Messages in this thread |  | | Subject | Re: cpu stopper threads and load balancing leads to deadlock | From | Mike Galbraith <> | Date | Wed, 18 Apr 2018 07:47:55 +0200 |
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On Tue, 2018-04-17 at 15:21 +0100, Matt Fleming wrote: > Hi guys, > > We've seen a bug in one of our SLE kernels where the cpu stopper > thread ("migration/15") is entering idle balance. This then triggers > active load balance. > > At the same time, a task on another CPU triggers a page fault and NUMA > balancing kicks in to try and migrate the task closer to the NUMA node > for that page (we're inside stop_two_cpus()). This faulting task is > spinning in try_to_wake_up() (inside smp_cond_load_acquire(&p->on_cpu, > !VAL)), waiting for "migration/15" to context switch. > > Unfortunately, because "migration/15" is doing active load balance > it's spinning waiting for the NUMA-page-faulting CPU's stopper lock, > which is already held (since it's inside stop_two_cpus()). > > Deadlock ensues. > > This seems like a situation that should be prohibited, but I cannot > find any code to prevent it. Is it OK for stopper threads to load > balance? Is there something that should prevent this situation from > happening?
I don't see anything to stop the deadlock either, would exclude stop class from playing idle balancer entirely, though I suppose you could check for caller being stop class in need_active_balance(). I don't think any RT class playing idle balancer is particularly wonderful.
-Mike
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