On Thu, 1 Apr 2021 at 21:30, Valentin Schneider
<valentin.schnei...@arm.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.you...@arm.com>
> Tested-by: Lingutla Chandrasekhar <clingu...@codeaurora.org>
> Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com>
Reviewed-by: Vincent Guittot <vincent.guit...@linaro.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
>
