Commit bced76ae authored Jan 11, 2012 by Peter Zijlstra Committed by Ingo Molnar Jan 11, 2012

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>

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;
		}

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