[tip:sched/core] sched/fair: Fix usage of find_idlest_group() when the local group is idlest

[tip-bot for Brendan Jackman]

Commit-ID:  93f50f90247e3e926bbe9830df089c64a5cec236
Gitweb:     https://git.kernel.org/tip/93f50f90247e3e926bbe9830df089c64a5cec236
Author:     Brendan Jackman <brendan.jackman@xxxxxxx>
AuthorDate: Thu, 5 Oct 2017 12:45:16 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 10 Oct 2017 11:45:36 +0200

sched/fair: Fix usage of find_idlest_group() when the 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@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Josef Bacik <jbacik@xxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20171005114516.18617-6-brendan.jackman@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 ed80d6b..56f343b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5917,7 +5917,7 @@ 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;