Re: [RFC PATCH v4 4/6] sched/cpufreq: Introduce sugov_cpu_ramp_boost

[Douglas Raillard <douglas.raillard@arm.com>]

On 1/23/20 3:55 PM, Rafael J. Wysocki wrote:
> On Wed, Jan 22, 2020 at 6:36 PM Douglas RAILLARD
> <douglas.raillard@arm.com> wrote:
>>
>> Use the utilization signals dynamic to detect when the utilization of a
>> set of tasks starts increasing because of a change in tasks' behavior.
>> This allows detecting when spending extra power for faster frequency
>> ramp up response would be beneficial to the reactivity of the system.
>>
>> This ramp boost is computed as the difference between util_avg and
>> util_est_enqueued. This number somehow represents a lower bound of how
>> much extra utilization this tasks is actually using, compared to our
>> best current stable knowledge of it (which is util_est_enqueued).
>>
>> When the set of runnable tasks changes, the boost is disabled as the
>> impact of blocked utilization on util_avg will make the delta with
>> util_est_enqueued not very informative.
>>
>> Signed-off-by: Douglas RAILLARD <douglas.raillard@arm.com>
>> ---
>>  kernel/sched/cpufreq_schedutil.c | 43 ++++++++++++++++++++++++++++++++
>>  1 file changed, 43 insertions(+)
>>
>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>> index 608963da4916..25a410a1ff6a 100644
>> --- a/kernel/sched/cpufreq_schedutil.c
>> +++ b/kernel/sched/cpufreq_schedutil.c
>> @@ -61,6 +61,10 @@ struct sugov_cpu {
>>         unsigned long           bw_dl;
>>         unsigned long           max;
>>
>> +       unsigned long           ramp_boost;
>> +       unsigned long           util_est_enqueued;
>> +       unsigned long           util_avg;
>> +
>>         /* The field below is for single-CPU policies only: */
>>  #ifdef CONFIG_NO_HZ_COMMON
>>         unsigned long           saved_idle_calls;
>> @@ -183,6 +187,42 @@ static void sugov_deferred_update(struct sugov_policy *sg_policy, u64 time,
>>         }
>>  }
>>
>> +static unsigned long sugov_cpu_ramp_boost(struct sugov_cpu *sg_cpu)
>> +{
>> +       return READ_ONCE(sg_cpu->ramp_boost);
>> +}
> 
> Where exactly is this function used?

In the next commit where the boost value is actually used to do
something. The function is introduced here to keep the
WRITE_ONCE/READ_ONCE pair together.

> 
>> +
>> +static unsigned long sugov_cpu_ramp_boost_update(struct sugov_cpu *sg_cpu)
>> +{
>> +       struct rq *rq = cpu_rq(sg_cpu->cpu);
>> +       unsigned long util_est_enqueued;
>> +       unsigned long util_avg;
>> +       unsigned long boost = 0;
>> +
>> +       util_est_enqueued = READ_ONCE(rq->cfs.avg.util_est.enqueued);
>> +       util_avg = READ_ONCE(rq->cfs.avg.util_avg);
>> +
>> +       /*
>> +        * Boost when util_avg becomes higher than the previous stable
>> +        * knowledge of the enqueued tasks' set util, which is CPU's
>> +        * util_est_enqueued.
>> +        *
>> +        * We try to spot changes in the workload itself, so we want to
>> +        * avoid the noise of tasks being enqueued/dequeued. To do that,
>> +        * we only trigger boosting when the "amount of work" enqueued
>> +        * is stable.
>> +        */
>> +       if (util_est_enqueued == sg_cpu->util_est_enqueued &&
>> +           util_avg >= sg_cpu->util_avg &&
>> +           util_avg > util_est_enqueued)
>> +               boost = util_avg - util_est_enqueued;
>> +
>> +       sg_cpu->util_est_enqueued = util_est_enqueued;
>> +       sg_cpu->util_avg = util_avg;
>> +       WRITE_ONCE(sg_cpu->ramp_boost, boost);
>> +       return boost;
>> +}
>> +
>>  /**
>>   * get_next_freq - Compute a new frequency for a given cpufreq policy.
>>   * @sg_policy: schedutil policy object to compute the new frequency for.
>> @@ -514,6 +554,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>>         busy = !sg_policy->need_freq_update && sugov_cpu_is_busy(sg_cpu);
>>
>>         util = sugov_get_util(sg_cpu);
>> +       sugov_cpu_ramp_boost_update(sg_cpu);
>>         max = sg_cpu->max;
>>         util = sugov_iowait_apply(sg_cpu, time, util, max);
>>         next_f = get_next_freq(sg_policy, util, max);
>> @@ -554,6 +595,8 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
>>                 unsigned long j_util, j_max;
>>
>>                 j_util = sugov_get_util(j_sg_cpu);
>> +               if (j_sg_cpu == sg_cpu)
>> +                       sugov_cpu_ramp_boost_update(sg_cpu);
>>                 j_max = j_sg_cpu->max;
>>                 j_util = sugov_iowait_apply(j_sg_cpu, time, j_util, j_max);
>>
>> --