Messages in this thread | | | Date | Tue, 17 Apr 2018 15:21:19 +0100 | From | Matt Fleming <> | Subject | cpu stopper threads and load balancing leads to deadlock |
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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?
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