[PATCH RESEND v3 5/5] sched/fair: Fix use of find_idlest_group when local group is idlest

Brendan Jackman
2017-10-05T11:45:48Z

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@arm.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.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 4a14ebca4d79..82a8e206657f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5955,7 +5955,7 @@ 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;
--
2.14.1