Calling task_tick_fair in CFS algorithm

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Why is there a for_each_sched_entity macro in task_tick_fair function called on system tick? According to me, only the run time characteristics of the current task should be updated. Why does it call entity_tick for the entire hierarchy of the current process?

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Benjamin Peterson On

CFS supports hierarchies of tasks, where entire subtrees can have their CPU usage accounting and quota aggregated. This features is used to implement cpu cgroups and CFS group scheduling. A interior node of the task tree can be thought of a as a virtual task than represents the CPU usage of its subtree.