Peng Zhang commented on YARN-3405:

Thanks, my mistake on the code detail. And I will verify it works now.

And If queue1 level has some other sibling queue(like queue-2) that equals to 
queue-1's usage/fairshare,  "candidateQueue" still may be not the queue-1 
itself, because they are equal by comparing, and will depends on the queue 
Then queue-1-2 still cannot preempt its sibling, and cause some live lock issue 
like above second scenario.

> FairScheduler's preemption cannot happen between sibling in some case
> ---------------------------------------------------------------------
>                 Key: YARN-3405
>                 URL: https://issues.apache.org/jira/browse/YARN-3405
>             Project: Hadoop YARN
>          Issue Type: Bug
>          Components: fairscheduler
>    Affects Versions: 2.7.0
>            Reporter: Peng Zhang
>            Priority: Critical
> Queue hierarchy described as below:
> {noformat}
>              root
>               |         
>            queue-1        
>           /      \
> queue-1-1        queue-1-2
> {noformat}
> 1. When queue-1-1 is active and it has been assigned with all resources.
> 2. When queue-1-2 is active, and it cause some new preemption request.
> 3. But when do preemption, it now starts from root, and found queue-1 is not 
> over fairshare, so no recursion preemption to queue-1-1.
> 4. Finally queue-1-2 will be waiting for resource release form queue-1-1 
> itself.

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