linux-kernel - Re: [PATCH v4 3/3] sched/fair: Introduce a CPU capacity comparison helper

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Date:   Fri, 2 Apr 2021 15:04:34 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Qais Yousef <qais.yousef@....com>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4 3/3] sched/fair: Introduce a CPU capacity comparison helper

On Thu, 1 Apr 2021 at 21:30, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> During load-balance, groups classified as group_misfit_task are filtered
> out if they do not pass
>
>   group_smaller_max_cpu_capacity(<candidate group>, <local group>);
>
> which itself employs fits_capacity() to compare the sgc->max_capacity of
> both groups.
>
> Due to the underlying margin, fits_capacity(X, 1024) will return false for
> any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
> {261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
> CPUs, misfit migration will never intentionally upmigrate it to a CPU of
> higher capacity due to the aforementioned margin.
>
> One may argue the 20% margin of fits_capacity() is excessive in the advent
> of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
> is that fits_capacity() is meant to compare a utilization value to a
> capacity value, whereas here it is being used to compare two capacity
> values. As CPU capacity and task utilization have different dynamics, a
> sensible approach here would be to add a new helper dedicated to comparing
> CPU capacities.
>
> While at it, replace group_smaller_{min, max}_cpu_capacity() with
> comparisons of the source group's min/max capacity and the destination
> CPU's capacity.
>
> Reviewed-by: Qais Yousef <qais.yousef@....com>
> Tested-by: Lingutla Chandrasekhar <clingutla@...eaurora.org>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c | 33 ++++++++++-----------------------
>  1 file changed, 10 insertions(+), 23 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d8077f82a380..c9c5c2697998 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
>   */
>  #define fits_capacity(cap, max)        ((cap) * 1280 < (max) * 1024)
>
> +/*
> + * The margin used when comparing CPU capacities.
> + * is 'cap1' noticeably greater than 'cap2'
> + *
> + * (default: ~5%)
> + */
> +#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
>  #endif
>
>  #ifdef CONFIG_CFS_BANDWIDTH
> @@ -8364,26 +8371,6 @@ group_is_overloaded(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
>         return false;
>  }
>
> -/*
> - * group_smaller_min_cpu_capacity: Returns true if sched_group sg has smaller
> - * per-CPU capacity than sched_group ref.
> - */
> -static inline bool
> -group_smaller_min_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
> -{
> -       return fits_capacity(sg->sgc->min_capacity, ref->sgc->min_capacity);
> -}
> -
> -/*
> - * group_smaller_max_cpu_capacity: Returns true if sched_group sg has smaller
> - * per-CPU capacity_orig than sched_group ref.
> - */
> -static inline bool
> -group_smaller_max_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
> -{
> -       return fits_capacity(sg->sgc->max_capacity, ref->sgc->max_capacity);
> -}
> -
>  static inline enum
>  group_type group_classify(unsigned int imbalance_pct,
>                           struct sched_group *group,
> @@ -8539,7 +8526,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>          * internally or be covered by avg_load imbalance (eventually).
>          */
>         if (sgs->group_type == group_misfit_task &&
> -           (!group_smaller_max_cpu_capacity(sg, sds->local) ||
> +           (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
>              sds->local_stat.group_type != group_has_spare))
>                 return false;
>
> @@ -8623,7 +8610,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>          */
>         if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
>             (sgs->group_type <= group_fully_busy) &&
> -           (group_smaller_min_cpu_capacity(sds->local, sg)))
> +           (capacity_greater(sg->sgc->min_capacity, capacity_of(env->dst_cpu))))
>                 return false;
>
>         return true;
> @@ -9423,7 +9410,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>                  * average load.
>                  */
>                 if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> -                   capacity_of(env->dst_cpu) < capacity &&
> +                   !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
>                     nr_running == 1)
>                         continue;
>
> --
> 2.25.1
>