Which kernel function manages balancing the RB-tree of the linux CFS?

Question

From what I have understood so far about the Linux CFS is that scheduling entities are indexed by their virtual runtime (vruntime) inside the RB-tree.

The scheduler regularly updates this vruntime by calling the update_curr function which basically increase the vruntime value of the current running entity.

What I don't understand is how does the scheduler keeps the RB-tree always well ordered. The update_curr function increases the value of vruntime but does not seem to check whether the entity should be moved back into the right of RB-tree. Which function performs this check?

score 1 · Accepted Answer · edited Aug 26 '20 at 21:24

While Ahmed is right about update_curr, I was still confused about the fact the tree could be unbalanced between the moment vruntime is updated and the eventual call to resched_curr. As a matter of fact, it turns out that the task that is currently scheduled is not part of the tree.

It is removed from the tree in set_next_entity(cfs_rq_of(se), se); which is called when pick_next_task_fair is invoked. So the tree is always balanced and the only function performing the balancing are enqueue_entity and dequeue_entity

score 0 · Answer 2 · answered Mar 23 '18 at 07:51

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update_curr doesn't just increase the value of vruntime it also calls account_cfs_rq_runtime which in turn eventually calls resched_curr which will fix the RB-tree (eventually)

answered Mar 23 '18 at 07:51

Ahmed Masud

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Does this mean that the tree is not fixed until the ```resched_curr``` call? – Tyrann Mar 25 '18 at 23:26
right... but resched_curr is guaranteed to be called. – Ahmed Masud Mar 27 '18 at 17:22

Which kernel function manages balancing the RB-tree of the linux CFS?

2 Answers2