linux-kernel - [tip:sched/core] sched/fair: Fix usage of find_idlest

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Date:   Tue, 10 Oct 2017 04:00:39 -0700
From:   tip-bot for Brendan Jackman <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     morten.rasmussen@....com, josef@...icpanda.com, hpa@...or.com,
        brendan.jackman@....com, torvalds@...ux-foundation.org,
        vincent.guittot@...aro.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        efault@....de, jbacik@...com, dietmar.eggemann@....com
Subject: [tip:sched/core] sched/fair: Fix usage of find_idlest_group() when
 no groups are allowed

Commit-ID:  6fee85ccbc76e8aeba43dc120c5fa3c5409a4e2c
Gitweb:     https://git.kernel.org/tip/6fee85ccbc76e8aeba43dc120c5fa3c5409a4e2c
Author:     Brendan Jackman <brendan.jackman@....com>
AuthorDate: Thu, 5 Oct 2017 12:45:15 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 10 Oct 2017 11:45:35 +0200

sched/fair: Fix usage of find_idlest_group() when no groups are allowed

When 'p' is not allowed on any of the CPUs in the sched_domain, we
currently return NULL from find_idlest_group(), and pointlessly
continue the search on lower sched_domain levels (where 'p' is also not
allowed) before returning prev_cpu regardless (as we have not updated
new_cpu).

Add an explicit check for this case, and add a comment to
find_idlest_group(). Now when find_idlest_group() returns NULL, it always
means that the local group is allowed and idlest.

Signed-off-by: Brendan Jackman <brendan.jackman@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Reviewed-by: Josef Bacik <jbacik@...com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Josef Bacik <josef@...icpanda.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Morten Rasmussen <morten.rasmussen@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20171005114516.18617-5-brendan.jackman@arm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cca1835..ed80d6b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5730,6 +5730,8 @@ static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
 /*
  * find_idlest_group finds and returns the least busy CPU group within the
  * domain.
+ *
+ * Assumes p is allowed on at least one CPU in sd.
  */
 static struct sched_group *
 find_idlest_group(struct sched_domain *sd, struct task_struct *p,
@@ -5917,6 +5919,9 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
 {
 	int new_cpu = prev_cpu;
 
+	if (!cpumask_intersects(sched_domain_span(sd), &p->cpus_allowed))
+		return prev_cpu;
+
 	while (sd) {
 		struct sched_group *group;
 		struct sched_domain *tmp;