Re: [PATCH v3] sched/freq: move call to cpufreq_update_util

[Rafael J. Wysocki]

On Wed, Nov 13, 2019 at 9:21 PM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> update_cfs_rq_load_avg() calls cfs_rq_util_change() everytime pelt decays,
> which might be inefficient when cpufreq driver has rate limitation.
>
> When a task is attached on a CPU, we have call path:
>
> update_load_avg()
>   update_cfs_rq_load_avg()
>     cfs_rq_util_change -- > trig frequency update
>   attach_entity_load_avg()
>     cfs_rq_util_change -- > trig frequency update
>
> The 1st frequency update will not take into account the utilization of the
> newly attached task and the 2nd one might be discard because of rate
> limitation of the cpufreq driver.
>
> update_cfs_rq_load_avg() is only called by update_blocked_averages()
> and update_load_avg() so we can move the call to
> cfs_rq_util_change/cpufreq_update_util() into these 2 functions. It's also
> interesting to notice that update_load_avg() already calls directly
> cfs_rq_util_change() for !SMP case.
>
> This changes will also ensure that cpufreq_update_util() is called even
> when there is no more CFS rq in the leaf_cfs_rq_list to update but only
> irq, rt or dl pelt signals.
>
> Reported-by: Doug Smythies <dsmythies@xxxxxxxxx>
> Fixes: 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load balancing path")
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

Looks reasonable to me:

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

> ---
>
> changes for v3:
> - fix typo
> - test the decay of root cfs_rq even for !CONFIG_FAIR_GROUP_SCHED case
>
>  kernel/sched/fair.c | 39 ++++++++++++++++++++++++++-------------
>  1 file changed, 26 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69a81a5..0a8f4ea 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3504,9 +3504,6 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
>         cfs_rq->load_last_update_time_copy = sa->last_update_time;
>  #endif
>
> -       if (decayed)
> -               cfs_rq_util_change(cfs_rq, 0);
> -
>         return decayed;
>  }
>
> @@ -3616,8 +3613,12 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>                 attach_entity_load_avg(cfs_rq, se, SCHED_CPUFREQ_MIGRATION);
>                 update_tg_load_avg(cfs_rq, 0);
>
> -       } else if (decayed && (flags & UPDATE_TG))
> -               update_tg_load_avg(cfs_rq, 0);
> +       } else if (decayed) {
> +               cfs_rq_util_change(cfs_rq, 0);
> +
> +               if (flags & UPDATE_TG)
> +                       update_tg_load_avg(cfs_rq, 0);
> +       }
>  }
>
>  #ifndef CONFIG_64BIT
> @@ -7543,6 +7544,7 @@ static void update_blocked_averages(int cpu)
>         const struct sched_class *curr_class;
>         struct rq_flags rf;
>         bool done = true;
> +       int decayed;
>
>         rq_lock_irqsave(rq, &rf);
>         update_rq_clock(rq);
> @@ -7552,9 +7554,9 @@ static void update_blocked_averages(int cpu)
>          * that RT, DL and IRQ signals have been updated before updating CFS.
>          */
>         curr_class = rq->curr->sched_class;
> -       update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> -       update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> -       update_irq_load_avg(rq, 0);
> +       decayed = update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> +       decayed |= update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> +       decayed |= update_irq_load_avg(rq, 0);
>
>         /* Don't need periodic decay once load/util_avg are null */
>         if (others_have_blocked(rq))
> @@ -7567,9 +7569,13 @@ static void update_blocked_averages(int cpu)
>         for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos) {
>                 struct sched_entity *se;
>
> -               if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq))
> +               if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) {
>                         update_tg_load_avg(cfs_rq, 0);
>
> +                       if (cfs_rq == &rq->cfs)
> +                               decayed = 1;
> +               }
> +
>                 /* Propagate pending load changes to the parent, if any: */
>                 se = cfs_rq->tg->se[cpu];
>                 if (se && !skip_blocked_update(se))
> @@ -7588,6 +7594,9 @@ static void update_blocked_averages(int cpu)
>         }
>
>         update_blocked_load_status(rq, !done);
> +
> +       if (decayed)
> +               cpufreq_update_util(rq, 0);
>         rq_unlock_irqrestore(rq, &rf);
>  }
>
> @@ -7644,6 +7653,7 @@ static inline void update_blocked_averages(int cpu)
>         struct cfs_rq *cfs_rq = &rq->cfs;
>         const struct sched_class *curr_class;
>         struct rq_flags rf;
> +       int decayed;
>
>         rq_lock_irqsave(rq, &rf);
>         update_rq_clock(rq);
> @@ -7653,13 +7663,16 @@ static inline void update_blocked_averages(int cpu)
>          * that RT, DL and IRQ signals have been updated before updating CFS.
>          */
>         curr_class = rq->curr->sched_class;
> -       update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> -       update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> -       update_irq_load_avg(rq, 0);
> +       decayed = update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> +       decayed |= update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> +       decayed |= update_irq_load_avg(rq, 0);
>
> -       update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
> +       decayed |= update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
>
>         update_blocked_load_status(rq, cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));
> +
> +       if (decayed)
> +               cpufreq_update_util(rq, 0);
>         rq_unlock_irqrestore(rq, &rf);
>  }
>
> --
> 2.7.4
>