linux-kernel - sched: Improve load balancing in the presence of idle CPUs

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Date:	Mon, 30 Mar 2015 11:55:29 -0700
From:	Jason Low <jason.low2@...com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>, peterz@...radead.org,
	mingo@...nel.org, Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	riel@...hat.com, daniel.lezcano@...aro.org,
	vincent.guittot@...aro.org, srikar@...ux.vnet.ibm.com,
	pjt@...gle.com, benh@...nel.crashing.org, efault@....de,
	linux-kernel@...r.kernel.org, iamjoonsoo.kim@....com,
	svaidy@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
	morten.rasmussen@....com, jason.low2@...com
Subject: sched: Improve load balancing in the presence of idle CPUs

Hi Preeti,

I noticed that another commit 4a725627f21d converted the check in
nohz_kick_needed() from idle_cpu() to rq->idle_balance, causing a
potentially outdated value to be used if this cpu is able to pull tasks
using rebalance_domains(), and nohz_kick_needed() directly returning
false.

Would this patch also help address some of the issue you are seeing?

Signed-off-by: Jason Low <jason.low2@...com>
---
 kernel/sched/fair.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fdae26e..ba8ec1a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7644,7 +7644,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 		 * balancing owner will pick it up.
 		 */
 		if (need_resched())
-			break;
+			goto end;
 
 		rq = cpu_rq(balance_cpu);
 
@@ -7687,7 +7687,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
 	int nr_busy, cpu = rq->cpu;
 	bool kick = false;
 
-	if (unlikely(rq->idle_balance))
+	if (unlikely(idle_cpu(cpu)))
 		return false;
 
        /*
-- 
1.7.2.5



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