Commit 9e0994c0 authored Oct 14, 2016 by Morten Rasmussen Committed by Ingo Molnar Nov 16, 2016

sched/fair: Avoid pulling tasks from non-overloaded higher capacity groups



For asymmetric CPU capacity systems it is counter-productive for
throughput if low capacity CPUs are pulling tasks from non-overloaded
CPUs with higher capacity. The assumption is that higher CPU capacity is
preferred over running alone in a group with lower CPU capacity.

This patch rejects higher CPU capacity groups with one or less task per
CPU as potential busiest group which could otherwise lead to a series of
failing load-balancing attempts leading to a force-migration.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: freedom.tan@mediatek.com
Cc: keita.kobayashi.ym@renesas.com
Cc: mgalbraith@suse.de
Cc: sgurrappadi@nvidia.com
Cc: vincent.guittot@linaro.org
Cc: yuyang.du@intel.com
Link: http://lkml.kernel.org/r/1476452472-24740-5-git-send-email-morten.rasmussen@arm.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent bf475ce0

kernel/sched/fair.c

+25 −0

Original line number	Diff line number	Diff line
		@@ -7073,6 +7073,17 @@ group_is_overloaded(struct lb_env env, struct sg_lb_stats sgs)
		return false;
		}

		/*
		* group_smaller_cpu_capacity: Returns true if sched_group sg has smaller
		* per-CPU capacity than sched_group ref.
		*/
		static inline bool
		group_smaller_cpu_capacity(struct sched_group sg, struct sched_group ref)
		{
		return sg->sgc->min_capacity * capacity_margin <
		ref->sgc->min_capacity * 1024;
		}

		static inline enum
		group_type group_classify(struct sched_group *group,
		struct sg_lb_stats *sgs)
		@@ -7176,6 +7187,20 @@ static bool update_sd_pick_busiest(struct lb_env *env,
		if (sgs->avg_load <= busiest->avg_load)
		return false;

		if (!(env->sd->flags & SD_ASYM_CPUCAPACITY))
		goto asym_packing;

		/*
		* Candidate sg has no more than one task per CPU and
		* has higher per-CPU capacity. Migrating tasks to less
		* capable CPUs may harm throughput. Maximize throughput,
		* power/energy consequences are not considered.
		*/
		if (sgs->sum_nr_running <= sgs->group_weight &&
		group_smaller_cpu_capacity(sds->local, sg))
		return false;

		asym_packing:
		/* This is the busiest node in its class. */
		if (!(env->sd->flags & SD_ASYM_PACKING))
		return true;

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