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

Eric and David reported dead machines and traced it to commit a195f00 ("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 <[email protected]> Tested-by: Eric Dumazet <[email protected]> Reported-by: David Ahern <[email protected]> [ logic cleanup as suggested by Eric ] Signed-off-by: Peter Zijlstra <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Martin Schwidefsky <[email protected]> Cc: Frederic Weisbecker <[email protected]> Cc: Suresh Siddha <[email protected]> Link: http://lkml.kernel.org/r/1326297936.2442.157.camel@twins Signed-off-by: Ingo Molnar <[email protected]>
kishon · Jan 11, 2012 · bced76a · bced76a
1 parent 6db9dc1
commit bced76a
Showing 1 changed file with 7 additions and 3 deletions.
diff --git 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 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 			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;
 		}