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>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 587a2f67ceb17329a884d996fbf08884f245cb44..8b384b8d2f1db681f0a62e2e745e06c963a6e20a 100644 (file)
--- 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++;