sched: Fix lockup by limiting load-balance retries on lock-break

Eric and David reported dead machines and traced it to commit
a195f004 ("sched: Fix load-balance lock-breaking"), it turns out
there's still a scenario where we can end up re-trying forever.

Since there is no strict forward progress guarantee in the
load-balance iteration we can get stuck re-retrying the same
task-set over and over.

Creating a forward progress guarantee with the existing
structure is somewhat non-trivial, for now simply terminate the
retry loop after a few tries.

Reported-by: Eric Dumazet <[e-mail]>
Tested-by: Eric Dumazet <[e-mail]>
Reported-by: David Ahern <[e-mail]>
[ logic cleanup as suggested by Eric ]
Signed-off-by: Peter Zijlstra <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Suresh Siddha <[e-mail]>
Link: http://lkml.kernel.org/r/1326297936.2442.157.camel@twins
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/sched/fair.c

···
 }
 
 #define LBF_ALL_PINNED	0x01
-#define LBF_NEED_BREAK	0x02
-#define LBF_ABORT	0x04
+#define LBF_NEED_BREAK	0x02	/* clears into HAD_BREAK */
+#define LBF_HAD_BREAK	0x04
+#define LBF_HAD_BREAKS	0x0C	/* count HAD_BREAKs overflows into ABORT */
+#define LBF_ABORT	0x10
 
 /*
  * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
···
 			goto out_balanced;
 
 		if (lb_flags & LBF_NEED_BREAK) {
-			lb_flags &= ~LBF_NEED_BREAK;
+			lb_flags += LBF_HAD_BREAK - LBF_NEED_BREAK;
+			if (lb_flags & LBF_ABORT)
+				goto out_balanced;
 			goto redo;
 		}