Messages in this thread Patch in this message | | | Subject | Re: [BUG] kernel freezes with latest tree | From | Peter Zijlstra <> | Date | Wed, 11 Jan 2012 17:05:36 +0100 |
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On Wed, 2012-01-11 at 16:56 +0100, Ingo Molnar wrote:
> Well, what happens if every CPU runs load_balance() and we keep > triggering: > > if (loops++ > sysctl_sched_nr_migrate) { > *lb_flags |= LBF_NEED_BREAK; > break; > } > > in this case load_balance() will do the retry: > > if (lb_flags & LBF_NEED_BREAK) { > lb_flags &= ~LBF_NEED_BREAK; > goto redo; > } > > but the retry starts the loop again: > > list_for_each_entry_safe(p, n, &busiest_cfs_rq->tasks, se.group_node) { > > so nobody is able to make progress: livelock/lockup.
Ah, right! Silly me. One possibility is to rotate that list, except that won't work for the cgroup case where we have another iteration.
OK, here's an updated patch..
--- Subject: sched: Limit load-balance retries on lock-break From: Peter Zijlstra <a.p.zijlstra@chello.nl> Date: Wed Jan 11 13:11:12 CET 2012
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> Reported-by: David Ahern <dsahern@gmail.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> [eric: logic cleanup] Tested-by: Eric Dumazet <eric.dumazet@gmail.com> Link: http://lkml.kernel.org/n/tip-ya9m8grb9wfc26uqnviq2wjq@git.kernel.org --- kernel/sched/fair.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3130,8 +3130,10 @@ task_hot(struct task_struct *p, u64 now, } #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, st 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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