lkml.org 
[lkml]   [2021]   [Feb]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 3/8] sched/fair: Tweak misfit-related capacity checks
On Thu, 28 Jan 2021 at 19:32, Valentin Schneider
<valentin.schneider@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>);

group_smaller_max_cpu_capacity and group_smaller_max_cpu_capacity are
removed in the next patch. Merge this and the next and directly remove
them

>
> 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.
>
> Introduce capacity_greater(), which uses a 5% margin. Use it to replace the
> existing capacity checks. Note that check_cpu_capacity() uses yet another
> margin (sd->imbalance_pct), and is left alone for now.
>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
> kernel/sched/fair.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7d01fa0bfc7e..58ce0b22fcb0 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 'cap' noticeably greater than 'ref'
> + *
> + * (default: ~5%)
> + */
> +#define capacity_greater(cap, ref) ((ref) * 1078 < (cap) * 1024)
> #endif
>
> #ifdef CONFIG_CFS_BANDWIDTH
> @@ -8253,7 +8260,7 @@ check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
> static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
> {
> return rq->misfit_task_load &&
> - (rq->cpu_capacity_orig < rq->rd->max_cpu_capacity ||
> + (capacity_greater(rq->rd->max_cpu_capacity, rq->cpu_capacity_orig) ||

Why do you add a margin here whereas there was no margin before ?

> check_cpu_capacity(rq, sd));
> }
>
> @@ -8352,7 +8359,7 @@ group_is_overloaded(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
> 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);
> + return capacity_greater(ref->sgc->min_capacity, sg->sgc->min_capacity);
> }
>
> /*
> @@ -8362,7 +8369,7 @@ group_smaller_min_cpu_capacity(struct sched_group *sg, struct 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);
> + return capacity_greater(ref->sgc->max_capacity, sg->sgc->max_capacity);
> }
>
> static inline enum
> @@ -9421,7 +9428,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
> * average load.
> */
> if (sd_has_asym_cpucapacity(env->sd) &&
> - capacity_of(env->dst_cpu) < capacity &&
> + !capacity_greater(capacity_of(env->dst_cpu), capacity) &&

same here

> nr_running == 1)
> continue;
>
> --
> 2.27.0
>

\
 
 \ /
  Last update: 2021-02-06 00:10    [W:0.293 / U:0.312 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site