linux-kernel - [PATCH v3 5/5] sched/fair: Fix use of find_idlest

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Message-Id: <20170831115802.26365-6-brendan.jackman@arm.com>
Date:   Thu, 31 Aug 2017 12:58:02 +0100
From:   Brendan Jackman <brendan.jackman@....com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Josef Bacik <josef@...icpanda.com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: [PATCH v3 5/5] sched/fair: Fix use of find_idlest_group when local group is idlest

find_idlest_group returns NULL when the local group is idlest. The
caller then continues the find_idlest_group search at a lower level
of the current CPU's sched_domain hierarchy. find_idlest_group_cpu is
not consulted and, crucially, @new_cpu is not updated. This means the
search is pointless and we return @prev_cpu from select_task_rq_fair.

This is fixed by initialising @new_cpu to @cpu instead of
@prev_cpu.

Signed-off-by: Brendan Jackman <brendan.jackman@....com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Josef Bacik <josef@...icpanda.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Morten Rasmussen <morten.rasmussen@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2608091..f93cb97 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5567,7 +5567,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
 static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p,
 				  int cpu, int prev_cpu, int sd_flag)
 {
-	int new_cpu = prev_cpu;
+	int new_cpu = cpu;
 
 	if (!cpumask_intersects(sched_domain_span(sd), &p->cpus_allowed))
 		return prev_cpu;
-- 
2.7.4