 5/5] sched/{core,cpufreq_schedutil}: add capacity
 clamping for RT/DL tasks

On 15/03/17 16:40, Joel Fernandes wrote:
> On Wed, Mar 15, 2017 at 9:24 AM, Juri Lelli <juri.lelli@....com> wrote:
> [..]
> >
> >> > However, trying to quickly summarize how that would work (for who is
> >> > already somewhat familiar with reclaiming bits):
> >> >
> >> >  - a task utilization contribution is accounted for (at rq level) as
> >> >    soon as it wakes up for the first time in a new period
> >> >  - its contribution is then removed after the 0lag time (or when the
> >> >    task gets throttled)
> >> >  - frequency transitions are triggered accordingly
> >> >
> >> > So, I don't see why triggering a go down request after the 0lag time
> >> > expired and quickly reacting to tasks waking up would have create
> >> > problems in your case?
> >>
> >> In my experience, the 'reacting to tasks' bit doesn't work very well.
> >
> > Humm.. but in this case we won't be 'reacting', we will be
> > 'anticipating' tasks' needs, right?
> 
> Are you saying we will start ramping frequency before the next
> activation so that we're ready for it?
> 

I'm saying that there is no need to ramp, simply select the frequency
that is needed for a task (or a set of them).

> If not, it sounds like it will only make the frequency request on the
> next activation when the Active bandwidth increases due to the task
> waking up. By then task has already started to run, right?
> 

When the task is enqueued back we select the frequency considering its
bandwidth request (and the bandwidth/utilization of the others). So,
when it actually starts running it will already have enough capacity to
finish in time.