Hi Douglas,
On 27-Jun 18:16, Douglas RAILLARD 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
> util_avg-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).
Maybe it's worth to call out here that at rq-level we don't have an
EWMA. However, the enqueued estimated utilization is derived by
considering the _task_util_est() which factors in the moving average
of tasks and thus makes the signal more stable even in case of tasks
switching between big and small activations.
> 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.raill...@arm.com>
> ---
> kernel/sched/cpufreq_schedutil.c | 42 ++++++++++++++++++++++++++++++++
> 1 file changed, 42 insertions(+)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c
> b/kernel/sched/cpufreq_schedutil.c
> index 7ffc6fe3b670..3eabfd815195 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -60,6 +60,9 @@ struct sugov_cpu {
> unsigned long bw_dl;
> unsigned long max;
>
> + unsigned long ramp_boost;
> + unsigned long util_est_enqueued;
> +
> /* The field below is for single-CPU policies only: */
> #ifdef CONFIG_NO_HZ_COMMON
> unsigned long saved_idle_calls;
> @@ -174,6 +177,41 @@ 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);
> +}
> +
> +static unsigned long sugov_cpu_ramp_boost_update(struct sugov_cpu *sg_cpu,
> + unsigned long util)
> +{
> + struct rq *rq = cpu_rq(sg_cpu->cpu);
Since you don't really need the rq below, maybe better:
struct sched_avg *sa = &cpu_rq(sg_cpu->cpu)->cfs.avg;
?
> + 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 > util_est_enqueued)
> + boost = util_avg - util_est_enqueued;
The above should be:
if (util_est_enqueued == sg_cpu->util_est_enqueue &&
util_avg > util_est_enqueued) {
boost = util_avg - util_est_enqueued;
}
but perhaps you can also go for a fast bailout with something like:
if (util_avg <= util_est_enqueued)
return 0;
if (util_est_enqueued == sg_cpu->util_est_enqueue)
boost = util_avg - util_est_enqueued;
Moreover: could it make sense to add a threshold on a minimal boost
value to return non zero?
> +
> + sg_cpu->util_est_enqueued = util_est_enqueued;
> + WRITE_ONCE(sg_cpu->ramp_boost, boost);
> + return boost;
You don't seem to use this returned value: should be void?
> +}
> +
> /**
> * get_next_freq - Compute a new frequency for a given cpufreq policy.
> * @sg_policy: schedutil policy object to compute the new frequency for.
> @@ -504,6 +542,7 @@ static void sugov_update_single(struct update_util_data
> *hook, u64 time,
> busy = sugov_cpu_is_busy(sg_cpu);
>
> util = sugov_get_util(sg_cpu);
> + sugov_cpu_ramp_boost_update(sg_cpu, util);
> max = sg_cpu->max;
> util = sugov_iowait_apply(sg_cpu, time, util, max);
> next_f = get_next_freq(sg_policy, util, max);
> @@ -544,6 +583,8 @@ 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_util);
> j_max = j_sg_cpu->max;
> j_util = sugov_iowait_apply(j_sg_cpu, time, j_util, j_max);
>
> @@ -553,6 +594,7 @@ static unsigned int sugov_next_freq_shared(struct
> sugov_cpu *sg_cpu, u64 time)
> }
> }
>
> +
> return get_next_freq(sg_policy, util, max);
> }
Best,
Patrick
--
#include <best/regards.h>
Patrick Bellasi
