Migrated CFS task getting an unfair advantage

From: Pavan Kondeti
Date: Wed Mar 09 2016 - 04:23:09 EST


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?


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