linux-kernel - Re: [RFCv5 PATCH 44/46] sched/fair: jump to max OPP when crossing UP threshold

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Date:	Wed, 08 Jul 2015 09:47:44 -0700
From:	Michael Turquette <mturquette@...libre.com>
To:	Morten Rasmussen <morten.rasmussen@....com>, peterz@...radead.org,
	mingo@...hat.com
Cc:	vincent.guittot@...aro.org, daniel.lezcano@...aro.org,
	"Dietmar Eggemann" <Dietmar.Eggemann@....com>, yuyang.du@...el.com,
	rjw@...ysocki.net, "Juri Lelli" <Juri.Lelli@....com>,
	sgurrappadi@...dia.com, pang.xunlei@....com.cn,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	"Juri Lelli" <juri.lelli@....com>
Subject: Re: [RFCv5 PATCH 44/46] sched/fair: jump to max OPP when crossing UP
 threshold

Quoting Morten Rasmussen (2015-07-07 11:24:27)
> From: Juri Lelli <juri.lelli@....com>
> 
> Since the true utilization of a long running task is not detectable while
> it is running and might be bigger than the current cpu capacity, create the
> maximum cpu capacity head room by requesting the maximum cpu capacity once
> the cpu usage plus the capacity margin exceeds the current capacity. This
> is also done to try to harm the performance of a task the least.
> 
> cc: Ingo Molnar <mingo@...hat.com>
> cc: Peter Zijlstra <peterz@...radead.org>
> 
> Signed-off-by: Juri Lelli <juri.lelli@....com>
> ---
>  kernel/sched/fair.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 323331f..c2d6de4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8586,6 +8586,25 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
>  
>         if (!rq->rd->overutilized && cpu_overutilized(task_cpu(curr)))
>                 rq->rd->overutilized = true;
> +
> +       /*
> +        * To make free room for a task that is building up its "real"
> +        * utilization and to harm its performance the least, request a
> +        * jump to max OPP as soon as get_cpu_usage() crosses the UP
> +        * threshold. The UP threshold is built relative to the current
> +        * capacity (OPP), by using same margin used to tell if a cpu
> +        * is overutilized (capacity_margin).
> +        */
> +       if (sched_energy_freq()) {
> +               int cpu = cpu_of(rq);
> +               unsigned long capacity_orig = capacity_orig_of(cpu);
> +               unsigned long capacity_curr = capacity_curr_of(cpu);
> +
> +               if (capacity_curr < capacity_orig &&
> +                   (capacity_curr * SCHED_LOAD_SCALE) <
> +                   (get_cpu_usage(cpu) * capacity_margin))
> +                       cpufreq_sched_set_cap(cpu, capacity_orig);

I'm sure that at some point the Product People are going to want to tune
the capacity value that is requested. Hard-coding the max
capacity/frequency in is a reasonable start, but at some point it would
be nice to fetch an intermediate capacity defined by the cpufreq driver
for this particular cpu. We have already seen that a lot in Android
devices using the interactive governor and it could be done from
cpufreq_sched_start().

Regards,
Mike

> +       }
>  }
>  
>  /*
> -- 
> 1.9.1
> 
> --
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