Commit 4aed8aa4 authored by Valentin Schneider's avatar Valentin Schneider Committed by Peter Zijlstra
Browse files

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: default avatarValentin Schneider <valentin.schneider@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: default avatarQais Yousef <qais.yousef@arm.com>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Tested-by: default avatarLingutla Chandrasekhar <clingutla@codeaurora.org>
Link: https://lkml.kernel.org/r/20210407220628.3798191-4-valentin.schneider@arm.com
parent 23fb06d9

kernel/sched/fair.c

+10 −23
Original line number Diff line number Diff line
@@ -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;