linux-kernel - [tip:sched/core] sched/fair: Avoid pulling tasks from non-overloaded higher capacity groups

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Date:   Wed, 16 Nov 2016 04:14:08 -0800
From:   tip-bot for Morten Rasmussen <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, hpa@...or.com,
        morten.rasmussen@....com, peterz@...radead.org, mingo@...nel.org
Subject: [tip:sched/core] sched/fair: Avoid pulling tasks from
 non-overloaded higher capacity groups

Commit-ID:  9e0994c0a1c1f82c705f1f66388e1bcffcee8bb9
Gitweb:     http://git.kernel.org/tip/9e0994c0a1c1f82c705f1f66388e1bcffcee8bb9
Author:     Morten Rasmussen <morten.rasmussen@....com>
AuthorDate: Fri, 14 Oct 2016 14:41:10 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 16 Nov 2016 10:29:06 +0100

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@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: dietmar.eggemann@....com
Cc: freedom.tan@...iatek.com
Cc: keita.kobayashi.ym@...esas.com
Cc: mgalbraith@...e.de
Cc: sgurrappadi@...dia.com
Cc: vincent.guittot@...aro.org
Cc: yuyang.du@...el.com
Link: http://lkml.kernel.org/r/1476452472-24740-5-git-send-email-morten.rasmussen@arm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index faf8f18..ee39bfd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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;