[tip:sched/core] sched/fair: Avoid pulling all tasks in idle balancing

[tip-bot for Yuyang Du <tipbot@zytor.com>]

Commit-ID:  985d3a4c11cd28251bcc7925aa2d7a9038910384
Gitweb:     http://git.kernel.org/tip/985d3a4c11cd28251bcc7925aa2d7a9038910384
Author:     Yuyang Du <yuyang.du@intel.com>
AuthorDate: Mon, 6 Jul 2015 06:11:51 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 3 Aug 2015 12:21:19 +0200

sched/fair: Avoid pulling all tasks in idle balancing

In idle balancing where a CPU going idle pulls tasks from another CPU,
a livelock may happen if the CPU pulls all tasks from another, makes
it idle, and this iterates. So just avoid this.

Reported-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Yuyang Du <yuyang.du@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150705221151.GF5197@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 587a2f6..8b384b8d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5863,6 +5863,13 @@ static int detach_tasks(struct lb_env *env)
 		return 0;
 
 	while (!list_empty(tasks)) {
+		/*
+		 * We don't want to steal all, otherwise we may be treated likewise,
+		 * which could at worst lead to a livelock crash.
+		 */
+		if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
+			break;
+
 		p = list_first_entry(tasks, struct task_struct, se.group_node);
 
 		env->loop++;