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sched/fair: Introduce a CPU capacity comparison helper 

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.

Also note that comparing capacity extrema of local and source sched_group's
doesn't make much sense when at the day of the day the imbalance will be
pulled by a known env->dst_cpu, whose capacity can be anywhere within the
local group's capacity extrema.

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.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Qais Yousef <qais.yousef@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
Link: https://lkml.kernel.org/r/20210407220628.3798191-4-valentin.schneider@arm.com
This commit is contained in:
Valentin Schneider 2021-04-07 23:06:28 +01:00 committed by Peter Zijlstra
parent 23fb06d960
commit 4aed8aa415
1 changed files with 10 additions and 23 deletions
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33
kernel/sched/fair.c
View File

@ -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
@ -8395,26 +8402,6 @@ 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,
@ -8570,7 +8557,7 @@ 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;
@ -8654,7 +8641,7 @@ 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;
@ -9454,7 +9441,7 @@ 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;