Messages in this thread | | | Subject | Re: cpu stopper threads and load balancing leads to deadlock | From | Mike Galbraith <> | Date | Thu, 19 Apr 2018 07:38:44 +0200 |
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On Wed, 2018-04-18 at 07:47 +0200, Mike Galbraith wrote: > 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...
Bah, insufficient: __do_softirq() -> rebalance_domains() still bites.
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