Messages in this thread | | | Date | Wed, 9 Mar 2016 14:52:57 +0530 | Subject | Migrated CFS task getting an unfair advantage | From | Pavan Kondeti <> |
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Hi
When a CFS task is enqueued during migration (load balance or change in affinity), its vruntime is normalized before updating the current and cfs_rq->min_vruntime. If the current entity is a low priority task or belongs to a cgroup that has lower cpu.shares and it is the only entity queued, there is a possibility of big update to the cfs_rq->min_vruntime. As the migrated task is normalized before this update, it gets an unfair advantage over tasks queued after this point. If the migrated task is a CPU hogger, the other CFS tasks queued on this CPU gets starved.
This problem can be simulated by running the below workload:
- create NR_CPU low prio CFS tasks affined to each CPU and put them under a cgroup with cpu.shares = 52 (1024/20). These tasks run forever. - create another CFS task which runs forever and periodically hops across all CPUs by changing the affinity. - We could see the destination CPU's cfs_rq->min_vruntime falls behind the migrated task by few hundreds of msec after migration.
If we add the migrated task to destination CPU cfs_rq's rb tree before updating the current in enqueue_entity(), the cfs_rq->min_vruntime does not go beyond the newly migrated task. Is this an acceptable solution?
Thanks, Pavan
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