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 <eric.dumazet@gmail.com>
Tested-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: David Ahern <dsahern@gmail.com>
[ logic cleanup as suggested by Eric ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1326297936.2442.157.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 7 insertions, 3 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8e42de9105f8..84adb2d66cbd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3130,8 +3130,10 @@ task_hot(struct task_struct *p, u64 now, struct sched_domain *sd)
 }
 
 #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?
@@ -4508,7 +4510,9 @@ redo:
                         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;
                 }