linux-kernel - [RFCv3 PATCH 48/48] sched: Disable energy-unfriendly nohz kicks

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Date:	Wed,  4 Feb 2015 18:31:25 +0000
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	peterz@...radead.org, mingo@...hat.com
Cc:	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	yuyang.du@...el.com, preeti@...ux.vnet.ibm.com,
	mturquette@...aro.org, nico@...aro.org, rjw@...ysocki.net,
	juri.lelli@....com, linux-kernel@...r.kernel.org
Subject: [RFCv3 PATCH 48/48] sched: Disable energy-unfriendly nohz kicks

With energy-aware scheduling enabled nohz_kick_needed() generates many
nohz idle-balance kicks which lead to nothing when multiple tasks get
packed on a single cpu to save energy. This causes unnecessary wake-ups
and hence wastes energy. Make these conditions depend on !energy_aware()
for now until the energy-aware nohz story gets sorted out.

cc: Ingo Molnar <mingo@...hat.com>
cc: Peter Zijlstra <peterz@...radead.org>

Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
---
 kernel/sched/fair.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1c248f8..cfe65ae 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8195,6 +8195,8 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 	clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
 }
 
+static int cpu_overutilized(int cpu, struct sched_domain *sd);
+
 /*
  * Current heuristic for kicking the idle load balancer in the presence
  * of an idle cpu in the system.
@@ -8234,12 +8236,13 @@ static inline bool nohz_kick_needed(struct rq *rq)
 	if (time_before(now, nohz.next_balance))
 		return false;
 
-	if (rq->nr_running >= 2)
+	sd = rcu_dereference(rq->sd);
+	if (rq->nr_running >= 2 && (!energy_aware() || cpu_overutilized(cpu, sd)))
 		return true;
 
 	rcu_read_lock();
 	sd = rcu_dereference(per_cpu(sd_busy, cpu));
-	if (sd) {
+	if (sd && !energy_aware()) {
 		sgc = sd->groups->sgc;
 		nr_busy = atomic_read(&sgc->nr_busy_cpus);
 
-- 
1.9.1

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