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Date: Thu, 1 Sep 2016 12:51:36 +0100
From: Patrick Bellasi <patrick.bellasi@....com>
To: Morten Rasmussen <morten.rasmussen@....com>
Cc: peterz@...radead.org, mingo@...hat.com, dietmar.eggemann@....com,
yuyang.du@...el.com, vincent.guittot@...aro.org,
mgalbraith@...e.de, sgurrappadi@...dia.com,
freedom.tan@...iatek.com, keita.kobayashi.ym@...esas.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/5] sched/fair: Track peak per-entity utilization
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@...hat.com>
> cc: Peter Zijlstra <peterz@...radead.org>
>
> Signed-off-by: Morten Rasmussen <morten.rasmussen@....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
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