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Commit bced76ae authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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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: default avatarEric Dumazet <eric.dumazet@gmail.com>
Tested-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
Reported-by: default avatarDavid Ahern <dsahern@gmail.com>
[ logic cleanup as suggested by Eric ]
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@elte.hu>
parent 6db9dc15

kernel/sched/fair.c

+7 −3
Original line number Diff line number Diff line
@@ -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;

		}