linux-kernel - Re: [PATCH v5 05/10] cpufreq/schedutil: get max utilization

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Date:   Mon, 28 May 2018 16:57:34 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Quentin Perret <quentin.perret@....com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Alessio Balsini <alessio.balsini@...tannapisa.it>
Subject: Re: [PATCH v5 05/10] cpufreq/schedutil: get max utilization

Hi Juri,

On 28 May 2018 at 12:12, Juri Lelli <juri.lelli@...hat.com> wrote:
> Hi Vincent,
>
> On 25/05/18 15:12, Vincent Guittot wrote:
>> Now that we have both the dl class bandwidth requirement and the dl class
>> utilization, we can use the max of the 2 values when agregating the
>> utilization of the CPU.
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>>  kernel/sched/sched.h | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 4526ba6..0eb07a8 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -2194,7 +2194,11 @@ static inline void cpufreq_update_util(struct rq *rq, unsigned int flags) {}
>>  #ifdef CONFIG_CPU_FREQ_GOV_SCHEDUTIL
>>  static inline unsigned long cpu_util_dl(struct rq *rq)
>>  {
>> -     return (rq->dl.running_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT;
>> +     unsigned long util = (rq->dl.running_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT;
>
> I'd be tempted to say the we actually want to cap to this one above
> instead of using the max (as you are proposing below) or the
> (theoretical) power reduction benefits of using DEADLINE for certain
> tasks might vanish.

The problem that I'm facing is that the sched_entity bandwidth is
removed after the 0-lag time and the rq->dl.running_bw goes back to
zero but if the DL task has preempted a CFS task, the utilization of
the CFS task will be lower than reality and schedutil will set a lower
OPP whereas the CPU is always running. The example with a RT task
described in the cover letter can be run with a DL task and will give
similar results.
avg_dl.util_avg tracks the utilization of the rq seen by the scheduler
whereas rq->dl.running_bw gives the minimum to match DL requirement.

>
>> +
>> +     util = max_t(unsigned long, util, READ_ONCE(rq->avg_dl.util_avg));
>> +
>> +     return util;
>
> Anyway, just a quick thought. I guess we should experiment with this a
> bit. Now, I don't unfortunately have a Arm platform at hand for testing.
> Claudio, Luca (now Cc-ed), would you be able to fire some tests with
> this change?
>
> Oh, adding Joel and Alessio as well that experimented with DEADLINE
> lately.
>
> Thanks,
>
> - Juri