Re: [PATCH v4 5/5] sched/fair: Track peak per-entity utilization

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 31-Aug 11:52, Morten Rasmussen wrote:
> When using PELT (per-entity load tracking) utilization to place tasks at
> wake-up using the decayed utilization (due to sleep) leads to
> under-estimation of true utilization of the task. This could mean
> putting the task on a cpu with less available capacity than is actually
> needed. This issue can be mitigated by using 'peak' utilization instead
> of the decayed utilization for placement decisions, e.g. at task
> wake-up.
> 
> The 'peak' utilization metric, util_peak, tracks util_avg when the task
> is running and retains its previous value while the task is
> blocked/waiting on the rq. It is instantly updated to track util_avg
> again as soon as the task running again.
> 
> cc: Ingo Molnar <mingo@redhat.com>
> cc: Peter Zijlstra <peterz@infradead.org>
> 
> Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
> ---
>  include/linux/sched.h |  2 +-
>  kernel/sched/fair.c   | 23 ++++++++++++++---------
>  2 files changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d75024053e9b..fff4e4b6e654 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1282,7 +1282,7 @@ struct load_weight {
>  struct sched_avg {
>  	u64 last_update_time, load_sum;
>  	u32 util_sum, period_contrib;
> -	unsigned long load_avg, util_avg;
> +	unsigned long load_avg, util_avg, util_peak;

By adding util_peak here (in sched_avg) we implicitly define a new
signal for CFS RQs as well, but in the rest of this patch it seems we
use it only for tasks?

Overall this seems to be a filtered signal on top of PELT but just for
tasks. Perhaps something similar can be useful for CPUs utilization as
well...

>  };
>  
>  #ifdef CONFIG_SCHEDSTATS
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 68d8b40c546b..27534e36555b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -692,6 +692,7 @@ void init_entity_runnable_average(struct sched_entity *se)
>  	 * At this point, util_avg won't be used in select_task_rq_fair anyway
>  	 */
>  	sa->util_avg = 0;
> +	sa->util_peak = 0;

For consistency with other sched_avg's signals, perhaps we should report
the value of util_peak from:

   kernel/sched/debug.c::{print_cfs_group_statproc_sched_show_task,proc_sched_show_task}

>  	sa->util_sum = 0;
>  	/* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */
>  }
> @@ -743,6 +744,7 @@ void post_init_entity_util_avg(struct sched_entity *se)
>  		} else {
>  			sa->util_avg = cap;
>  		}
> +		sa->util_peak = sa->util_avg;
>  		sa->util_sum = sa->util_avg * LOAD_AVG_MAX;
>  	}
>  
> @@ -2804,6 +2806,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>  		sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
>  	}
>  
> +	if (running || sa->util_avg > sa->util_peak)
> +		sa->util_peak = sa->util_avg;

Do we really need to update this new signal so often?

It seems that we use it only at wakeup time, is it not enough
to cache the util_avg value in dequeue_task_fair() in case of a
DEQUEUE_SLEEP?

> +
>  	return decayed;
>  }
>  
> @@ -5184,7 +5189,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
>  	return 1;
>  }
>  
> -static inline int task_util(struct task_struct *p);
> +static inline int task_util_peak(struct task_struct *p);
>  static int cpu_util_wake(int cpu, struct task_struct *p);
>  
>  static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
> @@ -5267,14 +5272,14 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>  	/*
>  	 * The cross-over point between using spare capacity or least load
>  	 * is too conservative for high utilization tasks on partially
> -	 * utilized systems if we require spare_capacity > task_util(p),
> +	 * utilized systems if we require spare_capacity > task_util_peak(p),
>  	 * so we allow for some task stuffing by using
> -	 * spare_capacity > task_util(p)/2.
> +	 * spare_capacity > task_util_peak(p)/2.
>  	 */
> -	if (this_spare > task_util(p) / 2 &&
> +	if (this_spare > task_util_peak(p) / 2 &&
>  	    imbalance*this_spare > 100*most_spare)
>  		return NULL;
> -	else if (most_spare > task_util(p) / 2)
> +	else if (most_spare > task_util_peak(p) / 2)
>  		return most_spare_sg;
>  
>  	if (!idlest || 100*this_load < imbalance*min_load)
> @@ -5432,9 +5437,9 @@ static int cpu_util(int cpu)
>  	return (util >= capacity) ? capacity : util;
>  }
>  
> -static inline int task_util(struct task_struct *p)
> +static inline int task_util_peak(struct task_struct *p)
>  {
> -	return p->se.avg.util_avg;
> +	return p->se.avg.util_peak;
>  }
>  
>  /*
> @@ -5450,7 +5455,7 @@ static int cpu_util_wake(int cpu, struct task_struct *p)
>  		return cpu_util(cpu);
>  
>  	capacity = capacity_orig_of(cpu);
> -	util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
> +	util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util_peak(p), 0);
>  
>  	return (util >= capacity) ? capacity : util;
>  }
> @@ -5476,7 +5481,7 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
>  	/* Bring task utilization in sync with prev_cpu */
>  	sync_entity_load_avg(&p->se);
>  
> -	return min_cap * 1024 < task_util(p) * capacity_margin;
> +	return min_cap * 1024 < task_util_peak(p) * capacity_margin;
>  }
>  
>  /*
> -- 
> 1.9.1
> 

-- 
#include <best/regards.h>

Patrick Bellasi